/*
** 2001 September 15
**
** The author disclaims copyright to this source code.  In place of
** a legal notice, here is a blessing:
**
**    May you do good and not evil.
**    May you find forgiveness for yourself and forgive others.
**    May you share freely, never taking more than you give.
**
*************************************************************************
** This is the implementation of the page cache subsystem or "pager".
** 
** The pager is used to access a database disk file.  It implements
** atomic commit and rollback through the use of a journal file that
** is separate from the database file.  The pager also implements file
** locking to prevent two processes from writing the same database
** file simultaneously, or one process from reading the database while
** another is writing.
*/
#include "sqliteInt.h"
// #include "wal.h"


// /******************* NOTES ON THE DESIGN OF THE PAGER ************************
// **
// ** This comment block describes invariants that hold when using a rollback
// ** journal.  These invariants do not apply for journal_mode=WAL,
// ** journal_mode=MEMORY, or journal_mode=OFF.
// **
// ** Within this comment block, a page is deemed to have been synced
// ** automatically as soon as it is written when PRAGMA synchronous=OFF.
// ** Otherwise, the page is not synced until the xSync method of the VFS
// ** is called successfully on the file containing the page.
// **
// ** Definition:  A page of the database file is said to be "overwriteable" if
// ** one or more of the following are true about the page:
// ** 
// **     (a)  The original content of the page as it was at the beginning of
// **          the transaction has been written into the rollback journal and
// **          synced.
// ** 
// **     (b)  The page was a freelist leaf page at the start of the transaction.
// ** 
// **     (c)  The page number is greater than the largest page that existed in
// **          the database file at the start of the transaction.
// ** 
// ** (1) A page of the database file is never overwritten unless one of the
// **     following are true:
// ** 
// **     (a) The page and all other pages on the same sector are overwriteable.
// ** 
// **     (b) The atomic page write optimization is enabled, and the entire
// **         transaction other than the update of the transaction sequence
// **         number consists of a single page change.
// ** 
// ** (2) The content of a page written into the rollback journal exactly matches
// **     both the content in the database when the rollback journal was written
// **     and the content in the database at the beginning of the current
// **     transaction.
// ** 
// ** (3) Writes to the database file are an integer multiple of the page size
// **     in length and are aligned on a page boundary.
// ** 
// ** (4) Reads from the database file are either aligned on a page boundary and
// **     an integer multiple of the page size in length or are taken from the
// **     first 100 bytes of the database file.
// ** 
// ** (5) All writes to the database file are synced prior to the rollback journal
// **     being deleted, truncated, or zeroed.
// ** 
// ** (6) If a super-journal file is used, then all writes to the database file
// **     are synced prior to the super-journal being deleted.
// ** 
// ** Definition: Two databases (or the same database at two points it time)
// ** are said to be "logically equivalent" if they give the same answer to
// ** all queries.  Note in particular the content of freelist leaf
// ** pages can be changed arbitrarily without affecting the logical equivalence
// ** of the database.
// ** 
// ** (7) At any time, if any subset, including the empty set and the total set,
// **     of the unsynced changes to a rollback journal are removed and the 
// **     journal is rolled back, the resulting database file will be logically
// **     equivalent to the database file at the beginning of the transaction.
// ** 
// ** (8) When a transaction is rolled back, the xTruncate method of the VFS
// **     is called to restore the database file to the same size it was at
// **     the beginning of the transaction.  (In some VFSes, the xTruncate
// **     method is a no-op, but that does not change the fact the SQLite will
// **     invoke it.)
// ** 
// ** (9) Whenever the database file is modified, at least one bit in the range
// **     of bytes from 24 through 39 inclusive will be changed prior to releasing
// **     the EXCLUSIVE lock, thus signaling other connections on the same
// **     database to flush their caches.
// **
// ** (10) The pattern of bits in bytes 24 through 39 shall not repeat in less
// **      than one billion transactions.
// **
// ** (11) A database file is well-formed at the beginning and at the conclusion
// **      of every transaction.
// **
// ** (12) An EXCLUSIVE lock is held on the database file when writing to
// **      the database file.
// **
// ** (13) A SHARED lock is held on the database file while reading any
// **      content out of the database file.
// **
// ******************************************************************************/

// /*
// ** Macros for troubleshooting.  Normally turned off
// */
// #if 0
// int sqlite3PagerTrace=1;  /* True to enable tracing */
// #define sqlite3DebugPrintf printf
// #define PAGERTRACE(X)     if( sqlite3PagerTrace ){ sqlite3DebugPrintf X; }
// #else
// #define PAGERTRACE(X)
// #endif

// /*
// ** The following two macros are used within the PAGERTRACE() macros above
// ** to print out file-descriptors. 
// **
// ** PAGERID() takes a pointer to a Pager struct as its argument. The
// ** associated file-descriptor is returned. FILEHANDLEID() takes an sqlite3_file
// ** struct as its argument.
// */
// #define PAGERID(p) (SQLITE_PTR_TO_INT(p->fd))
// #define FILEHANDLEID(fd) (SQLITE_PTR_TO_INT(fd))

// /*
// ** The Pager.eState variable stores the current 'state' of a pager. A
// ** pager may be in any one of the seven states shown in the following
// ** state diagram.
// **
// **                            OPEN <------+------+
// **                              |         |      |
// **                              V         |      |
// **               +---------> READER-------+      |
// **               |              |                |
// **               |              V                |
// **               |<-------WRITER_LOCKED------> ERROR
// **               |              |                ^  
// **               |              V                |
// **               |<------WRITER_CACHEMOD-------->|
// **               |              |                |
// **               |              V                |
// **               |<-------WRITER_DBMOD---------->|
// **               |              |                |
// **               |              V                |
// **               +<------WRITER_FINISHED-------->+
// **
// **
// ** List of state transitions and the C [function] that performs each:
// ** 
// **   OPEN              -> READER              [sqlite3PagerSharedLock]
// **   READER            -> OPEN                [pager_unlock]
// **
// **   READER            -> WRITER_LOCKED       [sqlite3PagerBegin]
// **   WRITER_LOCKED     -> WRITER_CACHEMOD     [pager_open_journal]
// **   WRITER_CACHEMOD   -> WRITER_DBMOD        [syncJournal]
// **   WRITER_DBMOD      -> WRITER_FINISHED     [sqlite3PagerCommitPhaseOne]
// **   WRITER_***        -> READER              [pager_end_transaction]
// **
// **   WRITER_***        -> ERROR               [pager_error]
// **   ERROR             -> OPEN                [pager_unlock]
// ** 
// **
// **  OPEN:
// **
// **    The pager starts up in this state. Nothing is guaranteed in this
// **    state - the file may or may not be locked and the database size is
// **    unknown. The database may not be read or written.
// **
// **    * No read or write transaction is active.
// **    * Any lock, or no lock at all, may be held on the database file.
// **    * The dbSize, dbOrigSize and dbFileSize variables may not be trusted.
// **
// **  READER:
// **
// **    In this state all the requirements for reading the database in 
// **    rollback (non-WAL) mode are met. Unless the pager is (or recently
// **    was) in exclusive-locking mode, a user-level read transaction is 
// **    open. The database size is known in this state.
// **
// **    A connection running with locking_mode=normal enters this state when
// **    it opens a read-transaction on the database and returns to state
// **    OPEN after the read-transaction is completed. However a connection
// **    running in locking_mode=exclusive (including temp databases) remains in
// **    this state even after the read-transaction is closed. The only way
// **    a locking_mode=exclusive connection can transition from READER to OPEN
// **    is via the ERROR state (see below).
// ** 
// **    * A read transaction may be active (but a write-transaction cannot).
// **    * A SHARED or greater lock is held on the database file.
// **    * The dbSize variable may be trusted (even if a user-level read 
// **      transaction is not active). The dbOrigSize and dbFileSize variables
// **      may not be trusted at this point.
// **    * If the database is a WAL database, then the WAL connection is open.
// **    * Even if a read-transaction is not open, it is guaranteed that 
// **      there is no hot-journal in the file-system.
// **
// **  WRITER_LOCKED:
// **
// **    The pager moves to this state from READER when a write-transaction
// **    is first opened on the database. In WRITER_LOCKED state, all locks 
// **    required to start a write-transaction are held, but no actual 
// **    modifications to the cache or database have taken place.
// **
// **    In rollback mode, a RESERVED or (if the transaction was opened with 
// **    BEGIN EXCLUSIVE) EXCLUSIVE lock is obtained on the database file when
// **    moving to this state, but the journal file is not written to or opened 
// **    to in this state. If the transaction is committed or rolled back while 
// **    in WRITER_LOCKED state, all that is required is to unlock the database 
// **    file.
// **
// **    IN WAL mode, WalBeginWriteTransaction() is called to lock the log file.
// **    If the connection is running with locking_mode=exclusive, an attempt
// **    is made to obtain an EXCLUSIVE lock on the database file.
// **
// **    * A write transaction is active.
// **    * If the connection is open in rollback-mode, a RESERVED or greater 
// **      lock is held on the database file.
// **    * If the connection is open in WAL-mode, a WAL write transaction
// **      is open (i.e. sqlite3WalBeginWriteTransaction() has been successfully
// **      called).
// **    * The dbSize, dbOrigSize and dbFileSize variables are all valid.
// **    * The contents of the pager cache have not been modified.
// **    * The journal file may or may not be open.
// **    * Nothing (not even the first header) has been written to the journal.
// **
// **  WRITER_CACHEMOD:
// **
// **    A pager moves from WRITER_LOCKED state to this state when a page is
// **    first modified by the upper layer. In rollback mode the journal file
// **    is opened (if it is not already open) and a header written to the
// **    start of it. The database file on disk has not been modified.
// **
// **    * A write transaction is active.
// **    * A RESERVED or greater lock is held on the database file.
// **    * The journal file is open and the first header has been written 
// **      to it, but the header has not been synced to disk.
// **    * The contents of the page cache have been modified.
// **
// **  WRITER_DBMOD:
// **
// **    The pager transitions from WRITER_CACHEMOD into WRITER_DBMOD state
// **    when it modifies the contents of the database file. WAL connections
// **    never enter this state (since they do not modify the database file,
// **    just the log file).
// **
// **    * A write transaction is active.
// **    * An EXCLUSIVE or greater lock is held on the database file.
// **    * The journal file is open and the first header has been written 
// **      and synced to disk.
// **    * The contents of the page cache have been modified (and possibly
// **      written to disk).
// **
// **  WRITER_FINISHED:
// **
// **    It is not possible for a WAL connection to enter this state.
// **
// **    A rollback-mode pager changes to WRITER_FINISHED state from WRITER_DBMOD
// **    state after the entire transaction has been successfully written into the
// **    database file. In this state the transaction may be committed simply
// **    by finalizing the journal file. Once in WRITER_FINISHED state, it is 
// **    not possible to modify the database further. At this point, the upper 
// **    layer must either commit or rollback the transaction.
// **
// **    * A write transaction is active.
// **    * An EXCLUSIVE or greater lock is held on the database file.
// **    * All writing and syncing of journal and database data has finished.
// **      If no error occurred, all that remains is to finalize the journal to
// **      commit the transaction. If an error did occur, the caller will need
// **      to rollback the transaction. 
// **
// **  ERROR:
// **
// **    The ERROR state is entered when an IO or disk-full error (including
// **    SQLITE_IOERR_NOMEM) occurs at a point in the code that makes it 
// **    difficult to be sure that the in-memory pager state (cache contents, 
// **    db size etc.) are consistent with the contents of the file-system.
// **
// **    Temporary pager files may enter the ERROR state, but in-memory pagers
// **    cannot.
// **
// **    For example, if an IO error occurs while performing a rollback, 
// **    the contents of the page-cache may be left in an inconsistent state.
// **    At this point it would be dangerous to change back to READER state
// **    (as usually happens after a rollback). Any subsequent readers might
// **    report database corruption (due to the inconsistent cache), and if
// **    they upgrade to writers, they may inadvertently corrupt the database
// **    file. To avoid this hazard, the pager switches into the ERROR state
// **    instead of READER following such an error.
// **
// **    Once it has entered the ERROR state, any attempt to use the pager
// **    to read or write data returns an error. Eventually, once all 
// **    outstanding transactions have been abandoned, the pager is able to
// **    transition back to OPEN state, discarding the contents of the 
// **    page-cache and any other in-memory state at the same time. Everything
// **    is reloaded from disk (and, if necessary, hot-journal rollback peformed)
// **    when a read-transaction is next opened on the pager (transitioning
// **    the pager into READER state). At that point the system has recovered 
// **    from the error.
// **
// **    Specifically, the pager jumps into the ERROR state if:
// **
// **      1. An error occurs while attempting a rollback. This happens in
// **         function sqlite3PagerRollback().
// **
// **      2. An error occurs while attempting to finalize a journal file
// **         following a commit in function sqlite3PagerCommitPhaseTwo().
// **
// **      3. An error occurs while attempting to write to the journal or
// **         database file in function pagerStress() in order to free up
// **         memory.
// **
// **    In other cases, the error is returned to the b-tree layer. The b-tree
// **    layer then attempts a rollback operation. If the error condition 
// **    persists, the pager enters the ERROR state via condition (1) above.
// **
// **    Condition (3) is necessary because it can be triggered by a read-only
// **    statement executed within a transaction. In this case, if the error
// **    code were simply returned to the user, the b-tree layer would not
// **    automatically attempt a rollback, as it assumes that an error in a
// **    read-only statement cannot leave the pager in an internally inconsistent 
// **    state.
// **
// **    * The Pager.errCode variable is set to something other than SQLITE_OK.
// **    * There are one or more outstanding references to pages (after the
// **      last reference is dropped the pager should move back to OPEN state).
// **    * The pager is not an in-memory pager.
// **    
// **
// ** Notes:
// **
// **   * A pager is never in WRITER_DBMOD or WRITER_FINISHED state if the
// **     connection is open in WAL mode. A WAL connection is always in one
// **     of the first four states.
// **
// **   * Normally, a connection open in exclusive mode is never in PAGER_OPEN
// **     state. There are two exceptions: immediately after exclusive-mode has
// **     been turned on (and before any read or write transactions are 
// **     executed), and when the pager is leaving the "error state".
// **
// **   * See also: assert_pager_state().
// */
// #define PAGER_OPEN                  0
// #define PAGER_READER                1
// #define PAGER_WRITER_LOCKED         2
// #define PAGER_WRITER_CACHEMOD       3
// #define PAGER_WRITER_DBMOD          4
// #define PAGER_WRITER_FINISHED       5
// #define PAGER_ERROR                 6

// /*
// ** The Pager.eLock variable is almost always set to one of the 
// ** following locking-states, according to the lock currently held on
// ** the database file: NO_LOCK, SHARED_LOCK, RESERVED_LOCK or EXCLUSIVE_LOCK.
// ** This variable is kept up to date as locks are taken and released by
// ** the pagerLockDb() and pagerUnlockDb() wrappers.
// **
// ** If the VFS xLock() or xUnlock() returns an error other than SQLITE_BUSY
// ** (i.e. one of the SQLITE_IOERR subtypes), it is not clear whether or not
// ** the operation was successful. In these circumstances pagerLockDb() and
// ** pagerUnlockDb() take a conservative approach - eLock is always updated
// ** when unlocking the file, and only updated when locking the file if the
// ** VFS call is successful. This way, the Pager.eLock variable may be set
// ** to a less exclusive (lower) value than the lock that is actually held
// ** at the system level, but it is never set to a more exclusive value.
// **
// ** This is usually safe. If an xUnlock fails or appears to fail, there may 
// ** be a few redundant xLock() calls or a lock may be held for longer than
// ** required, but nothing really goes wrong.
// **
// ** The exception is when the database file is unlocked as the pager moves
// ** from ERROR to OPEN state. At this point there may be a hot-journal file 
// ** in the file-system that needs to be rolled back (as part of an OPEN->SHARED
// ** transition, by the same pager or any other). If the call to xUnlock()
// ** fails at this point and the pager is left holding an EXCLUSIVE lock, this
// ** can confuse the call to xCheckReservedLock() call made later as part
// ** of hot-journal detection.
// **
// ** xCheckReservedLock() is defined as returning true "if there is a RESERVED 
// ** lock held by this process or any others". So xCheckReservedLock may 
// ** return true because the caller itself is holding an EXCLUSIVE lock (but
// ** doesn't know it because of a previous error in xUnlock). If this happens
// ** a hot-journal may be mistaken for a journal being created by an active
// ** transaction in another process, causing SQLite to read from the database
// ** without rolling it back.
// **
// ** To work around this, if a call to xUnlock() fails when unlocking the
// ** database in the ERROR state, Pager.eLock is set to UNKNOWN_LOCK. It
// ** is only changed back to a real locking state after a successful call
// ** to xLock(EXCLUSIVE). Also, the code to do the OPEN->SHARED state transition
// ** omits the check for a hot-journal if Pager.eLock is set to UNKNOWN_LOCK 
// ** lock. Instead, it assumes a hot-journal exists and obtains an EXCLUSIVE
// ** lock on the database file before attempting to roll it back. See function
// ** PagerSharedLock() for more detail.
// **
// ** Pager.eLock may only be set to UNKNOWN_LOCK when the pager is in 
// ** PAGER_OPEN state.
// */
// #define UNKNOWN_LOCK                (EXCLUSIVE_LOCK+1)

// /*
// ** The maximum allowed sector size. 64KiB. If the xSectorsize() method 
// ** returns a value larger than this, then MAX_SECTOR_SIZE is used instead.
// ** This could conceivably cause corruption following a power failure on
// ** such a system. This is currently an undocumented limit.
// */
// #define MAX_SECTOR_SIZE 0x10000


// /*
// ** An instance of the following structure is allocated for each active
// ** savepoint and statement transaction in the system. All such structures
// ** are stored in the Pager.aSavepoint[] array, which is allocated and
// ** resized using sqlite3Realloc().
// **
// ** When a savepoint is created, the PagerSavepoint.iHdrOffset field is
// ** set to 0. If a journal-header is written into the main journal while
// ** the savepoint is active, then iHdrOffset is set to the byte offset 
// ** immediately following the last journal record written into the main
// ** journal before the journal-header. This is required during savepoint
// ** rollback (see pagerPlaybackSavepoint()).
// */
// typedef struct PagerSavepoint PagerSavepoint;
// struct PagerSavepoint {
//   i64 iOffset;                 /* Starting offset in main journal */
//   i64 iHdrOffset;              /* See above */
//   Bitvec *pInSavepoint;        /* Set of pages in this savepoint */
//   Pgno nOrig;                  /* Original number of pages in file */
//   Pgno iSubRec;                /* Index of first record in sub-journal */
//   int bTruncateOnRelease;      /* If stmt journal may be truncated on RELEASE */
// #ifndef SQLITE_OMIT_WAL
//   u32 aWalData[WAL_SAVEPOINT_NDATA];        /* WAL savepoint context */
// #endif
// };

// /*
// ** Bits of the Pager.doNotSpill flag.  See further description below.
// */
// #define SPILLFLAG_OFF         0x01 /* Never spill cache.  Set via pragma */
// #define SPILLFLAG_ROLLBACK    0x02 /* Current rolling back, so do not spill */
// #define SPILLFLAG_NOSYNC      0x04 /* Spill is ok, but do not sync */

// /*
// ** An open page cache is an instance of struct Pager. A description of
// ** some of the more important member variables follows:
// **
// ** eState
// **
// **   The current 'state' of the pager object. See the comment and state
// **   diagram above for a description of the pager state.
// **
// ** eLock
// **
// **   For a real on-disk database, the current lock held on the database file -
// **   NO_LOCK, SHARED_LOCK, RESERVED_LOCK or EXCLUSIVE_LOCK.
// **
// **   For a temporary or in-memory database (neither of which require any
// **   locks), this variable is always set to EXCLUSIVE_LOCK. Since such
// **   databases always have Pager.exclusiveMode==1, this tricks the pager
// **   logic into thinking that it already has all the locks it will ever
// **   need (and no reason to release them).
// **
// **   In some (obscure) circumstances, this variable may also be set to
// **   UNKNOWN_LOCK. See the comment above the #define of UNKNOWN_LOCK for
// **   details.
// **
// ** changeCountDone
// **
// **   This boolean variable is used to make sure that the change-counter 
// **   (the 4-byte header field at byte offset 24 of the database file) is 
// **   not updated more often than necessary. 
// **
// **   It is set to true when the change-counter field is updated, which 
// **   can only happen if an exclusive lock is held on the database file.
// **   It is cleared (set to false) whenever an exclusive lock is 
// **   relinquished on the database file. Each time a transaction is committed,
// **   The changeCountDone flag is inspected. If it is true, the work of
// **   updating the change-counter is omitted for the current transaction.
// **
// **   This mechanism means that when running in exclusive mode, a connection 
// **   need only update the change-counter once, for the first transaction
// **   committed.
// **
// ** setSuper
// **
// **   When PagerCommitPhaseOne() is called to commit a transaction, it may
// **   (or may not) specify a super-journal name to be written into the 
// **   journal file before it is synced to disk.
// **
// **   Whether or not a journal file contains a super-journal pointer affects 
// **   the way in which the journal file is finalized after the transaction is 
// **   committed or rolled back when running in "journal_mode=PERSIST" mode.
// **   If a journal file does not contain a super-journal pointer, it is
// **   finalized by overwriting the first journal header with zeroes. If
// **   it does contain a super-journal pointer the journal file is finalized 
// **   by truncating it to zero bytes, just as if the connection were 
// **   running in "journal_mode=truncate" mode.
// **
// **   Journal files that contain super-journal pointers cannot be finalized
// **   simply by overwriting the first journal-header with zeroes, as the
// **   super-journal pointer could interfere with hot-journal rollback of any
// **   subsequently interrupted transaction that reuses the journal file.
// **
// **   The flag is cleared as soon as the journal file is finalized (either
// **   by PagerCommitPhaseTwo or PagerRollback). If an IO error prevents the
// **   journal file from being successfully finalized, the setSuper flag
// **   is cleared anyway (and the pager will move to ERROR state).
// **
// ** doNotSpill
// **
// **   This variables control the behavior of cache-spills  (calls made by
// **   the pcache module to the pagerStress() routine to write cached data
// **   to the file-system in order to free up memory).
// **
// **   When bits SPILLFLAG_OFF or SPILLFLAG_ROLLBACK of doNotSpill are set,
// **   writing to the database from pagerStress() is disabled altogether.
// **   The SPILLFLAG_ROLLBACK case is done in a very obscure case that
// **   comes up during savepoint rollback that requires the pcache module
// **   to allocate a new page to prevent the journal file from being written
// **   while it is being traversed by code in pager_playback().  The SPILLFLAG_OFF
// **   case is a user preference.
// ** 
// **   If the SPILLFLAG_NOSYNC bit is set, writing to the database from
// **   pagerStress() is permitted, but syncing the journal file is not.
// **   This flag is set by sqlite3PagerWrite() when the file-system sector-size
// **   is larger than the database page-size in order to prevent a journal sync
// **   from happening in between the journalling of two pages on the same sector. 
// **
// ** subjInMemory
// **
// **   This is a boolean variable. If true, then any required sub-journal
// **   is opened as an in-memory journal file. If false, then in-memory
// **   sub-journals are only used for in-memory pager files.
// **
// **   This variable is updated by the upper layer each time a new 
// **   write-transaction is opened.
// **
// ** dbSize, dbOrigSize, dbFileSize
// **
// **   Variable dbSize is set to the number of pages in the database file.
// **   It is valid in PAGER_READER and higher states (all states except for
// **   OPEN and ERROR). 
// **
// **   dbSize is set based on the size of the database file, which may be 
// **   larger than the size of the database (the value stored at offset
// **   28 of the database header by the btree). If the size of the file
// **   is not an integer multiple of the page-size, the value stored in
// **   dbSize is rounded down (i.e. a 5KB file with 2K page-size has dbSize==2).
// **   Except, any file that is greater than 0 bytes in size is considered
// **   to have at least one page. (i.e. a 1KB file with 2K page-size leads
// **   to dbSize==1).
// **
// **   During a write-transaction, if pages with page-numbers greater than
// **   dbSize are modified in the cache, dbSize is updated accordingly.
// **   Similarly, if the database is truncated using PagerTruncateImage(), 
// **   dbSize is updated.
// **
// **   Variables dbOrigSize and dbFileSize are valid in states 
// **   PAGER_WRITER_LOCKED and higher. dbOrigSize is a copy of the dbSize
// **   variable at the start of the transaction. It is used during rollback,
// **   and to determine whether or not pages need to be journalled before
// **   being modified.
// **
// **   Throughout a write-transaction, dbFileSize contains the size of
// **   the file on disk in pages. It is set to a copy of dbSize when the
// **   write-transaction is first opened, and updated when VFS calls are made
// **   to write or truncate the database file on disk. 
// **
// **   The only reason the dbFileSize variable is required is to suppress 
// **   unnecessary calls to xTruncate() after committing a transaction. If, 
// **   when a transaction is committed, the dbFileSize variable indicates 
// **   that the database file is larger than the database image (Pager.dbSize), 
// **   pager_truncate() is called. The pager_truncate() call uses xFilesize()
// **   to measure the database file on disk, and then truncates it if required.
// **   dbFileSize is not used when rolling back a transaction. In this case
// **   pager_truncate() is called unconditionally (which means there may be
// **   a call to xFilesize() that is not strictly required). In either case,
// **   pager_truncate() may cause the file to become smaller or larger.
// **
// ** dbHintSize
// **
// **   The dbHintSize variable is used to limit the number of calls made to
// **   the VFS xFileControl(FCNTL_SIZE_HINT) method. 
// **
// **   dbHintSize is set to a copy of the dbSize variable when a
// **   write-transaction is opened (at the same time as dbFileSize and
// **   dbOrigSize). If the xFileControl(FCNTL_SIZE_HINT) method is called,
// **   dbHintSize is increased to the number of pages that correspond to the
// **   size-hint passed to the method call. See pager_write_pagelist() for 
// **   details.
// **
// ** errCode
// **
// **   The Pager.errCode variable is only ever used in PAGER_ERROR state. It
// **   is set to zero in all other states. In PAGER_ERROR state, Pager.errCode 
// **   is always set to SQLITE_FULL, SQLITE_IOERR or one of the SQLITE_IOERR_XXX 
// **   sub-codes.
// **
// ** syncFlags, walSyncFlags
// **
// **   syncFlags is either SQLITE_SYNC_NORMAL (0x02) or SQLITE_SYNC_FULL (0x03).
// **   syncFlags is used for rollback mode.  walSyncFlags is used for WAL mode
// **   and contains the flags used to sync the checkpoint operations in the
// **   lower two bits, and sync flags used for transaction commits in the WAL
// **   file in bits 0x04 and 0x08.  In other words, to get the correct sync flags
// **   for checkpoint operations, use (walSyncFlags&0x03) and to get the correct
// **   sync flags for transaction commit, use ((walSyncFlags>>2)&0x03).  Note
// **   that with synchronous=NORMAL in WAL mode, transaction commit is not synced
// **   meaning that the 0x04 and 0x08 bits are both zero.
// */
// struct Pager {
//   sqlite3_vfs *pVfs;          /* OS functions to use for IO */
//   u8 exclusiveMode;           /* Boolean. True if locking_mode==EXCLUSIVE */
//   u8 journalMode;             /* One of the PAGER_JOURNALMODE_* values */
//   u8 useJournal;              /* Use a rollback journal on this file */
//   u8 noSync;                  /* Do not sync the journal if true */
//   u8 fullSync;                /* Do extra syncs of the journal for robustness */
//   u8 extraSync;               /* sync directory after journal delete */
//   u8 syncFlags;               /* SYNC_NORMAL or SYNC_FULL otherwise */
//   u8 walSyncFlags;            /* See description above */
//   u8 tempFile;                /* zFilename is a temporary or immutable file */
//   u8 noLock;                  /* Do not lock (except in WAL mode) */
//   u8 readOnly;                /* True for a read-only database */
//   u8 memDb;                   /* True to inhibit all file I/O */
//   u8 memVfs;                  /* VFS-implemented memory database */

//   /**************************************************************************
//   ** The following block contains those class members that change during
//   ** routine operation.  Class members not in this block are either fixed
//   ** when the pager is first created or else only change when there is a
//   ** significant mode change (such as changing the page_size, locking_mode,
//   ** or the journal_mode).  From another view, these class members describe
//   ** the "state" of the pager, while other class members describe the
//   ** "configuration" of the pager.
//   */
//   u8 eState;                  /* Pager state (OPEN, READER, WRITER_LOCKED..) */
//   u8 eLock;                   /* Current lock held on database file */
//   u8 changeCountDone;         /* Set after incrementing the change-counter */
//   u8 setSuper;                /* Super-jrnl name is written into jrnl */
//   u8 doNotSpill;              /* Do not spill the cache when non-zero */
//   u8 subjInMemory;            /* True to use in-memory sub-journals */
//   u8 bUseFetch;               /* True to use xFetch() */
//   u8 hasHeldSharedLock;       /* True if a shared lock has ever been held */
//   Pgno dbSize;                /* Number of pages in the database */
//   Pgno dbOrigSize;            /* dbSize before the current transaction */
//   Pgno dbFileSize;            /* Number of pages in the database file */
//   Pgno dbHintSize;            /* Value passed to FCNTL_SIZE_HINT call */
//   int errCode;                /* One of several kinds of errors */
//   int nRec;                   /* Pages journalled since last j-header written */
//   u32 cksumInit;              /* Quasi-random value added to every checksum */
//   u32 nSubRec;                /* Number of records written to sub-journal */
//   Bitvec *pInJournal;         /* One bit for each page in the database file */
//   sqlite3_file *fd;           /* File descriptor for database */
//   sqlite3_file *jfd;          /* File descriptor for main journal */
//   sqlite3_file *sjfd;         /* File descriptor for sub-journal */
//   i64 journalOff;             /* Current write offset in the journal file */
//   i64 journalHdr;             /* Byte offset to previous journal header */
//   sqlite3_backup *pBackup;    /* Pointer to list of ongoing backup processes */
//   PagerSavepoint *aSavepoint; /* Array of active savepoints */
//   int nSavepoint;             /* Number of elements in aSavepoint[] */
//   u32 iDataVersion;           /* Changes whenever database content changes */
//   char dbFileVers[16];        /* Changes whenever database file changes */

//   int nMmapOut;               /* Number of mmap pages currently outstanding */
//   sqlite3_int64 szMmap;       /* Desired maximum mmap size */
//   PgHdr *pMmapFreelist;       /* List of free mmap page headers (pDirty) */
//   /*
//   ** End of the routinely-changing class members
//   ***************************************************************************/

//   u16 nExtra;                 /* Add this many bytes to each in-memory page */
//   i16 nReserve;               /* Number of unused bytes at end of each page */
//   u32 vfsFlags;               /* Flags for sqlite3_vfs.xOpen() */
//   u32 sectorSize;             /* Assumed sector size during rollback */
//   Pgno mxPgno;                /* Maximum allowed size of the database */
//   i64 pageSize;               /* Number of bytes in a page */
//   i64 journalSizeLimit;       /* Size limit for persistent journal files */
//   char *zFilename;            /* Name of the database file */
//   char *zJournal;             /* Name of the journal file */
//   int (*xBusyHandler)(void*); /* Function to call when busy */
//   void *pBusyHandlerArg;      /* Context argument for xBusyHandler */
//   int aStat[4];               /* Total cache hits, misses, writes, spills */
// #ifdef SQLITE_TEST
//   int nRead;                  /* Database pages read */
// #endif
//   void (*xReiniter)(DbPage*); /* Call this routine when reloading pages */
//   int (*xGet)(Pager*,Pgno,DbPage**,int); /* Routine to fetch a patch */
//   char *pTmpSpace;            /* Pager.pageSize bytes of space for tmp use */
//   PCache *pPCache;            /* Pointer to page cache object */
// #ifndef SQLITE_OMIT_WAL
//   Wal *pWal;                  /* Write-ahead log used by "journal_mode=wal" */
//   char *zWal;                 /* File name for write-ahead log */
// #endif
// };

// /*
// ** Indexes for use with Pager.aStat[]. The Pager.aStat[] array contains
// ** the values accessed by passing SQLITE_DBSTATUS_CACHE_HIT, CACHE_MISS 
// ** or CACHE_WRITE to sqlite3_db_status().
// */
// #define PAGER_STAT_HIT   0
// #define PAGER_STAT_MISS  1
// #define PAGER_STAT_WRITE 2
// #define PAGER_STAT_SPILL 3

// /*
// ** The following global variables hold counters used for
// ** testing purposes only.  These variables do not exist in
// ** a non-testing build.  These variables are not thread-safe.
// */
// #ifdef SQLITE_TEST
// int sqlite3_pager_readdb_count = 0;    /* Number of full pages read from DB */
// int sqlite3_pager_writedb_count = 0;   /* Number of full pages written to DB */
// int sqlite3_pager_writej_count = 0;    /* Number of pages written to journal */
// # define PAGER_INCR(v)  v++
// #else
// # define PAGER_INCR(v)
// #endif



// /*
// ** Journal files begin with the following magic string.  The data
// ** was obtained from /dev/random.  It is used only as a sanity check.
// **
// ** Since version 2.8.0, the journal format contains additional sanity
// ** checking information.  If the power fails while the journal is being
// ** written, semi-random garbage data might appear in the journal
// ** file after power is restored.  If an attempt is then made
// ** to roll the journal back, the database could be corrupted.  The additional
// ** sanity checking data is an attempt to discover the garbage in the
// ** journal and ignore it.
// **
// ** The sanity checking information for the new journal format consists
// ** of a 32-bit checksum on each page of data.  The checksum covers both
// ** the page number and the pPager->pageSize bytes of data for the page.
// ** This cksum is initialized to a 32-bit random value that appears in the
// ** journal file right after the header.  The random initializer is important,
// ** because garbage data that appears at the end of a journal is likely
// ** data that was once in other files that have now been deleted.  If the
// ** garbage data came from an obsolete journal file, the checksums might
// ** be correct.  But by initializing the checksum to random value which
// ** is different for every journal, we minimize that risk.
// */
// static const unsigned char aJournalMagic[] = {
//   0xd9, 0xd5, 0x05, 0xf9, 0x20, 0xa1, 0x63, 0xd7,
// };

// /*
// ** The size of the of each page record in the journal is given by
// ** the following macro.
// */
// #define JOURNAL_PG_SZ(pPager)  ((pPager->pageSize) + 8)

// /*
// ** The journal header size for this pager. This is usually the same 
// ** size as a single disk sector. See also setSectorSize().
// */
// #define JOURNAL_HDR_SZ(pPager) (pPager->sectorSize)

// /*
// ** The macro MEMDB is true if we are dealing with an in-memory database.
// ** We do this as a macro so that if the SQLITE_OMIT_MEMORYDB macro is set,
// ** the value of MEMDB will be a constant and the compiler will optimize
// ** out code that would never execute.
// */
// #ifdef SQLITE_OMIT_MEMORYDB
// # define MEMDB 0
// #else
// # define MEMDB pPager->memDb
// #endif

// /*
// ** The macro USEFETCH is true if we are allowed to use the xFetch and xUnfetch
// ** interfaces to access the database using memory-mapped I/O.
// */
// #if SQLITE_MAX_MMAP_SIZE>0
// # define USEFETCH(x) ((x)->bUseFetch)
// #else
// # define USEFETCH(x) 0
// #endif

// /*
// ** The argument to this macro is a file descriptor (type sqlite3_file*).
// ** Return 0 if it is not open, or non-zero (but not 1) if it is.
// **
// ** This is so that expressions can be written as:
// **
// **   if( isOpen(pPager->jfd) ){ ...
// **
// ** instead of
// **
// **   if( pPager->jfd->pMethods ){ ...
// */
// #define isOpen(pFd) ((pFd)->pMethods!=0)

// #ifdef SQLITE_DIRECT_OVERFLOW_READ
// /*
// ** Return true if page pgno can be read directly from the database file
// ** by the b-tree layer. This is the case if:
// **
// **   * the database file is open,
// **   * there are no dirty pages in the cache, and
// **   * the desired page is not currently in the wal file.
// */
// int sqlite3PagerDirectReadOk(Pager *pPager, Pgno pgno){
//   if( pPager->fd->pMethods==0 ) return 0;
//   if( sqlite3PCacheIsDirty(pPager->pPCache) ) return 0;
// #ifndef SQLITE_OMIT_WAL
//   if( pPager->pWal ){
//     u32 iRead = 0;
//     int rc;
//     rc = sqlite3WalFindFrame(pPager->pWal, pgno, &iRead);
//     return (rc==SQLITE_OK && iRead==0);
//   }
// #endif
//   return 1;
// }
// #endif

// #ifndef SQLITE_OMIT_WAL
// # define pagerUseWal(x) ((x)->pWal!=0)
// #else
// # define pagerUseWal(x) 0
// # define pagerRollbackWal(x) 0
// # define pagerWalFrames(v,w,x,y) 0
// # define pagerOpenWalIfPresent(z) SQLITE_OK
// # define pagerBeginReadTransaction(z) SQLITE_OK
// #endif

// #ifndef NDEBUG 
// /*
// ** Usage:
// **
// **   assert( assert_pager_state(pPager) );
// **
// ** This function runs many asserts to try to find inconsistencies in
// ** the internal state of the Pager object.
// */
// static int assert_pager_state(Pager *p){
//   Pager *pPager = p;

//   /* State must be valid. */
//   assert( p->eState==PAGER_OPEN
//        || p->eState==PAGER_READER
//        || p->eState==PAGER_WRITER_LOCKED
//        || p->eState==PAGER_WRITER_CACHEMOD
//        || p->eState==PAGER_WRITER_DBMOD
//        || p->eState==PAGER_WRITER_FINISHED
//        || p->eState==PAGER_ERROR
//   );

//   /* Regardless of the current state, a temp-file connection always behaves
//   ** as if it has an exclusive lock on the database file. It never updates
//   ** the change-counter field, so the changeCountDone flag is always set.
//   */
//   assert( p->tempFile==0 || p->eLock==EXCLUSIVE_LOCK );
//   assert( p->tempFile==0 || pPager->changeCountDone );

//   /* If the useJournal flag is clear, the journal-mode must be "OFF". 
//   ** And if the journal-mode is "OFF", the journal file must not be open.
//   */
//   assert( p->journalMode==PAGER_JOURNALMODE_OFF || p->useJournal );
//   assert( p->journalMode!=PAGER_JOURNALMODE_OFF || !isOpen(p->jfd) );

//   /* Check that MEMDB implies noSync. And an in-memory journal. Since 
//   ** this means an in-memory pager performs no IO at all, it cannot encounter 
//   ** either SQLITE_IOERR or SQLITE_FULL during rollback or while finalizing 
//   ** a journal file. (although the in-memory journal implementation may 
//   ** return SQLITE_IOERR_NOMEM while the journal file is being written). It 
//   ** is therefore not possible for an in-memory pager to enter the ERROR 
//   ** state.
//   */
//   if( MEMDB ){
//     assert( !isOpen(p->fd) );
//     assert( p->noSync );
//     assert( p->journalMode==PAGER_JOURNALMODE_OFF 
//          || p->journalMode==PAGER_JOURNALMODE_MEMORY 
//     );
//     assert( p->eState!=PAGER_ERROR && p->eState!=PAGER_OPEN );
//     assert( pagerUseWal(p)==0 );
//   }

//   /* If changeCountDone is set, a RESERVED lock or greater must be held
//   ** on the file.
//   */
//   assert( pPager->changeCountDone==0 || pPager->eLock>=RESERVED_LOCK );
//   assert( p->eLock!=PENDING_LOCK );

//   switch( p->eState ){
//     case PAGER_OPEN:
//       assert( !MEMDB );
//       assert( pPager->errCode==SQLITE_OK );
//       assert( sqlite3PcacheRefCount(pPager->pPCache)==0 || pPager->tempFile );
//       break;

//     case PAGER_READER:
//       assert( pPager->errCode==SQLITE_OK );
//       assert( p->eLock!=UNKNOWN_LOCK );
//       assert( p->eLock>=SHARED_LOCK );
//       break;

//     case PAGER_WRITER_LOCKED:
//       assert( p->eLock!=UNKNOWN_LOCK );
//       assert( pPager->errCode==SQLITE_OK );
//       if( !pagerUseWal(pPager) ){
//         assert( p->eLock>=RESERVED_LOCK );
//       }
//       assert( pPager->dbSize==pPager->dbOrigSize );
//       assert( pPager->dbOrigSize==pPager->dbFileSize );
//       assert( pPager->dbOrigSize==pPager->dbHintSize );
//       assert( pPager->setSuper==0 );
//       break;

//     case PAGER_WRITER_CACHEMOD:
//       assert( p->eLock!=UNKNOWN_LOCK );
//       assert( pPager->errCode==SQLITE_OK );
//       if( !pagerUseWal(pPager) ){
//         /* It is possible that if journal_mode=wal here that neither the
//         ** journal file nor the WAL file are open. This happens during
//         ** a rollback transaction that switches from journal_mode=off
//         ** to journal_mode=wal.
//         */
//         assert( p->eLock>=RESERVED_LOCK );
//         assert( isOpen(p->jfd) 
//              || p->journalMode==PAGER_JOURNALMODE_OFF 
//              || p->journalMode==PAGER_JOURNALMODE_WAL 
//         );
//       }
//       assert( pPager->dbOrigSize==pPager->dbFileSize );
//       assert( pPager->dbOrigSize==pPager->dbHintSize );
//       break;

//     case PAGER_WRITER_DBMOD:
//       assert( p->eLock==EXCLUSIVE_LOCK );
//       assert( pPager->errCode==SQLITE_OK );
//       assert( !pagerUseWal(pPager) );
//       assert( p->eLock>=EXCLUSIVE_LOCK );
//       assert( isOpen(p->jfd) 
//            || p->journalMode==PAGER_JOURNALMODE_OFF 
//            || p->journalMode==PAGER_JOURNALMODE_WAL 
//            || (sqlite3OsDeviceCharacteristics(p->fd)&SQLITE_IOCAP_BATCH_ATOMIC)
//       );
//       assert( pPager->dbOrigSize<=pPager->dbHintSize );
//       break;

//     case PAGER_WRITER_FINISHED:
//       assert( p->eLock==EXCLUSIVE_LOCK );
//       assert( pPager->errCode==SQLITE_OK );
//       assert( !pagerUseWal(pPager) );
//       assert( isOpen(p->jfd) 
//            || p->journalMode==PAGER_JOURNALMODE_OFF 
//            || p->journalMode==PAGER_JOURNALMODE_WAL 
//            || (sqlite3OsDeviceCharacteristics(p->fd)&SQLITE_IOCAP_BATCH_ATOMIC)
//       );
//       break;

//     case PAGER_ERROR:
//       /* There must be at least one outstanding reference to the pager if
//       ** in ERROR state. Otherwise the pager should have already dropped
//       ** back to OPEN state.
//       */
//       assert( pPager->errCode!=SQLITE_OK );
//       assert( sqlite3PcacheRefCount(pPager->pPCache)>0 || pPager->tempFile );
//       break;
//   }

//   return 1;
// }
// #endif /* ifndef NDEBUG */

// #ifdef SQLITE_DEBUG 
// /*
// ** Return a pointer to a human readable string in a static buffer
// ** containing the state of the Pager object passed as an argument. This
// ** is intended to be used within debuggers. For example, as an alternative
// ** to "print *pPager" in gdb:
// **
// ** (gdb) printf "%s", print_pager_state(pPager)
// **
// ** This routine has external linkage in order to suppress compiler warnings
// ** about an unused function.  It is enclosed within SQLITE_DEBUG and so does
// ** not appear in normal builds.
// */
// char *print_pager_state(Pager *p){
//   static char zRet[1024];

//   sqlite3_snprintf(1024, zRet,
//       "Filename:      %s\n"
//       "State:         %s errCode=%d\n"
//       "Lock:          %s\n"
//       "Locking mode:  locking_mode=%s\n"
//       "Journal mode:  journal_mode=%s\n"
//       "Backing store: tempFile=%d memDb=%d useJournal=%d\n"
//       "Journal:       journalOff=%lld journalHdr=%lld\n"
//       "Size:          dbsize=%d dbOrigSize=%d dbFileSize=%d\n"
//       , p->zFilename
//       , p->eState==PAGER_OPEN            ? "OPEN" :
//         p->eState==PAGER_READER          ? "READER" :
//         p->eState==PAGER_WRITER_LOCKED   ? "WRITER_LOCKED" :
//         p->eState==PAGER_WRITER_CACHEMOD ? "WRITER_CACHEMOD" :
//         p->eState==PAGER_WRITER_DBMOD    ? "WRITER_DBMOD" :
//         p->eState==PAGER_WRITER_FINISHED ? "WRITER_FINISHED" :
//         p->eState==PAGER_ERROR           ? "ERROR" : "?error?"
//       , (int)p->errCode
//       , p->eLock==NO_LOCK         ? "NO_LOCK" :
//         p->eLock==RESERVED_LOCK   ? "RESERVED" :
//         p->eLock==EXCLUSIVE_LOCK  ? "EXCLUSIVE" :
//         p->eLock==SHARED_LOCK     ? "SHARED" :
//         p->eLock==UNKNOWN_LOCK    ? "UNKNOWN" : "?error?"
//       , p->exclusiveMode ? "exclusive" : "normal"
//       , p->journalMode==PAGER_JOURNALMODE_MEMORY   ? "memory" :
//         p->journalMode==PAGER_JOURNALMODE_OFF      ? "off" :
//         p->journalMode==PAGER_JOURNALMODE_DELETE   ? "delete" :
//         p->journalMode==PAGER_JOURNALMODE_PERSIST  ? "persist" :
//         p->journalMode==PAGER_JOURNALMODE_TRUNCATE ? "truncate" :
//         p->journalMode==PAGER_JOURNALMODE_WAL      ? "wal" : "?error?"
//       , (int)p->tempFile, (int)p->memDb, (int)p->useJournal
//       , p->journalOff, p->journalHdr
//       , (int)p->dbSize, (int)p->dbOrigSize, (int)p->dbFileSize
//   );

//   return zRet;
// }
// #endif

// /* Forward references to the various page getters */
// static int getPageNormal(Pager*,Pgno,DbPage**,int);
// static int getPageError(Pager*,Pgno,DbPage**,int);
// #if SQLITE_MAX_MMAP_SIZE>0
// static int getPageMMap(Pager*,Pgno,DbPage**,int);
// #endif

// /*
// ** Set the Pager.xGet method for the appropriate routine used to fetch
// ** content from the pager.
// */
// static void setGetterMethod(Pager *pPager){
//   if( pPager->errCode ){
//     pPager->xGet = getPageError;
// #if SQLITE_MAX_MMAP_SIZE>0
//   }else if( USEFETCH(pPager) ){
//     pPager->xGet = getPageMMap;
// #endif /* SQLITE_MAX_MMAP_SIZE>0 */
//   }else{
//     pPager->xGet = getPageNormal;
//   }
// }

// /*
// ** Return true if it is necessary to write page *pPg into the sub-journal.
// ** A page needs to be written into the sub-journal if there exists one
// ** or more open savepoints for which:
// **
// **   * The page-number is less than or equal to PagerSavepoint.nOrig, and
// **   * The bit corresponding to the page-number is not set in
// **     PagerSavepoint.pInSavepoint.
// */
// static int subjRequiresPage(PgHdr *pPg){
//   Pager *pPager = pPg->pPager;
//   PagerSavepoint *p;
//   Pgno pgno = pPg->pgno;
//   int i;
//   for(i=0; i<pPager->nSavepoint; i++){
//     p = &pPager->aSavepoint[i];
//     if( p->nOrig>=pgno && 0==sqlite3BitvecTestNotNull(p->pInSavepoint, pgno) ){
//       for(i=i+1; i<pPager->nSavepoint; i++){
//         pPager->aSavepoint[i].bTruncateOnRelease = 0;
//       }
//       return 1;
//     }
//   }
//   return 0;
// }

// #ifdef SQLITE_DEBUG
// /*
// ** Return true if the page is already in the journal file.
// */
// static int pageInJournal(Pager *pPager, PgHdr *pPg){
//   return sqlite3BitvecTest(pPager->pInJournal, pPg->pgno);
// }
// #endif

// /*
// ** Read a 32-bit integer from the given file descriptor.  Store the integer
// ** that is read in *pRes.  Return SQLITE_OK if everything worked, or an
// ** error code is something goes wrong.
// **
// ** All values are stored on disk as big-endian.
// */
// static int read32bits(sqlite3_file *fd, i64 offset, u32 *pRes){
//   unsigned char ac[4];
//   int rc = sqlite3OsRead(fd, ac, sizeof(ac), offset);
//   if( rc==SQLITE_OK ){
//     *pRes = sqlite3Get4byte(ac);
//   }
//   return rc;
// }

// /*
// ** Write a 32-bit integer into a string buffer in big-endian byte order.
// */
// #define put32bits(A,B)  sqlite3Put4byte((u8*)A,B)


// /*
// ** Write a 32-bit integer into the given file descriptor.  Return SQLITE_OK
// ** on success or an error code is something goes wrong.
// */
// static int write32bits(sqlite3_file *fd, i64 offset, u32 val){
//   char ac[4];
//   put32bits(ac, val);
//   return sqlite3OsWrite(fd, ac, 4, offset);
// }

// /*
// ** Unlock the database file to level eLock, which must be either NO_LOCK
// ** or SHARED_LOCK. Regardless of whether or not the call to xUnlock()
// ** succeeds, set the Pager.eLock variable to match the (attempted) new lock.
// **
// ** Except, if Pager.eLock is set to UNKNOWN_LOCK when this function is
// ** called, do not modify it. See the comment above the #define of 
// ** UNKNOWN_LOCK for an explanation of this.
// */
// static int pagerUnlockDb(Pager *pPager, int eLock){
//   int rc = SQLITE_OK;

//   assert( !pPager->exclusiveMode || pPager->eLock==eLock );
//   assert( eLock==NO_LOCK || eLock==SHARED_LOCK );
//   assert( eLock!=NO_LOCK || pagerUseWal(pPager)==0 );
//   if( isOpen(pPager->fd) ){
//     assert( pPager->eLock>=eLock );
//     rc = pPager->noLock ? SQLITE_OK : sqlite3OsUnlock(pPager->fd, eLock);
//     if( pPager->eLock!=UNKNOWN_LOCK ){
//       pPager->eLock = (u8)eLock;
//     }
//     IOTRACE(("UNLOCK %p %d\n", pPager, eLock))
//   }
//   pPager->changeCountDone = pPager->tempFile; /* ticket fb3b3024ea238d5c */
//   return rc;
// }

// /*
// ** Lock the database file to level eLock, which must be either SHARED_LOCK,
// ** RESERVED_LOCK or EXCLUSIVE_LOCK. If the caller is successful, set the
// ** Pager.eLock variable to the new locking state. 
// **
// ** Except, if Pager.eLock is set to UNKNOWN_LOCK when this function is 
// ** called, do not modify it unless the new locking state is EXCLUSIVE_LOCK. 
// ** See the comment above the #define of UNKNOWN_LOCK for an explanation 
// ** of this.
// */
// static int pagerLockDb(Pager *pPager, int eLock){
//   int rc = SQLITE_OK;

//   assert( eLock==SHARED_LOCK || eLock==RESERVED_LOCK || eLock==EXCLUSIVE_LOCK );
//   if( pPager->eLock<eLock || pPager->eLock==UNKNOWN_LOCK ){
//     rc = pPager->noLock ? SQLITE_OK : sqlite3OsLock(pPager->fd, eLock);
//     if( rc==SQLITE_OK && (pPager->eLock!=UNKNOWN_LOCK||eLock==EXCLUSIVE_LOCK) ){
//       pPager->eLock = (u8)eLock;
//       IOTRACE(("LOCK %p %d\n", pPager, eLock))
//     }
//   }
//   return rc;
// }

// /*
// ** This function determines whether or not the atomic-write or
// ** atomic-batch-write optimizations can be used with this pager. The
// ** atomic-write optimization can be used if:
// **
// **  (a) the value returned by OsDeviceCharacteristics() indicates that
// **      a database page may be written atomically, and
// **  (b) the value returned by OsSectorSize() is less than or equal
// **      to the page size.
// **
// ** If it can be used, then the value returned is the size of the journal 
// ** file when it contains rollback data for exactly one page.
// **
// ** The atomic-batch-write optimization can be used if OsDeviceCharacteristics()
// ** returns a value with the SQLITE_IOCAP_BATCH_ATOMIC bit set. -1 is
// ** returned in this case.
// **
// ** If neither optimization can be used, 0 is returned.
// */
// static int jrnlBufferSize(Pager *pPager){
//   assert( !MEMDB );

// #if defined(SQLITE_ENABLE_ATOMIC_WRITE) \
//  || defined(SQLITE_ENABLE_BATCH_ATOMIC_WRITE)
//   int dc;                           /* Device characteristics */

//   assert( isOpen(pPager->fd) );
//   dc = sqlite3OsDeviceCharacteristics(pPager->fd);
// #else
//   UNUSED_PARAMETER(pPager);
// #endif

// #ifdef SQLITE_ENABLE_BATCH_ATOMIC_WRITE
//   if( pPager->dbSize>0 && (dc&SQLITE_IOCAP_BATCH_ATOMIC) ){
//     return -1;
//   }
// #endif

// #ifdef SQLITE_ENABLE_ATOMIC_WRITE
//   {
//     int nSector = pPager->sectorSize;
//     int szPage = pPager->pageSize;

//     assert(SQLITE_IOCAP_ATOMIC512==(512>>8));
//     assert(SQLITE_IOCAP_ATOMIC64K==(65536>>8));
//     if( 0==(dc&(SQLITE_IOCAP_ATOMIC|(szPage>>8)) || nSector>szPage) ){
//       return 0;
//     }
//   }

//   return JOURNAL_HDR_SZ(pPager) + JOURNAL_PG_SZ(pPager);
// #endif

//   return 0;
// }

// /*
// ** If SQLITE_CHECK_PAGES is defined then we do some sanity checking
// ** on the cache using a hash function.  This is used for testing
// ** and debugging only.
// */
// #ifdef SQLITE_CHECK_PAGES
// /*
// ** Return a 32-bit hash of the page data for pPage.
// */
// static u32 pager_datahash(int nByte, unsigned char *pData){
//   u32 hash = 0;
//   int i;
//   for(i=0; i<nByte; i++){
//     hash = (hash*1039) + pData[i];
//   }
//   return hash;
// }
// static u32 pager_pagehash(PgHdr *pPage){
//   return pager_datahash(pPage->pPager->pageSize, (unsigned char *)pPage->pData);
// }
// static void pager_set_pagehash(PgHdr *pPage){
//   pPage->pageHash = pager_pagehash(pPage);
// }

// /*
// ** The CHECK_PAGE macro takes a PgHdr* as an argument. If SQLITE_CHECK_PAGES
// ** is defined, and NDEBUG is not defined, an assert() statement checks
// ** that the page is either dirty or still matches the calculated page-hash.
// */
// #define CHECK_PAGE(x) checkPage(x)
// static void checkPage(PgHdr *pPg){
//   Pager *pPager = pPg->pPager;
//   assert( pPager->eState!=PAGER_ERROR );
//   assert( (pPg->flags&PGHDR_DIRTY) || pPg->pageHash==pager_pagehash(pPg) );
// }

// #else
// #define pager_datahash(X,Y)  0
// #define pager_pagehash(X)  0
// #define pager_set_pagehash(X)
// #define CHECK_PAGE(x)
// #endif  /* SQLITE_CHECK_PAGES */

// /*
// ** When this is called the journal file for pager pPager must be open.
// ** This function attempts to read a super-journal file name from the 
// ** end of the file and, if successful, copies it into memory supplied 
// ** by the caller. See comments above writeSuperJournal() for the format
// ** used to store a super-journal file name at the end of a journal file.
// **
// ** zSuper must point to a buffer of at least nSuper bytes allocated by
// ** the caller. This should be sqlite3_vfs.mxPathname+1 (to ensure there is
// ** enough space to write the super-journal name). If the super-journal
// ** name in the journal is longer than nSuper bytes (including a
// ** nul-terminator), then this is handled as if no super-journal name
// ** were present in the journal.
// **
// ** If a super-journal file name is present at the end of the journal
// ** file, then it is copied into the buffer pointed to by zSuper. A
// ** nul-terminator byte is appended to the buffer following the
// ** super-journal file name.
// **
// ** If it is determined that no super-journal file name is present 
// ** zSuper[0] is set to 0 and SQLITE_OK returned.
// **
// ** If an error occurs while reading from the journal file, an SQLite
// ** error code is returned.
// */
// static int readSuperJournal(sqlite3_file *pJrnl, char *zSuper, u32 nSuper){
//   int rc;                    /* Return code */
//   u32 len;                   /* Length in bytes of super-journal name */
//   i64 szJ;                   /* Total size in bytes of journal file pJrnl */
//   u32 cksum;                 /* MJ checksum value read from journal */
//   u32 u;                     /* Unsigned loop counter */
//   unsigned char aMagic[8];   /* A buffer to hold the magic header */
//   zSuper[0] = '\0';

//   if( SQLITE_OK!=(rc = sqlite3OsFileSize(pJrnl, &szJ))
//    || szJ<16
//    || SQLITE_OK!=(rc = read32bits(pJrnl, szJ-16, &len))
//    || len>=nSuper 
//    || len>szJ-16
//    || len==0 
//    || SQLITE_OK!=(rc = read32bits(pJrnl, szJ-12, &cksum))
//    || SQLITE_OK!=(rc = sqlite3OsRead(pJrnl, aMagic, 8, szJ-8))
//    || memcmp(aMagic, aJournalMagic, 8)
//    || SQLITE_OK!=(rc = sqlite3OsRead(pJrnl, zSuper, len, szJ-16-len))
//   ){
//     return rc;
//   }

//   /* See if the checksum matches the super-journal name */
//   for(u=0; u<len; u++){
//     cksum -= zSuper[u];
//   }
//   if( cksum ){
//     /* If the checksum doesn't add up, then one or more of the disk sectors
//     ** containing the super-journal filename is corrupted. This means
//     ** definitely roll back, so just return SQLITE_OK and report a (nul)
//     ** super-journal filename.
//     */
//     len = 0;
//   }
//   zSuper[len] = '\0';
//   zSuper[len+1] = '\0';
   
//   return SQLITE_OK;
// }

// /*
// ** Return the offset of the sector boundary at or immediately 
// ** following the value in pPager->journalOff, assuming a sector 
// ** size of pPager->sectorSize bytes.
// **
// ** i.e for a sector size of 512:
// **
// **   Pager.journalOff          Return value
// **   ---------------------------------------
// **   0                         0
// **   512                       512
// **   100                       512
// **   2000                      2048
// ** 
// */
// static i64 journalHdrOffset(Pager *pPager){
//   i64 offset = 0;
//   i64 c = pPager->journalOff;
//   if( c ){
//     offset = ((c-1)/JOURNAL_HDR_SZ(pPager) + 1) * JOURNAL_HDR_SZ(pPager);
//   }
//   assert( offset%JOURNAL_HDR_SZ(pPager)==0 );
//   assert( offset>=c );
//   assert( (offset-c)<JOURNAL_HDR_SZ(pPager) );
//   return offset;
// }

// /*
// ** The journal file must be open when this function is called.
// **
// ** This function is a no-op if the journal file has not been written to
// ** within the current transaction (i.e. if Pager.journalOff==0).
// **
// ** If doTruncate is non-zero or the Pager.journalSizeLimit variable is
// ** set to 0, then truncate the journal file to zero bytes in size. Otherwise,
// ** zero the 28-byte header at the start of the journal file. In either case, 
// ** if the pager is not in no-sync mode, sync the journal file immediately 
// ** after writing or truncating it.
// **
// ** If Pager.journalSizeLimit is set to a positive, non-zero value, and
// ** following the truncation or zeroing described above the size of the 
// ** journal file in bytes is larger than this value, then truncate the
// ** journal file to Pager.journalSizeLimit bytes. The journal file does
// ** not need to be synced following this operation.
// **
// ** If an IO error occurs, abandon processing and return the IO error code.
// ** Otherwise, return SQLITE_OK.
// */
// static int zeroJournalHdr(Pager *pPager, int doTruncate){
//   int rc = SQLITE_OK;                               /* Return code */
//   assert( isOpen(pPager->jfd) );
//   assert( !sqlite3JournalIsInMemory(pPager->jfd) );
//   if( pPager->journalOff ){
//     const i64 iLimit = pPager->journalSizeLimit;    /* Local cache of jsl */

//     IOTRACE(("JZEROHDR %p\n", pPager))
//     if( doTruncate || iLimit==0 ){
//       rc = sqlite3OsTruncate(pPager->jfd, 0);
//     }else{
//       static const char zeroHdr[28] = {0};
//       rc = sqlite3OsWrite(pPager->jfd, zeroHdr, sizeof(zeroHdr), 0);
//     }
//     if( rc==SQLITE_OK && !pPager->noSync ){
//       rc = sqlite3OsSync(pPager->jfd, SQLITE_SYNC_DATAONLY|pPager->syncFlags);
//     }

//     /* At this point the transaction is committed but the write lock 
//     ** is still held on the file. If there is a size limit configured for 
//     ** the persistent journal and the journal file currently consumes more
//     ** space than that limit allows for, truncate it now. There is no need
//     ** to sync the file following this operation.
//     */
//     if( rc==SQLITE_OK && iLimit>0 ){
//       i64 sz;
//       rc = sqlite3OsFileSize(pPager->jfd, &sz);
//       if( rc==SQLITE_OK && sz>iLimit ){
//         rc = sqlite3OsTruncate(pPager->jfd, iLimit);
//       }
//     }
//   }
//   return rc;
// }

// /*
// ** The journal file must be open when this routine is called. A journal
// ** header (JOURNAL_HDR_SZ bytes) is written into the journal file at the
// ** current location.
// **
// ** The format for the journal header is as follows:
// ** - 8 bytes: Magic identifying journal format.
// ** - 4 bytes: Number of records in journal, or -1 no-sync mode is on.
// ** - 4 bytes: Random number used for page hash.
// ** - 4 bytes: Initial database page count.
// ** - 4 bytes: Sector size used by the process that wrote this journal.
// ** - 4 bytes: Database page size.
// ** 
// ** Followed by (JOURNAL_HDR_SZ - 28) bytes of unused space.
// */
// static int writeJournalHdr(Pager *pPager){
//   int rc = SQLITE_OK;                 /* Return code */
//   char *zHeader = pPager->pTmpSpace;  /* Temporary space used to build header */
//   u32 nHeader = (u32)pPager->pageSize;/* Size of buffer pointed to by zHeader */
//   u32 nWrite;                         /* Bytes of header sector written */
//   int ii;                             /* Loop counter */

//   assert( isOpen(pPager->jfd) );      /* Journal file must be open. */

//   if( nHeader>JOURNAL_HDR_SZ(pPager) ){
//     nHeader = JOURNAL_HDR_SZ(pPager);
//   }

//   /* If there are active savepoints and any of them were created 
//   ** since the most recent journal header was written, update the 
//   ** PagerSavepoint.iHdrOffset fields now.
//   */
//   for(ii=0; ii<pPager->nSavepoint; ii++){
//     if( pPager->aSavepoint[ii].iHdrOffset==0 ){
//       pPager->aSavepoint[ii].iHdrOffset = pPager->journalOff;
//     }
//   }

//   pPager->journalHdr = pPager->journalOff = journalHdrOffset(pPager);

//   /* 
//   ** Write the nRec Field - the number of page records that follow this
//   ** journal header. Normally, zero is written to this value at this time.
//   ** After the records are added to the journal (and the journal synced, 
//   ** if in full-sync mode), the zero is overwritten with the true number
//   ** of records (see syncJournal()).
//   **
//   ** A faster alternative is to write 0xFFFFFFFF to the nRec field. When
//   ** reading the journal this value tells SQLite to assume that the
//   ** rest of the journal file contains valid page records. This assumption
//   ** is dangerous, as if a failure occurred whilst writing to the journal
//   ** file it may contain some garbage data. There are two scenarios
//   ** where this risk can be ignored:
//   **
//   **   * When the pager is in no-sync mode. Corruption can follow a
//   **     power failure in this case anyway.
//   **
//   **   * When the SQLITE_IOCAP_SAFE_APPEND flag is set. This guarantees
//   **     that garbage data is never appended to the journal file.
//   */
//   assert( isOpen(pPager->fd) || pPager->noSync );
//   if( pPager->noSync || (pPager->journalMode==PAGER_JOURNALMODE_MEMORY)
//    || (sqlite3OsDeviceCharacteristics(pPager->fd)&SQLITE_IOCAP_SAFE_APPEND) 
//   ){
//     memcpy(zHeader, aJournalMagic, sizeof(aJournalMagic));
//     put32bits(&zHeader[sizeof(aJournalMagic)], 0xffffffff);
//   }else{
//     memset(zHeader, 0, sizeof(aJournalMagic)+4);
//   }

//   /* The random check-hash initializer */ 
//   sqlite3_randomness(sizeof(pPager->cksumInit), &pPager->cksumInit);
//   put32bits(&zHeader[sizeof(aJournalMagic)+4], pPager->cksumInit);
//   /* The initial database size */
//   put32bits(&zHeader[sizeof(aJournalMagic)+8], pPager->dbOrigSize);
//   /* The assumed sector size for this process */
//   put32bits(&zHeader[sizeof(aJournalMagic)+12], pPager->sectorSize);

//   /* The page size */
//   put32bits(&zHeader[sizeof(aJournalMagic)+16], pPager->pageSize);

//   /* Initializing the tail of the buffer is not necessary.  Everything
//   ** works find if the following memset() is omitted.  But initializing
//   ** the memory prevents valgrind from complaining, so we are willing to
//   ** take the performance hit.
//   */
//   memset(&zHeader[sizeof(aJournalMagic)+20], 0,
//          nHeader-(sizeof(aJournalMagic)+20));

//   /* In theory, it is only necessary to write the 28 bytes that the 
//   ** journal header consumes to the journal file here. Then increment the 
//   ** Pager.journalOff variable by JOURNAL_HDR_SZ so that the next 
//   ** record is written to the following sector (leaving a gap in the file
//   ** that will be implicitly filled in by the OS).
//   **
//   ** However it has been discovered that on some systems this pattern can 
//   ** be significantly slower than contiguously writing data to the file,
//   ** even if that means explicitly writing data to the block of 
//   ** (JOURNAL_HDR_SZ - 28) bytes that will not be used. So that is what
//   ** is done. 
//   **
//   ** The loop is required here in case the sector-size is larger than the 
//   ** database page size. Since the zHeader buffer is only Pager.pageSize
//   ** bytes in size, more than one call to sqlite3OsWrite() may be required
//   ** to populate the entire journal header sector.
//   */ 
//   for(nWrite=0; rc==SQLITE_OK&&nWrite<JOURNAL_HDR_SZ(pPager); nWrite+=nHeader){
//     IOTRACE(("JHDR %p %lld %d\n", pPager, pPager->journalHdr, nHeader))
//     rc = sqlite3OsWrite(pPager->jfd, zHeader, nHeader, pPager->journalOff);
//     assert( pPager->journalHdr <= pPager->journalOff );
//     pPager->journalOff += nHeader;
//   }

//   return rc;
// }

// /*
// ** The journal file must be open when this is called. A journal header file
// ** (JOURNAL_HDR_SZ bytes) is read from the current location in the journal
// ** file. The current location in the journal file is given by
// ** pPager->journalOff. See comments above function writeJournalHdr() for
// ** a description of the journal header format.
// **
// ** If the header is read successfully, *pNRec is set to the number of
// ** page records following this header and *pDbSize is set to the size of the
// ** database before the transaction began, in pages. Also, pPager->cksumInit
// ** is set to the value read from the journal header. SQLITE_OK is returned
// ** in this case.
// **
// ** If the journal header file appears to be corrupted, SQLITE_DONE is
// ** returned and *pNRec and *PDbSize are undefined.  If JOURNAL_HDR_SZ bytes
// ** cannot be read from the journal file an error code is returned.
// */
// static int readJournalHdr(
//   Pager *pPager,               /* Pager object */
//   int isHot,
//   i64 journalSize,             /* Size of the open journal file in bytes */
//   u32 *pNRec,                  /* OUT: Value read from the nRec field */
//   u32 *pDbSize                 /* OUT: Value of original database size field */
// ){
//   int rc;                      /* Return code */
//   unsigned char aMagic[8];     /* A buffer to hold the magic header */
//   i64 iHdrOff;                 /* Offset of journal header being read */

//   assert( isOpen(pPager->jfd) );      /* Journal file must be open. */

//   /* Advance Pager.journalOff to the start of the next sector. If the
//   ** journal file is too small for there to be a header stored at this
//   ** point, return SQLITE_DONE.
//   */
//   pPager->journalOff = journalHdrOffset(pPager);
//   if( pPager->journalOff+JOURNAL_HDR_SZ(pPager) > journalSize ){
//     return SQLITE_DONE;
//   }
//   iHdrOff = pPager->journalOff;

//   /* Read in the first 8 bytes of the journal header. If they do not match
//   ** the  magic string found at the start of each journal header, return
//   ** SQLITE_DONE. If an IO error occurs, return an error code. Otherwise,
//   ** proceed.
//   */
//   if( isHot || iHdrOff!=pPager->journalHdr ){
//     rc = sqlite3OsRead(pPager->jfd, aMagic, sizeof(aMagic), iHdrOff);
//     if( rc ){
//       return rc;
//     }
//     if( memcmp(aMagic, aJournalMagic, sizeof(aMagic))!=0 ){
//       return SQLITE_DONE;
//     }
//   }

//   /* Read the first three 32-bit fields of the journal header: The nRec
//   ** field, the checksum-initializer and the database size at the start
//   ** of the transaction. Return an error code if anything goes wrong.
//   */
//   if( SQLITE_OK!=(rc = read32bits(pPager->jfd, iHdrOff+8, pNRec))
//    || SQLITE_OK!=(rc = read32bits(pPager->jfd, iHdrOff+12, &pPager->cksumInit))
//    || SQLITE_OK!=(rc = read32bits(pPager->jfd, iHdrOff+16, pDbSize))
//   ){
//     return rc;
//   }

//   if( pPager->journalOff==0 ){
//     u32 iPageSize;               /* Page-size field of journal header */
//     u32 iSectorSize;             /* Sector-size field of journal header */

//     /* Read the page-size and sector-size journal header fields. */
//     if( SQLITE_OK!=(rc = read32bits(pPager->jfd, iHdrOff+20, &iSectorSize))
//      || SQLITE_OK!=(rc = read32bits(pPager->jfd, iHdrOff+24, &iPageSize))
//     ){
//       return rc;
//     }

//     /* Versions of SQLite prior to 3.5.8 set the page-size field of the
//     ** journal header to zero. In this case, assume that the Pager.pageSize
//     ** variable is already set to the correct page size.
//     */
//     if( iPageSize==0 ){
//       iPageSize = pPager->pageSize;
//     }

//     /* Check that the values read from the page-size and sector-size fields
//     ** are within range. To be 'in range', both values need to be a power
//     ** of two greater than or equal to 512 or 32, and not greater than their 
//     ** respective compile time maximum limits.
//     */
//     if( iPageSize<512                  || iSectorSize<32
//      || iPageSize>SQLITE_MAX_PAGE_SIZE || iSectorSize>MAX_SECTOR_SIZE
//      || ((iPageSize-1)&iPageSize)!=0   || ((iSectorSize-1)&iSectorSize)!=0 
//     ){
//       /* If the either the page-size or sector-size in the journal-header is 
//       ** invalid, then the process that wrote the journal-header must have 
//       ** crashed before the header was synced. In this case stop reading 
//       ** the journal file here.
//       */
//       return SQLITE_DONE;
//     }

//     /* Update the page-size to match the value read from the journal. 
//     ** Use a testcase() macro to make sure that malloc failure within 
//     ** PagerSetPagesize() is tested.
//     */
//     rc = sqlite3PagerSetPagesize(pPager, &iPageSize, -1);
//     testcase( rc!=SQLITE_OK );

//     /* Update the assumed sector-size to match the value used by 
//     ** the process that created this journal. If this journal was
//     ** created by a process other than this one, then this routine
//     ** is being called from within pager_playback(). The local value
//     ** of Pager.sectorSize is restored at the end of that routine.
//     */
//     pPager->sectorSize = iSectorSize;
//   }

//   pPager->journalOff += JOURNAL_HDR_SZ(pPager);
//   return rc;
// }


// /*
// ** Write the supplied super-journal name into the journal file for pager
// ** pPager at the current location. The super-journal name must be the last
// ** thing written to a journal file. If the pager is in full-sync mode, the
// ** journal file descriptor is advanced to the next sector boundary before
// ** anything is written. The format is:
// **
// **   + 4 bytes: PAGER_MJ_PGNO.
// **   + N bytes: super-journal filename in utf-8.
// **   + 4 bytes: N (length of super-journal name in bytes, no nul-terminator).
// **   + 4 bytes: super-journal name checksum.
// **   + 8 bytes: aJournalMagic[].
// **
// ** The super-journal page checksum is the sum of the bytes in thesuper-journal
// ** name, where each byte is interpreted as a signed 8-bit integer.
// **
// ** If zSuper is a NULL pointer (occurs for a single database transaction), 
// ** this call is a no-op.
// */
// static int writeSuperJournal(Pager *pPager, const char *zSuper){
//   int rc;                          /* Return code */
//   int nSuper;                      /* Length of string zSuper */
//   i64 iHdrOff;                     /* Offset of header in journal file */
//   i64 jrnlSize;                    /* Size of journal file on disk */
//   u32 cksum = 0;                   /* Checksum of string zSuper */

//   assert( pPager->setSuper==0 );
//   assert( !pagerUseWal(pPager) );

//   if( !zSuper 
//    || pPager->journalMode==PAGER_JOURNALMODE_MEMORY 
//    || !isOpen(pPager->jfd)
//   ){
//     return SQLITE_OK;
//   }
//   pPager->setSuper = 1;
//   assert( pPager->journalHdr <= pPager->journalOff );

//   /* Calculate the length in bytes and the checksum of zSuper */
//   for(nSuper=0; zSuper[nSuper]; nSuper++){
//     cksum += zSuper[nSuper];
//   }

//   /* If in full-sync mode, advance to the next disk sector before writing
//   ** the super-journal name. This is in case the previous page written to
//   ** the journal has already been synced.
//   */
//   if( pPager->fullSync ){
//     pPager->journalOff = journalHdrOffset(pPager);
//   }
//   iHdrOff = pPager->journalOff;

//   /* Write the super-journal data to the end of the journal file. If
//   ** an error occurs, return the error code to the caller.
//   */
//   if( (0 != (rc = write32bits(pPager->jfd, iHdrOff, PAGER_MJ_PGNO(pPager))))
//    || (0 != (rc = sqlite3OsWrite(pPager->jfd, zSuper, nSuper, iHdrOff+4)))
//    || (0 != (rc = write32bits(pPager->jfd, iHdrOff+4+nSuper, nSuper)))
//    || (0 != (rc = write32bits(pPager->jfd, iHdrOff+4+nSuper+4, cksum)))
//    || (0 != (rc = sqlite3OsWrite(pPager->jfd, aJournalMagic, 8,
//                                  iHdrOff+4+nSuper+8)))
//   ){
//     return rc;
//   }
//   pPager->journalOff += (nSuper+20);

//   /* If the pager is in peristent-journal mode, then the physical 
//   ** journal-file may extend past the end of the super-journal name
//   ** and 8 bytes of magic data just written to the file. This is 
//   ** dangerous because the code to rollback a hot-journal file
//   ** will not be able to find the super-journal name to determine 
//   ** whether or not the journal is hot. 
//   **
//   ** Easiest thing to do in this scenario is to truncate the journal 
//   ** file to the required size.
//   */ 
//   if( SQLITE_OK==(rc = sqlite3OsFileSize(pPager->jfd, &jrnlSize))
//    && jrnlSize>pPager->journalOff
//   ){
//     rc = sqlite3OsTruncate(pPager->jfd, pPager->journalOff);
//   }
//   return rc;
// }

// /*
// ** Discard the entire contents of the in-memory page-cache.
// */
// static void pager_reset(Pager *pPager){
//   pPager->iDataVersion++;
//   sqlite3BackupRestart(pPager->pBackup);
//   sqlite3PcacheClear(pPager->pPCache);
// }

// /*
// ** Return the pPager->iDataVersion value
// */
// u32 sqlite3PagerDataVersion(Pager *pPager){
//   return pPager->iDataVersion;
// }

// /*
// ** Free all structures in the Pager.aSavepoint[] array and set both
// ** Pager.aSavepoint and Pager.nSavepoint to zero. Close the sub-journal
// ** if it is open and the pager is not in exclusive mode.
// */
// static void releaseAllSavepoints(Pager *pPager){
//   int ii;               /* Iterator for looping through Pager.aSavepoint */
//   for(ii=0; ii<pPager->nSavepoint; ii++){
//     sqlite3BitvecDestroy(pPager->aSavepoint[ii].pInSavepoint);
//   }
//   if( !pPager->exclusiveMode || sqlite3JournalIsInMemory(pPager->sjfd) ){
//     sqlite3OsClose(pPager->sjfd);
//   }
//   sqlite3_free(pPager->aSavepoint);
//   pPager->aSavepoint = 0;
//   pPager->nSavepoint = 0;
//   pPager->nSubRec = 0;
// }

// /*
// ** Set the bit number pgno in the PagerSavepoint.pInSavepoint 
// ** bitvecs of all open savepoints. Return SQLITE_OK if successful
// ** or SQLITE_NOMEM if a malloc failure occurs.
// */
// static int addToSavepointBitvecs(Pager *pPager, Pgno pgno){
//   int ii;                   /* Loop counter */
//   int rc = SQLITE_OK;       /* Result code */

//   for(ii=0; ii<pPager->nSavepoint; ii++){
//     PagerSavepoint *p = &pPager->aSavepoint[ii];
//     if( pgno<=p->nOrig ){
//       rc |= sqlite3BitvecSet(p->pInSavepoint, pgno);
//       testcase( rc==SQLITE_NOMEM );
//       assert( rc==SQLITE_OK || rc==SQLITE_NOMEM );
//     }
//   }
//   return rc;
// }

// /*
// ** This function is a no-op if the pager is in exclusive mode and not
// ** in the ERROR state. Otherwise, it switches the pager to PAGER_OPEN
// ** state.
// **
// ** If the pager is not in exclusive-access mode, the database file is
// ** completely unlocked. If the file is unlocked and the file-system does
// ** not exhibit the UNDELETABLE_WHEN_OPEN property, the journal file is
// ** closed (if it is open).
// **
// ** If the pager is in ERROR state when this function is called, the 
// ** contents of the pager cache are discarded before switching back to 
// ** the OPEN state. Regardless of whether the pager is in exclusive-mode
// ** or not, any journal file left in the file-system will be treated
// ** as a hot-journal and rolled back the next time a read-transaction
// ** is opened (by this or by any other connection).
// */
// static void pager_unlock(Pager *pPager){

//   assert( pPager->eState==PAGER_READER 
//        || pPager->eState==PAGER_OPEN 
//        || pPager->eState==PAGER_ERROR 
//   );

//   sqlite3BitvecDestroy(pPager->pInJournal);
//   pPager->pInJournal = 0;
//   releaseAllSavepoints(pPager);

//   if( pagerUseWal(pPager) ){
//     assert( !isOpen(pPager->jfd) );
//     sqlite3WalEndReadTransaction(pPager->pWal);
//     pPager->eState = PAGER_OPEN;
//   }else if( !pPager->exclusiveMode ){
//     int rc;                       /* Error code returned by pagerUnlockDb() */
//     int iDc = isOpen(pPager->fd)?sqlite3OsDeviceCharacteristics(pPager->fd):0;

//     /* If the operating system support deletion of open files, then
//     ** close the journal file when dropping the database lock.  Otherwise
//     ** another connection with journal_mode=delete might delete the file
//     ** out from under us.
//     */
//     assert( (PAGER_JOURNALMODE_MEMORY   & 5)!=1 );
//     assert( (PAGER_JOURNALMODE_OFF      & 5)!=1 );
//     assert( (PAGER_JOURNALMODE_WAL      & 5)!=1 );
//     assert( (PAGER_JOURNALMODE_DELETE   & 5)!=1 );
//     assert( (PAGER_JOURNALMODE_TRUNCATE & 5)==1 );
//     assert( (PAGER_JOURNALMODE_PERSIST  & 5)==1 );
//     if( 0==(iDc & SQLITE_IOCAP_UNDELETABLE_WHEN_OPEN)
//      || 1!=(pPager->journalMode & 5)
//     ){
//       sqlite3OsClose(pPager->jfd);
//     }

//     /* If the pager is in the ERROR state and the call to unlock the database
//     ** file fails, set the current lock to UNKNOWN_LOCK. See the comment
//     ** above the #define for UNKNOWN_LOCK for an explanation of why this
//     ** is necessary.
//     */
//     rc = pagerUnlockDb(pPager, NO_LOCK);
//     if( rc!=SQLITE_OK && pPager->eState==PAGER_ERROR ){
//       pPager->eLock = UNKNOWN_LOCK;
//     }

//     /* The pager state may be changed from PAGER_ERROR to PAGER_OPEN here
//     ** without clearing the error code. This is intentional - the error
//     ** code is cleared and the cache reset in the block below.
//     */
//     assert( pPager->errCode || pPager->eState!=PAGER_ERROR );
//     pPager->eState = PAGER_OPEN;
//   }

//   /* If Pager.errCode is set, the contents of the pager cache cannot be
//   ** trusted. Now that there are no outstanding references to the pager,
//   ** it can safely move back to PAGER_OPEN state. This happens in both
//   ** normal and exclusive-locking mode.
//   */
//   assert( pPager->errCode==SQLITE_OK || !MEMDB );
//   if( pPager->errCode ){
//     if( pPager->tempFile==0 ){
//       pager_reset(pPager);
//       pPager->changeCountDone = 0;
//       pPager->eState = PAGER_OPEN;
//     }else{
//       pPager->eState = (isOpen(pPager->jfd) ? PAGER_OPEN : PAGER_READER);
//     }
//     if( USEFETCH(pPager) ) sqlite3OsUnfetch(pPager->fd, 0, 0);
//     pPager->errCode = SQLITE_OK;
//     setGetterMethod(pPager);
//   }

//   pPager->journalOff = 0;
//   pPager->journalHdr = 0;
//   pPager->setSuper = 0;
// }

// /*
// ** This function is called whenever an IOERR or FULL error that requires
// ** the pager to transition into the ERROR state may ahve occurred.
// ** The first argument is a pointer to the pager structure, the second 
// ** the error-code about to be returned by a pager API function. The 
// ** value returned is a copy of the second argument to this function. 
// **
// ** If the second argument is SQLITE_FULL, SQLITE_IOERR or one of the
// ** IOERR sub-codes, the pager enters the ERROR state and the error code
// ** is stored in Pager.errCode. While the pager remains in the ERROR state,
// ** all major API calls on the Pager will immediately return Pager.errCode.
// **
// ** The ERROR state indicates that the contents of the pager-cache 
// ** cannot be trusted. This state can be cleared by completely discarding 
// ** the contents of the pager-cache. If a transaction was active when
// ** the persistent error occurred, then the rollback journal may need
// ** to be replayed to restore the contents of the database file (as if
// ** it were a hot-journal).
// */
// static int pager_error(Pager *pPager, int rc){
//   int rc2 = rc & 0xff;
//   assert( rc==SQLITE_OK || !MEMDB );
//   assert(
//        pPager->errCode==SQLITE_FULL ||
//        pPager->errCode==SQLITE_OK ||
//        (pPager->errCode & 0xff)==SQLITE_IOERR
//   );
//   if( rc2==SQLITE_FULL || rc2==SQLITE_IOERR ){
//     pPager->errCode = rc;
//     pPager->eState = PAGER_ERROR;
//     setGetterMethod(pPager);
//   }
//   return rc;
// }

// static int pager_truncate(Pager *pPager, Pgno nPage);

// /*
// ** The write transaction open on pPager is being committed (bCommit==1)
// ** or rolled back (bCommit==0).
// **
// ** Return TRUE if and only if all dirty pages should be flushed to disk.
// **
// ** Rules:
// **
// **   *  For non-TEMP databases, always sync to disk.  This is necessary
// **      for transactions to be durable.
// **
// **   *  Sync TEMP database only on a COMMIT (not a ROLLBACK) when the backing
// **      file has been created already (via a spill on pagerStress()) and
// **      when the number of dirty pages in memory exceeds 25% of the total
// **      cache size.
// */
// static int pagerFlushOnCommit(Pager *pPager, int bCommit){
//   if( pPager->tempFile==0 ) return 1;
//   if( !bCommit ) return 0;
//   if( !isOpen(pPager->fd) ) return 0;
//   return (sqlite3PCachePercentDirty(pPager->pPCache)>=25);
// }

// /*
// ** This routine ends a transaction. A transaction is usually ended by 
// ** either a COMMIT or a ROLLBACK operation. This routine may be called 
// ** after rollback of a hot-journal, or if an error occurs while opening
// ** the journal file or writing the very first journal-header of a
// ** database transaction.
// ** 
// ** This routine is never called in PAGER_ERROR state. If it is called
// ** in PAGER_NONE or PAGER_SHARED state and the lock held is less
// ** exclusive than a RESERVED lock, it is a no-op.
// **
// ** Otherwise, any active savepoints are released.
// **
// ** If the journal file is open, then it is "finalized". Once a journal 
// ** file has been finalized it is not possible to use it to roll back a 
// ** transaction. Nor will it be considered to be a hot-journal by this
// ** or any other database connection. Exactly how a journal is finalized
// ** depends on whether or not the pager is running in exclusive mode and
// ** the current journal-mode (Pager.journalMode value), as follows:
// **
// **   journalMode==MEMORY
// **     Journal file descriptor is simply closed. This destroys an 
// **     in-memory journal.
// **
// **   journalMode==TRUNCATE
// **     Journal file is truncated to zero bytes in size.
// **
// **   journalMode==PERSIST
// **     The first 28 bytes of the journal file are zeroed. This invalidates
// **     the first journal header in the file, and hence the entire journal
// **     file. An invalid journal file cannot be rolled back.
// **
// **   journalMode==DELETE
// **     The journal file is closed and deleted using sqlite3OsDelete().
// **
// **     If the pager is running in exclusive mode, this method of finalizing
// **     the journal file is never used. Instead, if the journalMode is
// **     DELETE and the pager is in exclusive mode, the method described under
// **     journalMode==PERSIST is used instead.
// **
// ** After the journal is finalized, the pager moves to PAGER_READER state.
// ** If running in non-exclusive rollback mode, the lock on the file is 
// ** downgraded to a SHARED_LOCK.
// **
// ** SQLITE_OK is returned if no error occurs. If an error occurs during
// ** any of the IO operations to finalize the journal file or unlock the
// ** database then the IO error code is returned to the user. If the 
// ** operation to finalize the journal file fails, then the code still
// ** tries to unlock the database file if not in exclusive mode. If the
// ** unlock operation fails as well, then the first error code related
// ** to the first error encountered (the journal finalization one) is
// ** returned.
// */
// static int pager_end_transaction(Pager *pPager, int hasSuper, int bCommit){
//   int rc = SQLITE_OK;      /* Error code from journal finalization operation */
//   int rc2 = SQLITE_OK;     /* Error code from db file unlock operation */

//   /* Do nothing if the pager does not have an open write transaction
//   ** or at least a RESERVED lock. This function may be called when there
//   ** is no write-transaction active but a RESERVED or greater lock is
//   ** held under two circumstances:
//   **
//   **   1. After a successful hot-journal rollback, it is called with
//   **      eState==PAGER_NONE and eLock==EXCLUSIVE_LOCK.
//   **
//   **   2. If a connection with locking_mode=exclusive holding an EXCLUSIVE 
//   **      lock switches back to locking_mode=normal and then executes a
//   **      read-transaction, this function is called with eState==PAGER_READER 
//   **      and eLock==EXCLUSIVE_LOCK when the read-transaction is closed.
//   */
//   assert( assert_pager_state(pPager) );
//   assert( pPager->eState!=PAGER_ERROR );
//   if( pPager->eState<PAGER_WRITER_LOCKED && pPager->eLock<RESERVED_LOCK ){
//     return SQLITE_OK;
//   }

//   releaseAllSavepoints(pPager);
//   assert( isOpen(pPager->jfd) || pPager->pInJournal==0 
//       || (sqlite3OsDeviceCharacteristics(pPager->fd)&SQLITE_IOCAP_BATCH_ATOMIC)
//   );
//   if( isOpen(pPager->jfd) ){
//     assert( !pagerUseWal(pPager) );

//     /* Finalize the journal file. */
//     if( sqlite3JournalIsInMemory(pPager->jfd) ){
//       /* assert( pPager->journalMode==PAGER_JOURNALMODE_MEMORY ); */
//       sqlite3OsClose(pPager->jfd);
//     }else if( pPager->journalMode==PAGER_JOURNALMODE_TRUNCATE ){
//       if( pPager->journalOff==0 ){
//         rc = SQLITE_OK;
//       }else{
//         rc = sqlite3OsTruncate(pPager->jfd, 0);
//         if( rc==SQLITE_OK && pPager->fullSync ){
//           /* Make sure the new file size is written into the inode right away.
//           ** Otherwise the journal might resurrect following a power loss and
//           ** cause the last transaction to roll back.  See
//           ** https://bugzilla.mozilla.org/show_bug.cgi?id=1072773
//           */
//           rc = sqlite3OsSync(pPager->jfd, pPager->syncFlags);
//         }
//       }
//       pPager->journalOff = 0;
//     }else if( pPager->journalMode==PAGER_JOURNALMODE_PERSIST
//       || (pPager->exclusiveMode && pPager->journalMode!=PAGER_JOURNALMODE_WAL)
//     ){
//       rc = zeroJournalHdr(pPager, hasSuper||pPager->tempFile);
//       pPager->journalOff = 0;
//     }else{
//       /* This branch may be executed with Pager.journalMode==MEMORY if
//       ** a hot-journal was just rolled back. In this case the journal
//       ** file should be closed and deleted. If this connection writes to
//       ** the database file, it will do so using an in-memory journal.
//       */
//       int bDelete = !pPager->tempFile;
//       assert( sqlite3JournalIsInMemory(pPager->jfd)==0 );
//       assert( pPager->journalMode==PAGER_JOURNALMODE_DELETE 
//            || pPager->journalMode==PAGER_JOURNALMODE_MEMORY 
//            || pPager->journalMode==PAGER_JOURNALMODE_WAL 
//       );
//       sqlite3OsClose(pPager->jfd);
//       if( bDelete ){
//         rc = sqlite3OsDelete(pPager->pVfs, pPager->zJournal, pPager->extraSync);
//       }
//     }
//   }

// #ifdef SQLITE_CHECK_PAGES
//   sqlite3PcacheIterateDirty(pPager->pPCache, pager_set_pagehash);
//   if( pPager->dbSize==0 && sqlite3PcacheRefCount(pPager->pPCache)>0 ){
//     PgHdr *p = sqlite3PagerLookup(pPager, 1);
//     if( p ){
//       p->pageHash = 0;
//       sqlite3PagerUnrefNotNull(p);
//     }
//   }
// #endif

//   sqlite3BitvecDestroy(pPager->pInJournal);
//   pPager->pInJournal = 0;
//   pPager->nRec = 0;
//   if( rc==SQLITE_OK ){
//     if( MEMDB || pagerFlushOnCommit(pPager, bCommit) ){
//       sqlite3PcacheCleanAll(pPager->pPCache);
//     }else{
//       sqlite3PcacheClearWritable(pPager->pPCache);
//     }
//     sqlite3PcacheTruncate(pPager->pPCache, pPager->dbSize);
//   }

//   if( pagerUseWal(pPager) ){
//     /* Drop the WAL write-lock, if any. Also, if the connection was in 
//     ** locking_mode=exclusive mode but is no longer, drop the EXCLUSIVE 
//     ** lock held on the database file.
//     */
//     rc2 = sqlite3WalEndWriteTransaction(pPager->pWal);
//     assert( rc2==SQLITE_OK );
//   }else if( rc==SQLITE_OK && bCommit && pPager->dbFileSize>pPager->dbSize ){
//     /* This branch is taken when committing a transaction in rollback-journal
//     ** mode if the database file on disk is larger than the database image.
//     ** At this point the journal has been finalized and the transaction 
//     ** successfully committed, but the EXCLUSIVE lock is still held on the
//     ** file. So it is safe to truncate the database file to its minimum
//     ** required size.  */
//     assert( pPager->eLock==EXCLUSIVE_LOCK );
//     rc = pager_truncate(pPager, pPager->dbSize);
//   }

//   if( rc==SQLITE_OK && bCommit ){
//     rc = sqlite3OsFileControl(pPager->fd, SQLITE_FCNTL_COMMIT_PHASETWO, 0);
//     if( rc==SQLITE_NOTFOUND ) rc = SQLITE_OK;
//   }

//   if( !pPager->exclusiveMode 
//    && (!pagerUseWal(pPager) || sqlite3WalExclusiveMode(pPager->pWal, 0))
//   ){
//     rc2 = pagerUnlockDb(pPager, SHARED_LOCK);
//   }
//   pPager->eState = PAGER_READER;
//   pPager->setSuper = 0;

//   return (rc==SQLITE_OK?rc2:rc);
// }

// /*
// ** Execute a rollback if a transaction is active and unlock the 
// ** database file. 
// **
// ** If the pager has already entered the ERROR state, do not attempt 
// ** the rollback at this time. Instead, pager_unlock() is called. The
// ** call to pager_unlock() will discard all in-memory pages, unlock
// ** the database file and move the pager back to OPEN state. If this 
// ** means that there is a hot-journal left in the file-system, the next 
// ** connection to obtain a shared lock on the pager (which may be this one) 
// ** will roll it back.
// **
// ** If the pager has not already entered the ERROR state, but an IO or
// ** malloc error occurs during a rollback, then this will itself cause 
// ** the pager to enter the ERROR state. Which will be cleared by the
// ** call to pager_unlock(), as described above.
// */
// static void pagerUnlockAndRollback(Pager *pPager){
//   if( pPager->eState!=PAGER_ERROR && pPager->eState!=PAGER_OPEN ){
//     assert( assert_pager_state(pPager) );
//     if( pPager->eState>=PAGER_WRITER_LOCKED ){
//       sqlite3BeginBenignMalloc();
//       sqlite3PagerRollback(pPager);
//       sqlite3EndBenignMalloc();
//     }else if( !pPager->exclusiveMode ){
//       assert( pPager->eState==PAGER_READER );
//       pager_end_transaction(pPager, 0, 0);
//     }
//   }
//   pager_unlock(pPager);
// }

// /*
// ** Parameter aData must point to a buffer of pPager->pageSize bytes
// ** of data. Compute and return a checksum based ont the contents of the 
// ** page of data and the current value of pPager->cksumInit.
// **
// ** This is not a real checksum. It is really just the sum of the 
// ** random initial value (pPager->cksumInit) and every 200th byte
// ** of the page data, starting with byte offset (pPager->pageSize%200).
// ** Each byte is interpreted as an 8-bit unsigned integer.
// **
// ** Changing the formula used to compute this checksum results in an
// ** incompatible journal file format.
// **
// ** If journal corruption occurs due to a power failure, the most likely 
// ** scenario is that one end or the other of the record will be changed. 
// ** It is much less likely that the two ends of the journal record will be
// ** correct and the middle be corrupt.  Thus, this "checksum" scheme,
// ** though fast and simple, catches the mostly likely kind of corruption.
// */
// static u32 pager_cksum(Pager *pPager, const u8 *aData){
//   u32 cksum = pPager->cksumInit;         /* Checksum value to return */
//   int i = pPager->pageSize-200;          /* Loop counter */
//   while( i>0 ){
//     cksum += aData[i];
//     i -= 200;
//   }
//   return cksum;
// }

// /*
// ** Read a single page from either the journal file (if isMainJrnl==1) or
// ** from the sub-journal (if isMainJrnl==0) and playback that page.
// ** The page begins at offset *pOffset into the file. The *pOffset
// ** value is increased to the start of the next page in the journal.
// **
// ** The main rollback journal uses checksums - the statement journal does 
// ** not.
// **
// ** If the page number of the page record read from the (sub-)journal file
// ** is greater than the current value of Pager.dbSize, then playback is
// ** skipped and SQLITE_OK is returned.
// **
// ** If pDone is not NULL, then it is a record of pages that have already
// ** been played back.  If the page at *pOffset has already been played back
// ** (if the corresponding pDone bit is set) then skip the playback.
// ** Make sure the pDone bit corresponding to the *pOffset page is set
// ** prior to returning.
// **
// ** If the page record is successfully read from the (sub-)journal file
// ** and played back, then SQLITE_OK is returned. If an IO error occurs
// ** while reading the record from the (sub-)journal file or while writing
// ** to the database file, then the IO error code is returned. If data
// ** is successfully read from the (sub-)journal file but appears to be
// ** corrupted, SQLITE_DONE is returned. Data is considered corrupted in
// ** two circumstances:
// ** 
// **   * If the record page-number is illegal (0 or PAGER_MJ_PGNO), or
// **   * If the record is being rolled back from the main journal file
// **     and the checksum field does not match the record content.
// **
// ** Neither of these two scenarios are possible during a savepoint rollback.
// **
// ** If this is a savepoint rollback, then memory may have to be dynamically
// ** allocated by this function. If this is the case and an allocation fails,
// ** SQLITE_NOMEM is returned.
// */
// static int pager_playback_one_page(
//   Pager *pPager,                /* The pager being played back */
//   i64 *pOffset,                 /* Offset of record to playback */
//   Bitvec *pDone,                /* Bitvec of pages already played back */
//   int isMainJrnl,               /* 1 -> main journal. 0 -> sub-journal. */
//   int isSavepnt                 /* True for a savepoint rollback */
// ){
//   int rc;
//   PgHdr *pPg;                   /* An existing page in the cache */
//   Pgno pgno;                    /* The page number of a page in journal */
//   u32 cksum;                    /* Checksum used for sanity checking */
//   char *aData;                  /* Temporary storage for the page */
//   sqlite3_file *jfd;            /* The file descriptor for the journal file */
//   int isSynced;                 /* True if journal page is synced */

//   assert( (isMainJrnl&~1)==0 );      /* isMainJrnl is 0 or 1 */
//   assert( (isSavepnt&~1)==0 );       /* isSavepnt is 0 or 1 */
//   assert( isMainJrnl || pDone );     /* pDone always used on sub-journals */
//   assert( isSavepnt || pDone==0 );   /* pDone never used on non-savepoint */

//   aData = pPager->pTmpSpace;
//   assert( aData );         /* Temp storage must have already been allocated */
//   assert( pagerUseWal(pPager)==0 || (!isMainJrnl && isSavepnt) );

//   /* Either the state is greater than PAGER_WRITER_CACHEMOD (a transaction 
//   ** or savepoint rollback done at the request of the caller) or this is
//   ** a hot-journal rollback. If it is a hot-journal rollback, the pager
//   ** is in state OPEN and holds an EXCLUSIVE lock. Hot-journal rollback
//   ** only reads from the main journal, not the sub-journal.
//   */
//   assert( pPager->eState>=PAGER_WRITER_CACHEMOD
//        || (pPager->eState==PAGER_OPEN && pPager->eLock==EXCLUSIVE_LOCK)
//   );
//   assert( pPager->eState>=PAGER_WRITER_CACHEMOD || isMainJrnl );

//   /* Read the page number and page data from the journal or sub-journal
//   ** file. Return an error code to the caller if an IO error occurs.
//   */
//   jfd = isMainJrnl ? pPager->jfd : pPager->sjfd;
//   rc = read32bits(jfd, *pOffset, &pgno);
//   if( rc!=SQLITE_OK ) return rc;
//   rc = sqlite3OsRead(jfd, (u8*)aData, pPager->pageSize, (*pOffset)+4);
//   if( rc!=SQLITE_OK ) return rc;
//   *pOffset += pPager->pageSize + 4 + isMainJrnl*4;

//   /* Sanity checking on the page.  This is more important that I originally
//   ** thought.  If a power failure occurs while the journal is being written,
//   ** it could cause invalid data to be written into the journal.  We need to
//   ** detect this invalid data (with high probability) and ignore it.
//   */
//   if( pgno==0 || pgno==PAGER_MJ_PGNO(pPager) ){
//     assert( !isSavepnt );
//     return SQLITE_DONE;
//   }
//   if( pgno>(Pgno)pPager->dbSize || sqlite3BitvecTest(pDone, pgno) ){
//     return SQLITE_OK;
//   }
//   if( isMainJrnl ){
//     rc = read32bits(jfd, (*pOffset)-4, &cksum);
//     if( rc ) return rc;
//     if( !isSavepnt && pager_cksum(pPager, (u8*)aData)!=cksum ){
//       return SQLITE_DONE;
//     }
//   }

//   /* If this page has already been played back before during the current
//   ** rollback, then don't bother to play it back again.
//   */
//   if( pDone && (rc = sqlite3BitvecSet(pDone, pgno))!=SQLITE_OK ){
//     return rc;
//   }

//   /* When playing back page 1, restore the nReserve setting
//   */
//   if( pgno==1 && pPager->nReserve!=((u8*)aData)[20] ){
//     pPager->nReserve = ((u8*)aData)[20];
//   }

//   /* If the pager is in CACHEMOD state, then there must be a copy of this
//   ** page in the pager cache. In this case just update the pager cache,
//   ** not the database file. The page is left marked dirty in this case.
//   **
//   ** An exception to the above rule: If the database is in no-sync mode
//   ** and a page is moved during an incremental vacuum then the page may
//   ** not be in the pager cache. Later: if a malloc() or IO error occurs
//   ** during a Movepage() call, then the page may not be in the cache
//   ** either. So the condition described in the above paragraph is not
//   ** assert()able.
//   **
//   ** If in WRITER_DBMOD, WRITER_FINISHED or OPEN state, then we update the
//   ** pager cache if it exists and the main file. The page is then marked 
//   ** not dirty. Since this code is only executed in PAGER_OPEN state for
//   ** a hot-journal rollback, it is guaranteed that the page-cache is empty
//   ** if the pager is in OPEN state.
//   **
//   ** Ticket #1171:  The statement journal might contain page content that is
//   ** different from the page content at the start of the transaction.
//   ** This occurs when a page is changed prior to the start of a statement
//   ** then changed again within the statement.  When rolling back such a
//   ** statement we must not write to the original database unless we know
//   ** for certain that original page contents are synced into the main rollback
//   ** journal.  Otherwise, a power loss might leave modified data in the
//   ** database file without an entry in the rollback journal that can
//   ** restore the database to its original form.  Two conditions must be
//   ** met before writing to the database files. (1) the database must be
//   ** locked.  (2) we know that the original page content is fully synced
//   ** in the main journal either because the page is not in cache or else
//   ** the page is marked as needSync==0.
//   **
//   ** 2008-04-14:  When attempting to vacuum a corrupt database file, it
//   ** is possible to fail a statement on a database that does not yet exist.
//   ** Do not attempt to write if database file has never been opened.
//   */
//   if( pagerUseWal(pPager) ){
//     pPg = 0;
//   }else{
//     pPg = sqlite3PagerLookup(pPager, pgno);
//   }
//   assert( pPg || !MEMDB );
//   assert( pPager->eState!=PAGER_OPEN || pPg==0 || pPager->tempFile );
//   PAGERTRACE(("PLAYBACK %d page %d hash(%08x) %s\n",
//            PAGERID(pPager), pgno, pager_datahash(pPager->pageSize, (u8*)aData),
//            (isMainJrnl?"main-journal":"sub-journal")
//   ));
//   if( isMainJrnl ){
//     isSynced = pPager->noSync || (*pOffset <= pPager->journalHdr);
//   }else{
//     isSynced = (pPg==0 || 0==(pPg->flags & PGHDR_NEED_SYNC));
//   }
//   if( isOpen(pPager->fd)
//    && (pPager->eState>=PAGER_WRITER_DBMOD || pPager->eState==PAGER_OPEN)
//    && isSynced
//   ){
//     i64 ofst = (pgno-1)*(i64)pPager->pageSize;
//     testcase( !isSavepnt && pPg!=0 && (pPg->flags&PGHDR_NEED_SYNC)!=0 );
//     assert( !pagerUseWal(pPager) );

//     /* Write the data read from the journal back into the database file.
//     ** This is usually safe even for an encrypted database - as the data
//     ** was encrypted before it was written to the journal file. The exception
//     ** is if the data was just read from an in-memory sub-journal. In that
//     ** case it must be encrypted here before it is copied into the database
//     ** file.  */
//     rc = sqlite3OsWrite(pPager->fd, (u8 *)aData, pPager->pageSize, ofst);

//     if( pgno>pPager->dbFileSize ){
//       pPager->dbFileSize = pgno;
//     }
//     if( pPager->pBackup ){
//       sqlite3BackupUpdate(pPager->pBackup, pgno, (u8*)aData);
//     }
//   }else if( !isMainJrnl && pPg==0 ){
//     /* If this is a rollback of a savepoint and data was not written to
//     ** the database and the page is not in-memory, there is a potential
//     ** problem. When the page is next fetched by the b-tree layer, it 
//     ** will be read from the database file, which may or may not be 
//     ** current. 
//     **
//     ** There are a couple of different ways this can happen. All are quite
//     ** obscure. When running in synchronous mode, this can only happen 
//     ** if the page is on the free-list at the start of the transaction, then
//     ** populated, then moved using sqlite3PagerMovepage().
//     **
//     ** The solution is to add an in-memory page to the cache containing
//     ** the data just read from the sub-journal. Mark the page as dirty 
//     ** and if the pager requires a journal-sync, then mark the page as 
//     ** requiring a journal-sync before it is written.
//     */
//     assert( isSavepnt );
//     assert( (pPager->doNotSpill & SPILLFLAG_ROLLBACK)==0 );
//     pPager->doNotSpill |= SPILLFLAG_ROLLBACK;
//     rc = sqlite3PagerGet(pPager, pgno, &pPg, 1);
//     assert( (pPager->doNotSpill & SPILLFLAG_ROLLBACK)!=0 );
//     pPager->doNotSpill &= ~SPILLFLAG_ROLLBACK;
//     if( rc!=SQLITE_OK ) return rc;
//     sqlite3PcacheMakeDirty(pPg);
//   }
//   if( pPg ){
//     /* No page should ever be explicitly rolled back that is in use, except
//     ** for page 1 which is held in use in order to keep the lock on the
//     ** database active. However such a page may be rolled back as a result
//     ** of an internal error resulting in an automatic call to
//     ** sqlite3PagerRollback().
//     */
//     void *pData;
//     pData = pPg->pData;
//     memcpy(pData, (u8*)aData, pPager->pageSize);
//     pPager->xReiniter(pPg);
//     /* It used to be that sqlite3PcacheMakeClean(pPg) was called here.  But
//     ** that call was dangerous and had no detectable benefit since the cache
//     ** is normally cleaned by sqlite3PcacheCleanAll() after rollback and so
//     ** has been removed. */
//     pager_set_pagehash(pPg);

//     /* If this was page 1, then restore the value of Pager.dbFileVers.
//     ** Do this before any decoding. */
//     if( pgno==1 ){
//       memcpy(&pPager->dbFileVers, &((u8*)pData)[24],sizeof(pPager->dbFileVers));
//     }
//     sqlite3PcacheRelease(pPg);
//   }
//   return rc;
// }

// /*
// ** Parameter zSuper is the name of a super-journal file. A single journal
// ** file that referred to the super-journal file has just been rolled back.
// ** This routine checks if it is possible to delete the super-journal file,
// ** and does so if it is.
// **
// ** Argument zSuper may point to Pager.pTmpSpace. So that buffer is not 
// ** available for use within this function.
// **
// ** When a super-journal file is created, it is populated with the names 
// ** of all of its child journals, one after another, formatted as utf-8 
// ** encoded text. The end of each child journal file is marked with a 
// ** nul-terminator byte (0x00). i.e. the entire contents of a super-journal
// ** file for a transaction involving two databases might be:
// **
// **   "/home/bill/a.db-journal\x00/home/bill/b.db-journal\x00"
// **
// ** A super-journal file may only be deleted once all of its child 
// ** journals have been rolled back.
// **
// ** This function reads the contents of the super-journal file into 
// ** memory and loops through each of the child journal names. For
// ** each child journal, it checks if:
// **
// **   * if the child journal exists, and if so
// **   * if the child journal contains a reference to super-journal 
// **     file zSuper
// **
// ** If a child journal can be found that matches both of the criteria
// ** above, this function returns without doing anything. Otherwise, if
// ** no such child journal can be found, file zSuper is deleted from
// ** the file-system using sqlite3OsDelete().
// **
// ** If an IO error within this function, an error code is returned. This
// ** function allocates memory by calling sqlite3Malloc(). If an allocation
// ** fails, SQLITE_NOMEM is returned. Otherwise, if no IO or malloc errors 
// ** occur, SQLITE_OK is returned.
// **
// ** TODO: This function allocates a single block of memory to load
// ** the entire contents of the super-journal file. This could be
// ** a couple of kilobytes or so - potentially larger than the page 
// ** size.
// */
// static int pager_delsuper(Pager *pPager, const char *zSuper){
//   sqlite3_vfs *pVfs = pPager->pVfs;
//   int rc;                   /* Return code */
//   sqlite3_file *pSuper;     /* Malloc'd super-journal file descriptor */
//   sqlite3_file *pJournal;   /* Malloc'd child-journal file descriptor */
//   char *zSuperJournal = 0;  /* Contents of super-journal file */
//   i64 nSuperJournal;        /* Size of super-journal file */
//   char *zJournal;           /* Pointer to one journal within MJ file */
//   char *zSuperPtr;          /* Space to hold super-journal filename */
//   char *zFree = 0;          /* Free this buffer */
//   int nSuperPtr;            /* Amount of space allocated to zSuperPtr[] */

//   /* Allocate space for both the pJournal and pSuper file descriptors.
//   ** If successful, open the super-journal file for reading.
//   */
//   pSuper = (sqlite3_file *)sqlite3MallocZero(pVfs->szOsFile * 2);
//   if( !pSuper ){
//     rc = SQLITE_NOMEM;
//     pJournal = 0;
//   }else{
//     const int flags = (SQLITE_OPEN_READONLY|SQLITE_OPEN_SUPER_JOURNAL);
//     rc = sqlite3OsOpen(pVfs, zSuper, pSuper, flags, 0);
//     pJournal = (sqlite3_file *)(((u8 *)pSuper) + pVfs->szOsFile);
//   }
//   if( rc!=SQLITE_OK ) goto delsuper_out;

//   /* Load the entire super-journal file into space obtained from
//   ** sqlite3_malloc() and pointed to by zSuperJournal.   Also obtain
//   ** sufficient space (in zSuperPtr) to hold the names of super-journal
//   ** files extracted from regular rollback-journals.
//   */
//   rc = sqlite3OsFileSize(pSuper, &nSuperJournal);
//   if( rc!=SQLITE_OK ) goto delsuper_out;
//   nSuperPtr = pVfs->mxPathname+1;
//   zFree = sqlite3Malloc(4 + nSuperJournal + nSuperPtr + 2);
//   if( !zFree ){
//     rc = SQLITE_NOMEM;
//     goto delsuper_out;
//   }
//   zFree[0] = zFree[1] = zFree[2] = zFree[3] = 0;
//   zSuperJournal = &zFree[4];
//   zSuperPtr = &zSuperJournal[nSuperJournal+2];
//   rc = sqlite3OsRead(pSuper, zSuperJournal, (int)nSuperJournal, 0);
//   if( rc!=SQLITE_OK ) goto delsuper_out;
//   zSuperJournal[nSuperJournal] = 0;
//   zSuperJournal[nSuperJournal+1] = 0;

//   zJournal = zSuperJournal;
//   while( (zJournal-zSuperJournal)<nSuperJournal ){
//     int exists;
//     rc = sqlite3OsAccess(pVfs, zJournal, SQLITE_ACCESS_EXISTS, &exists);
//     if( rc!=SQLITE_OK ){
//       goto delsuper_out;
//     }
//     if( exists ){
//       /* One of the journals pointed to by the super-journal exists.
//       ** Open it and check if it points at the super-journal. If
//       ** so, return without deleting the super-journal file.
//       ** NB:  zJournal is really a MAIN_JOURNAL.  But call it a 
//       ** SUPER_JOURNAL here so that the VFS will not send the zJournal
//       ** name into sqlite3_database_file_object().
//       */
//       int c;
//       int flags = (SQLITE_OPEN_READONLY|SQLITE_OPEN_SUPER_JOURNAL);
//       rc = sqlite3OsOpen(pVfs, zJournal, pJournal, flags, 0);
//       if( rc!=SQLITE_OK ){
//         goto delsuper_out;
//       }

//       rc = readSuperJournal(pJournal, zSuperPtr, nSuperPtr);
//       sqlite3OsClose(pJournal);
//       if( rc!=SQLITE_OK ){
//         goto delsuper_out;
//       }

//       c = zSuperPtr[0]!=0 && strcmp(zSuperPtr, zSuper)==0;
//       if( c ){
//         /* We have a match. Do not delete the super-journal file. */
//         goto delsuper_out;
//       }
//     }
//     zJournal += (sqlite3Strlen30(zJournal)+1);
//   }
 
//   sqlite3OsClose(pSuper);
//   rc = sqlite3OsDelete(pVfs, zSuper, 0);

// delsuper_out:
//   sqlite3_free(zFree);
//   if( pSuper ){
//     sqlite3OsClose(pSuper);
//     assert( !isOpen(pJournal) );
//     sqlite3_free(pSuper);
//   }
//   return rc;
// }


// /*
// ** This function is used to change the actual size of the database 
// ** file in the file-system. This only happens when committing a transaction,
// ** or rolling back a transaction (including rolling back a hot-journal).
// **
// ** If the main database file is not open, or the pager is not in either
// ** DBMOD or OPEN state, this function is a no-op. Otherwise, the size 
// ** of the file is changed to nPage pages (nPage*pPager->pageSize bytes). 
// ** If the file on disk is currently larger than nPage pages, then use the VFS
// ** xTruncate() method to truncate it.
// **
// ** Or, it might be the case that the file on disk is smaller than 
// ** nPage pages. Some operating system implementations can get confused if 
// ** you try to truncate a file to some size that is larger than it 
// ** currently is, so detect this case and write a single zero byte to 
// ** the end of the new file instead.
// **
// ** If successful, return SQLITE_OK. If an IO error occurs while modifying
// ** the database file, return the error code to the caller.
// */
// static int pager_truncate(Pager *pPager, Pgno nPage){
//   int rc = SQLITE_OK;
//   assert( pPager->eState!=PAGER_ERROR );
//   assert( pPager->eState!=PAGER_READER );
  
//   if( isOpen(pPager->fd) 
//    && (pPager->eState>=PAGER_WRITER_DBMOD || pPager->eState==PAGER_OPEN) 
//   ){
//     i64 currentSize, newSize;
//     int szPage = pPager->pageSize;
//     assert( pPager->eLock==EXCLUSIVE_LOCK );
//     /* TODO: Is it safe to use Pager.dbFileSize here? */
//     rc = sqlite3OsFileSize(pPager->fd, &currentSize);
//     newSize = szPage*(i64)nPage;
//     if( rc==SQLITE_OK && currentSize!=newSize ){
//       if( currentSize>newSize ){
//         rc = sqlite3OsTruncate(pPager->fd, newSize);
//       }else if( (currentSize+szPage)<=newSize ){
//         char *pTmp = pPager->pTmpSpace;
//         memset(pTmp, 0, szPage);
//         testcase( (newSize-szPage) == currentSize );
//         testcase( (newSize-szPage) >  currentSize );
//         rc = sqlite3OsWrite(pPager->fd, pTmp, szPage, newSize-szPage);
//       }
//       if( rc==SQLITE_OK ){
//         pPager->dbFileSize = nPage;
//       }
//     }
//   }
//   return rc;
// }

// /*
// ** Return a sanitized version of the sector-size of OS file pFile. The
// ** return value is guaranteed to lie between 32 and MAX_SECTOR_SIZE.
// */
// int sqlite3SectorSize(sqlite3_file *pFile){
//   int iRet = sqlite3OsSectorSize(pFile);
//   if( iRet<32 ){
//     iRet = 512;
//   }else if( iRet>MAX_SECTOR_SIZE ){
//     assert( MAX_SECTOR_SIZE>=512 );
//     iRet = MAX_SECTOR_SIZE;
//   }
//   return iRet;
// }

// /*
// ** Set the value of the Pager.sectorSize variable for the given
// ** pager based on the value returned by the xSectorSize method
// ** of the open database file. The sector size will be used 
// ** to determine the size and alignment of journal header and 
// ** super-journal pointers within created journal files.
// **
// ** For temporary files the effective sector size is always 512 bytes.
// **
// ** Otherwise, for non-temporary files, the effective sector size is
// ** the value returned by the xSectorSize() method rounded up to 32 if
// ** it is less than 32, or rounded down to MAX_SECTOR_SIZE if it
// ** is greater than MAX_SECTOR_SIZE.
// **
// ** If the file has the SQLITE_IOCAP_POWERSAFE_OVERWRITE property, then set
// ** the effective sector size to its minimum value (512).  The purpose of
// ** pPager->sectorSize is to define the "blast radius" of bytes that
// ** might change if a crash occurs while writing to a single byte in
// ** that range.  But with POWERSAFE_OVERWRITE, the blast radius is zero
// ** (that is what POWERSAFE_OVERWRITE means), so we minimize the sector
// ** size.  For backwards compatibility of the rollback journal file format,
// ** we cannot reduce the effective sector size below 512.
// */
// static void setSectorSize(Pager *pPager){
//   assert( isOpen(pPager->fd) || pPager->tempFile );

//   if( pPager->tempFile
//    || (sqlite3OsDeviceCharacteristics(pPager->fd) & 
//               SQLITE_IOCAP_POWERSAFE_OVERWRITE)!=0
//   ){
//     /* Sector size doesn't matter for temporary files. Also, the file
//     ** may not have been opened yet, in which case the OsSectorSize()
//     ** call will segfault. */
//     pPager->sectorSize = 512;
//   }else{
//     pPager->sectorSize = sqlite3SectorSize(pPager->fd);
//   }
// }

// /*
// ** Playback the journal and thus restore the database file to
// ** the state it was in before we started making changes.  
// **
// ** The journal file format is as follows: 
// **
// **  (1)  8 byte prefix.  A copy of aJournalMagic[].
// **  (2)  4 byte big-endian integer which is the number of valid page records
// **       in the journal.  If this value is 0xffffffff, then compute the
// **       number of page records from the journal size.
// **  (3)  4 byte big-endian integer which is the initial value for the 
// **       sanity checksum.
// **  (4)  4 byte integer which is the number of pages to truncate the
// **       database to during a rollback.
// **  (5)  4 byte big-endian integer which is the sector size.  The header
// **       is this many bytes in size.
// **  (6)  4 byte big-endian integer which is the page size.
// **  (7)  zero padding out to the next sector size.
// **  (8)  Zero or more pages instances, each as follows:
// **        +  4 byte page number.
// **        +  pPager->pageSize bytes of data.
// **        +  4 byte checksum
// **
// ** When we speak of the journal header, we mean the first 7 items above.
// ** Each entry in the journal is an instance of the 8th item.
// **
// ** Call the value from the second bullet "nRec".  nRec is the number of
// ** valid page entries in the journal.  In most cases, you can compute the
// ** value of nRec from the size of the journal file.  But if a power
// ** failure occurred while the journal was being written, it could be the
// ** case that the size of the journal file had already been increased but
// ** the extra entries had not yet made it safely to disk.  In such a case,
// ** the value of nRec computed from the file size would be too large.  For
// ** that reason, we always use the nRec value in the header.
// **
// ** If the nRec value is 0xffffffff it means that nRec should be computed
// ** from the file size.  This value is used when the user selects the
// ** no-sync option for the journal.  A power failure could lead to corruption
// ** in this case.  But for things like temporary table (which will be
// ** deleted when the power is restored) we don't care.  
// **
// ** If the file opened as the journal file is not a well-formed
// ** journal file then all pages up to the first corrupted page are rolled
// ** back (or no pages if the journal header is corrupted). The journal file
// ** is then deleted and SQLITE_OK returned, just as if no corruption had
// ** been encountered.
// **
// ** If an I/O or malloc() error occurs, the journal-file is not deleted
// ** and an error code is returned.
// **
// ** The isHot parameter indicates that we are trying to rollback a journal
// ** that might be a hot journal.  Or, it could be that the journal is 
// ** preserved because of JOURNALMODE_PERSIST or JOURNALMODE_TRUNCATE.
// ** If the journal really is hot, reset the pager cache prior rolling
// ** back any content.  If the journal is merely persistent, no reset is
// ** needed.
// */
// static int pager_playback(Pager *pPager, int isHot){
//   sqlite3_vfs *pVfs = pPager->pVfs;
//   i64 szJ;                 /* Size of the journal file in bytes */
//   u32 nRec;                /* Number of Records in the journal */
//   u32 u;                   /* Unsigned loop counter */
//   Pgno mxPg = 0;           /* Size of the original file in pages */
//   int rc;                  /* Result code of a subroutine */
//   int res = 1;             /* Value returned by sqlite3OsAccess() */
//   char *zSuper = 0;        /* Name of super-journal file if any */
//   int needPagerReset;      /* True to reset page prior to first page rollback */
//   int nPlayback = 0;       /* Total number of pages restored from journal */
//   u32 savedPageSize = pPager->pageSize;

//   /* Figure out how many records are in the journal.  Abort early if
//   ** the journal is empty.
//   */
//   assert( isOpen(pPager->jfd) );
//   rc = sqlite3OsFileSize(pPager->jfd, &szJ);
//   if( rc!=SQLITE_OK ){
//     goto end_playback;
//   }

//   /* Read the super-journal name from the journal, if it is present.
//   ** If a super-journal file name is specified, but the file is not
//   ** present on disk, then the journal is not hot and does not need to be
//   ** played back.
//   **
//   ** TODO: Technically the following is an error because it assumes that
//   ** buffer Pager.pTmpSpace is (mxPathname+1) bytes or larger. i.e. that
//   ** (pPager->pageSize >= pPager->pVfs->mxPathname+1). Using os_unix.c,
//   ** mxPathname is 512, which is the same as the minimum allowable value
//   ** for pageSize.
//   */
//   zSuper = pPager->pTmpSpace;
//   rc = readSuperJournal(pPager->jfd, zSuper, pPager->pVfs->mxPathname+1);
//   if( rc==SQLITE_OK && zSuper[0] ){
//     rc = sqlite3OsAccess(pVfs, zSuper, SQLITE_ACCESS_EXISTS, &res);
//   }
//   zSuper = 0;
//   if( rc!=SQLITE_OK || !res ){
//     goto end_playback;
//   }
//   pPager->journalOff = 0;
//   needPagerReset = isHot;

//   /* This loop terminates either when a readJournalHdr() or 
//   ** pager_playback_one_page() call returns SQLITE_DONE or an IO error 
//   ** occurs. 
//   */
//   while( 1 ){
//     /* Read the next journal header from the journal file.  If there are
//     ** not enough bytes left in the journal file for a complete header, or
//     ** it is corrupted, then a process must have failed while writing it.
//     ** This indicates nothing more needs to be rolled back.
//     */
//     rc = readJournalHdr(pPager, isHot, szJ, &nRec, &mxPg);
//     if( rc!=SQLITE_OK ){ 
//       if( rc==SQLITE_DONE ){
//         rc = SQLITE_OK;
//       }
//       goto end_playback;
//     }

//     /* If nRec is 0xffffffff, then this journal was created by a process
//     ** working in no-sync mode. This means that the rest of the journal
//     ** file consists of pages, there are no more journal headers. Compute
//     ** the value of nRec based on this assumption.
//     */
//     if( nRec==0xffffffff ){
//       assert( pPager->journalOff==JOURNAL_HDR_SZ(pPager) );
//       nRec = (int)((szJ - JOURNAL_HDR_SZ(pPager))/JOURNAL_PG_SZ(pPager));
//     }

//     /* If nRec is 0 and this rollback is of a transaction created by this
//     ** process and if this is the final header in the journal, then it means
//     ** that this part of the journal was being filled but has not yet been
//     ** synced to disk.  Compute the number of pages based on the remaining
//     ** size of the file.
//     **
//     ** The third term of the test was added to fix ticket #2565.
//     ** When rolling back a hot journal, nRec==0 always means that the next
//     ** chunk of the journal contains zero pages to be rolled back.  But
//     ** when doing a ROLLBACK and the nRec==0 chunk is the last chunk in
//     ** the journal, it means that the journal might contain additional
//     ** pages that need to be rolled back and that the number of pages 
//     ** should be computed based on the journal file size.
//     */
//     if( nRec==0 && !isHot &&
//         pPager->journalHdr+JOURNAL_HDR_SZ(pPager)==pPager->journalOff ){
//       nRec = (int)((szJ - pPager->journalOff) / JOURNAL_PG_SZ(pPager));
//     }

//     /* If this is the first header read from the journal, truncate the
//     ** database file back to its original size.
//     */
//     if( pPager->journalOff==JOURNAL_HDR_SZ(pPager) ){
//       rc = pager_truncate(pPager, mxPg);
//       if( rc!=SQLITE_OK ){
//         goto end_playback;
//       }
//       pPager->dbSize = mxPg;
//     }

//     /* Copy original pages out of the journal and back into the 
//     ** database file and/or page cache.
//     */
//     for(u=0; u<nRec; u++){
//       if( needPagerReset ){
//         pager_reset(pPager);
//         needPagerReset = 0;
//       }
//       rc = pager_playback_one_page(pPager,&pPager->journalOff,0,1,0);
//       if( rc==SQLITE_OK ){
//         nPlayback++;
//       }else{
//         if( rc==SQLITE_DONE ){
//           pPager->journalOff = szJ;
//           break;
//         }else if( rc==SQLITE_IOERR_SHORT_READ ){
//           /* If the journal has been truncated, simply stop reading and
//           ** processing the journal. This might happen if the journal was
//           ** not completely written and synced prior to a crash.  In that
//           ** case, the database should have never been written in the
//           ** first place so it is OK to simply abandon the rollback. */
//           rc = SQLITE_OK;
//           goto end_playback;
//         }else{
//           /* If we are unable to rollback, quit and return the error
//           ** code.  This will cause the pager to enter the error state
//           ** so that no further harm will be done.  Perhaps the next
//           ** process to come along will be able to rollback the database.
//           */
//           goto end_playback;
//         }
//       }
//     }
//   }
//   /*NOTREACHED*/
//   assert( 0 );

// end_playback:
//   if( rc==SQLITE_OK ){
//     rc = sqlite3PagerSetPagesize(pPager, &savedPageSize, -1);
//   }
//   /* Following a rollback, the database file should be back in its original
//   ** state prior to the start of the transaction, so invoke the
//   ** SQLITE_FCNTL_DB_UNCHANGED file-control method to disable the
//   ** assertion that the transaction counter was modified.
//   */
// #ifdef SQLITE_DEBUG
//   sqlite3OsFileControlHint(pPager->fd,SQLITE_FCNTL_DB_UNCHANGED,0);
// #endif

//   /* If this playback is happening automatically as a result of an IO or 
//   ** malloc error that occurred after the change-counter was updated but 
//   ** before the transaction was committed, then the change-counter 
//   ** modification may just have been reverted. If this happens in exclusive 
//   ** mode, then subsequent transactions performed by the connection will not
//   ** update the change-counter at all. This may lead to cache inconsistency
//   ** problems for other processes at some point in the future. So, just
//   ** in case this has happened, clear the changeCountDone flag now.
//   */
//   pPager->changeCountDone = pPager->tempFile;

//   if( rc==SQLITE_OK ){
//     /* Leave 4 bytes of space before the super-journal filename in memory.
//     ** This is because it may end up being passed to sqlite3OsOpen(), in
//     ** which case it requires 4 0x00 bytes in memory immediately before
//     ** the filename. */
//     zSuper = &pPager->pTmpSpace[4];
//     rc = readSuperJournal(pPager->jfd, zSuper, pPager->pVfs->mxPathname+1);
//     testcase( rc!=SQLITE_OK );
//   }
//   if( rc==SQLITE_OK
//    && (pPager->eState>=PAGER_WRITER_DBMOD || pPager->eState==PAGER_OPEN)
//   ){
//     rc = sqlite3PagerSync(pPager, 0);
//   }
//   if( rc==SQLITE_OK ){
//     rc = pager_end_transaction(pPager, zSuper[0]!='\0', 0);
//     testcase( rc!=SQLITE_OK );
//   }
//   if( rc==SQLITE_OK && zSuper[0] && res ){
//     /* If there was a super-journal and this routine will return success,
//     ** see if it is possible to delete the super-journal.
//     */
//     assert( zSuper==&pPager->pTmpSpace[4] );
//     memset(&zSuper[-4], 0, 4);
//     rc = pager_delsuper(pPager, zSuper);
//     testcase( rc!=SQLITE_OK );
//   }
//   if( isHot && nPlayback ){
//     sqlite3_log(SQLITE_NOTICE_RECOVER_ROLLBACK, "recovered %d pages from %s",
//                 nPlayback, pPager->zJournal);
//   }

//   /* The Pager.sectorSize variable may have been updated while rolling
//   ** back a journal created by a process with a different sector size
//   ** value. Reset it to the correct value for this process.
//   */
//   setSectorSize(pPager);
//   return rc;
// }


// /*
// ** Read the content for page pPg out of the database file (or out of
// ** the WAL if that is where the most recent copy if found) into 
// ** pPg->pData. A shared lock or greater must be held on the database
// ** file before this function is called.
// **
// ** If page 1 is read, then the value of Pager.dbFileVers[] is set to
// ** the value read from the database file.
// **
// ** If an IO error occurs, then the IO error is returned to the caller.
// ** Otherwise, SQLITE_OK is returned.
// */
// static int readDbPage(PgHdr *pPg){
//   Pager *pPager = pPg->pPager; /* Pager object associated with page pPg */
//   int rc = SQLITE_OK;          /* Return code */

// #ifndef SQLITE_OMIT_WAL
//   u32 iFrame = 0;              /* Frame of WAL containing pgno */

//   assert( pPager->eState>=PAGER_READER && !MEMDB );
//   assert( isOpen(pPager->fd) );

//   if( pagerUseWal(pPager) ){
//     rc = sqlite3WalFindFrame(pPager->pWal, pPg->pgno, &iFrame);
//     if( rc ) return rc;
//   }
//   if( iFrame ){
//     rc = sqlite3WalReadFrame(pPager->pWal, iFrame,pPager->pageSize,pPg->pData);
//   }else
// #endif
//   {
//     i64 iOffset = (pPg->pgno-1)*(i64)pPager->pageSize;
//     rc = sqlite3OsRead(pPager->fd, pPg->pData, pPager->pageSize, iOffset);
//     if( rc==SQLITE_IOERR_SHORT_READ ){
//       rc = SQLITE_OK;
//     }
//   }

//   if( pPg->pgno==1 ){
//     if( rc ){
//       /* If the read is unsuccessful, set the dbFileVers[] to something
//       ** that will never be a valid file version.  dbFileVers[] is a copy
//       ** of bytes 24..39 of the database.  Bytes 28..31 should always be
//       ** zero or the size of the database in page. Bytes 32..35 and 35..39
//       ** should be page numbers which are never 0xffffffff.  So filling
//       ** pPager->dbFileVers[] with all 0xff bytes should suffice.
//       **
//       ** For an encrypted database, the situation is more complex:  bytes
//       ** 24..39 of the database are white noise.  But the probability of
//       ** white noise equaling 16 bytes of 0xff is vanishingly small so
//       ** we should still be ok.
//       */
//       memset(pPager->dbFileVers, 0xff, sizeof(pPager->dbFileVers));
//     }else{
//       u8 *dbFileVers = &((u8*)pPg->pData)[24];
//       memcpy(&pPager->dbFileVers, dbFileVers, sizeof(pPager->dbFileVers));
//     }
//   }
//   PAGER_INCR(sqlite3_pager_readdb_count);
//   PAGER_INCR(pPager->nRead);
//   IOTRACE(("PGIN %p %d\n", pPager, pPg->pgno));
//   PAGERTRACE(("FETCH %d page %d hash(%08x)\n",
//                PAGERID(pPager), pPg->pgno, pager_pagehash(pPg)));

//   return rc;
// }

// /*
// ** Update the value of the change-counter at offsets 24 and 92 in
// ** the header and the sqlite version number at offset 96.
// **
// ** This is an unconditional update.  See also the pager_incr_changecounter()
// ** routine which only updates the change-counter if the update is actually
// ** needed, as determined by the pPager->changeCountDone state variable.
// */
// static void pager_write_changecounter(PgHdr *pPg){
//   u32 change_counter;
//   if( NEVER(pPg==0) ) return;

//   /* Increment the value just read and write it back to byte 24. */
//   change_counter = sqlite3Get4byte((u8*)pPg->pPager->dbFileVers)+1;
//   put32bits(((char*)pPg->pData)+24, change_counter);

//   /* Also store the SQLite version number in bytes 96..99 and in
//   ** bytes 92..95 store the change counter for which the version number
//   ** is valid. */
//   put32bits(((char*)pPg->pData)+92, change_counter);
//   put32bits(((char*)pPg->pData)+96, SQLITE_VERSION_NUMBER);
// }

// #ifndef SQLITE_OMIT_WAL
// /*
// ** This function is invoked once for each page that has already been 
// ** written into the log file when a WAL transaction is rolled back.
// ** Parameter iPg is the page number of said page. The pCtx argument 
// ** is actually a pointer to the Pager structure.
// **
// ** If page iPg is present in the cache, and has no outstanding references,
// ** it is discarded. Otherwise, if there are one or more outstanding
// ** references, the page content is reloaded from the database. If the
// ** attempt to reload content from the database is required and fails, 
// ** return an SQLite error code. Otherwise, SQLITE_OK.
// */
// static int pagerUndoCallback(void *pCtx, Pgno iPg){
//   int rc = SQLITE_OK;
//   Pager *pPager = (Pager *)pCtx;
//   PgHdr *pPg;

//   assert( pagerUseWal(pPager) );
//   pPg = sqlite3PagerLookup(pPager, iPg);
//   if( pPg ){
//     if( sqlite3PcachePageRefcount(pPg)==1 ){
//       sqlite3PcacheDrop(pPg);
//     }else{
//       rc = readDbPage(pPg);
//       if( rc==SQLITE_OK ){
//         pPager->xReiniter(pPg);
//       }
//       sqlite3PagerUnrefNotNull(pPg);
//     }
//   }

//   /* Normally, if a transaction is rolled back, any backup processes are
//   ** updated as data is copied out of the rollback journal and into the
//   ** database. This is not generally possible with a WAL database, as
//   ** rollback involves simply truncating the log file. Therefore, if one
//   ** or more frames have already been written to the log (and therefore 
//   ** also copied into the backup databases) as part of this transaction,
//   ** the backups must be restarted.
//   */
//   sqlite3BackupRestart(pPager->pBackup);

//   return rc;
// }

// /*
// ** This function is called to rollback a transaction on a WAL database.
// */
// static int pagerRollbackWal(Pager *pPager){
//   int rc;                         /* Return Code */
//   PgHdr *pList;                   /* List of dirty pages to revert */

//   /* For all pages in the cache that are currently dirty or have already
//   ** been written (but not committed) to the log file, do one of the 
//   ** following:
//   **
//   **   + Discard the cached page (if refcount==0), or
//   **   + Reload page content from the database (if refcount>0).
//   */
//   pPager->dbSize = pPager->dbOrigSize;
//   rc = sqlite3WalUndo(pPager->pWal, pagerUndoCallback, (void *)pPager);
//   pList = sqlite3PcacheDirtyList(pPager->pPCache);
//   while( pList && rc==SQLITE_OK ){
//     PgHdr *pNext = pList->pDirty;
//     rc = pagerUndoCallback((void *)pPager, pList->pgno);
//     pList = pNext;
//   }

//   return rc;
// }

// /*
// ** This function is a wrapper around sqlite3WalFrames(). As well as logging
// ** the contents of the list of pages headed by pList (connected by pDirty),
// ** this function notifies any active backup processes that the pages have
// ** changed. 
// **
// ** The list of pages passed into this routine is always sorted by page number.
// ** Hence, if page 1 appears anywhere on the list, it will be the first page.
// */ 
// static int pagerWalFrames(
//   Pager *pPager,                  /* Pager object */
//   PgHdr *pList,                   /* List of frames to log */
//   Pgno nTruncate,                 /* Database size after this commit */
//   int isCommit                    /* True if this is a commit */
// ){
//   int rc;                         /* Return code */
//   int nList;                      /* Number of pages in pList */
//   PgHdr *p;                       /* For looping over pages */

//   assert( pPager->pWal );
//   assert( pList );
// #ifdef SQLITE_DEBUG
//   /* Verify that the page list is in accending order */
//   for(p=pList; p && p->pDirty; p=p->pDirty){
//     assert( p->pgno < p->pDirty->pgno );
//   }
// #endif

//   assert( pList->pDirty==0 || isCommit );
//   if( isCommit ){
//     /* If a WAL transaction is being committed, there is no point in writing
//     ** any pages with page numbers greater than nTruncate into the WAL file.
//     ** They will never be read by any client. So remove them from the pDirty
//     ** list here. */
//     PgHdr **ppNext = &pList;
//     nList = 0;
//     for(p=pList; (*ppNext = p)!=0; p=p->pDirty){
//       if( p->pgno<=nTruncate ){
//         ppNext = &p->pDirty;
//         nList++;
//       }
//     }
//     assert( pList );
//   }else{
//     nList = 1;
//   }
//   pPager->aStat[PAGER_STAT_WRITE] += nList;

//   if( pList->pgno==1 ) pager_write_changecounter(pList);
//   rc = sqlite3WalFrames(pPager->pWal, 
//       pPager->pageSize, pList, nTruncate, isCommit, pPager->walSyncFlags
//   );
//   if( rc==SQLITE_OK && pPager->pBackup ){
//     for(p=pList; p; p=p->pDirty){
//       sqlite3BackupUpdate(pPager->pBackup, p->pgno, (u8 *)p->pData);
//     }
//   }

// #ifdef SQLITE_CHECK_PAGES
//   pList = sqlite3PcacheDirtyList(pPager->pPCache);
//   for(p=pList; p; p=p->pDirty){
//     pager_set_pagehash(p);
//   }
// #endif

//   return rc;
// }

// /*
// ** Begin a read transaction on the WAL.
// **
// ** This routine used to be called "pagerOpenSnapshot()" because it essentially
// ** makes a snapshot of the database at the current point in time and preserves
// ** that snapshot for use by the reader in spite of concurrently changes by
// ** other writers or checkpointers.
// */
// static int pagerBeginReadTransaction(Pager *pPager){
//   int rc;                         /* Return code */
//   int changed = 0;                /* True if cache must be reset */

//   assert( pagerUseWal(pPager) );
//   assert( pPager->eState==PAGER_OPEN || pPager->eState==PAGER_READER );

//   /* sqlite3WalEndReadTransaction() was not called for the previous
//   ** transaction in locking_mode=EXCLUSIVE.  So call it now.  If we
//   ** are in locking_mode=NORMAL and EndRead() was previously called,
//   ** the duplicate call is harmless.
//   */
//   sqlite3WalEndReadTransaction(pPager->pWal);

//   rc = sqlite3WalBeginReadTransaction(pPager->pWal, &changed);
//   if( rc!=SQLITE_OK || changed ){
//     pager_reset(pPager);
//     if( USEFETCH(pPager) ) sqlite3OsUnfetch(pPager->fd, 0, 0);
//   }

//   return rc;
// }
// #endif

// /*
// ** This function is called as part of the transition from PAGER_OPEN
// ** to PAGER_READER state to determine the size of the database file
// ** in pages (assuming the page size currently stored in Pager.pageSize).
// **
// ** If no error occurs, SQLITE_OK is returned and the size of the database
// ** in pages is stored in *pnPage. Otherwise, an error code (perhaps
// ** SQLITE_IOERR_FSTAT) is returned and *pnPage is left unmodified.
// */
// static int pagerPagecount(Pager *pPager, Pgno *pnPage){
//   Pgno nPage;                     /* Value to return via *pnPage */

//   /* Query the WAL sub-system for the database size. The WalDbsize()
//   ** function returns zero if the WAL is not open (i.e. Pager.pWal==0), or
//   ** if the database size is not available. The database size is not
//   ** available from the WAL sub-system if the log file is empty or
//   ** contains no valid committed transactions.
//   */
//   assert( pPager->eState==PAGER_OPEN );
//   assert( pPager->eLock>=SHARED_LOCK );
//   assert( isOpen(pPager->fd) );
//   assert( pPager->tempFile==0 );
//   nPage = sqlite3WalDbsize(pPager->pWal);

//   /* If the number of pages in the database is not available from the
//   ** WAL sub-system, determine the page count based on the size of
//   ** the database file.  If the size of the database file is not an
//   ** integer multiple of the page-size, round up the result.
//   */
//   if( nPage==0 && ALWAYS(isOpen(pPager->fd)) ){
//     i64 n = 0;                    /* Size of db file in bytes */
//     int rc = sqlite3OsFileSize(pPager->fd, &n);
//     if( rc!=SQLITE_OK ){
//       return rc;
//     }
//     nPage = (Pgno)((n+pPager->pageSize-1) / pPager->pageSize);
//   }

//   /* If the current number of pages in the file is greater than the
//   ** configured maximum pager number, increase the allowed limit so
//   ** that the file can be read.
//   */
//   if( nPage>pPager->mxPgno ){
//     pPager->mxPgno = (Pgno)nPage;
//   }

//   *pnPage = nPage;
//   return SQLITE_OK;
// }

// #ifndef SQLITE_OMIT_WAL
// /*
// ** Check if the *-wal file that corresponds to the database opened by pPager
// ** exists if the database is not empy, or verify that the *-wal file does
// ** not exist (by deleting it) if the database file is empty.
// **
// ** If the database is not empty and the *-wal file exists, open the pager
// ** in WAL mode.  If the database is empty or if no *-wal file exists and
// ** if no error occurs, make sure Pager.journalMode is not set to
// ** PAGER_JOURNALMODE_WAL.
// **
// ** Return SQLITE_OK or an error code.
// **
// ** The caller must hold a SHARED lock on the database file to call this
// ** function. Because an EXCLUSIVE lock on the db file is required to delete 
// ** a WAL on a none-empty database, this ensures there is no race condition 
// ** between the xAccess() below and an xDelete() being executed by some 
// ** other connection.
// */
// static int pagerOpenWalIfPresent(Pager *pPager){
//   int rc = SQLITE_OK;
//   assert( pPager->eState==PAGER_OPEN );
//   assert( pPager->eLock>=SHARED_LOCK );

//   if( !pPager->tempFile ){
//     int isWal;                    /* True if WAL file exists */
//     rc = sqlite3OsAccess(
//         pPager->pVfs, pPager->zWal, SQLITE_ACCESS_EXISTS, &isWal
//     );
//     if( rc==SQLITE_OK ){
//       if( isWal ){
//         Pgno nPage;                   /* Size of the database file */

//         rc = pagerPagecount(pPager, &nPage);
//         if( rc ) return rc;
//         if( nPage==0 ){
//           rc = sqlite3OsDelete(pPager->pVfs, pPager->zWal, 0);
//         }else{
//           testcase( sqlite3PcachePagecount(pPager->pPCache)==0 );
//           rc = sqlite3PagerOpenWal(pPager, 0);
//         }
//       }else if( pPager->journalMode==PAGER_JOURNALMODE_WAL ){
//         pPager->journalMode = PAGER_JOURNALMODE_DELETE;
//       }
//     }
//   }
//   return rc;
// }
// #endif

// /*
// ** Playback savepoint pSavepoint. Or, if pSavepoint==NULL, then playback
// ** the entire super-journal file. The case pSavepoint==NULL occurs when 
// ** a ROLLBACK TO command is invoked on a SAVEPOINT that is a transaction 
// ** savepoint.
// **
// ** When pSavepoint is not NULL (meaning a non-transaction savepoint is 
// ** being rolled back), then the rollback consists of up to three stages,
// ** performed in the order specified:
// **
// **   * Pages are played back from the main journal starting at byte
// **     offset PagerSavepoint.iOffset and continuing to 
// **     PagerSavepoint.iHdrOffset, or to the end of the main journal
// **     file if PagerSavepoint.iHdrOffset is zero.
// **
// **   * If PagerSavepoint.iHdrOffset is not zero, then pages are played
// **     back starting from the journal header immediately following 
// **     PagerSavepoint.iHdrOffset to the end of the main journal file.
// **
// **   * Pages are then played back from the sub-journal file, starting
// **     with the PagerSavepoint.iSubRec and continuing to the end of
// **     the journal file.
// **
// ** Throughout the rollback process, each time a page is rolled back, the
// ** corresponding bit is set in a bitvec structure (variable pDone in the
// ** implementation below). This is used to ensure that a page is only
// ** rolled back the first time it is encountered in either journal.
// **
// ** If pSavepoint is NULL, then pages are only played back from the main
// ** journal file. There is no need for a bitvec in this case.
// **
// ** In either case, before playback commences the Pager.dbSize variable
// ** is reset to the value that it held at the start of the savepoint 
// ** (or transaction). No page with a page-number greater than this value
// ** is played back. If one is encountered it is simply skipped.
// */
// static int pagerPlaybackSavepoint(Pager *pPager, PagerSavepoint *pSavepoint){
//   i64 szJ;                 /* Effective size of the main journal */
//   i64 iHdrOff;             /* End of first segment of main-journal records */
//   int rc = SQLITE_OK;      /* Return code */
//   Bitvec *pDone = 0;       /* Bitvec to ensure pages played back only once */

//   assert( pPager->eState!=PAGER_ERROR );
//   assert( pPager->eState>=PAGER_WRITER_LOCKED );

//   /* Allocate a bitvec to use to store the set of pages rolled back */
//   if( pSavepoint ){
//     pDone = sqlite3BitvecCreate(pSavepoint->nOrig);
//     if( !pDone ){
//       return SQLITE_NOMEM;
//     }
//   }

//   /* Set the database size back to the value it was before the savepoint 
//   ** being reverted was opened.
//   */
//   pPager->dbSize = pSavepoint ? pSavepoint->nOrig : pPager->dbOrigSize;
//   pPager->changeCountDone = pPager->tempFile;

//   if( !pSavepoint && pagerUseWal(pPager) ){
//     return pagerRollbackWal(pPager);
//   }

//   /* Use pPager->journalOff as the effective size of the main rollback
//   ** journal.  The actual file might be larger than this in
//   ** PAGER_JOURNALMODE_TRUNCATE or PAGER_JOURNALMODE_PERSIST.  But anything
//   ** past pPager->journalOff is off-limits to us.
//   */
//   szJ = pPager->journalOff;
//   assert( pagerUseWal(pPager)==0 || szJ==0 );

//   /* Begin by rolling back records from the main journal starting at
//   ** PagerSavepoint.iOffset and continuing to the next journal header.
//   ** There might be records in the main journal that have a page number
//   ** greater than the current database size (pPager->dbSize) but those
//   ** will be skipped automatically.  Pages are added to pDone as they
//   ** are played back.
//   */
//   if( pSavepoint && !pagerUseWal(pPager) ){
//     iHdrOff = pSavepoint->iHdrOffset ? pSavepoint->iHdrOffset : szJ;
//     pPager->journalOff = pSavepoint->iOffset;
//     while( rc==SQLITE_OK && pPager->journalOff<iHdrOff ){
//       rc = pager_playback_one_page(pPager, &pPager->journalOff, pDone, 1, 1);
//     }
//     assert( rc!=SQLITE_DONE );
//   }else{
//     pPager->journalOff = 0;
//   }

//   /* Continue rolling back records out of the main journal starting at
//   ** the first journal header seen and continuing until the effective end
//   ** of the main journal file.  Continue to skip out-of-range pages and
//   ** continue adding pages rolled back to pDone.
//   */
//   while( rc==SQLITE_OK && pPager->journalOff<szJ ){
//     u32 ii;            /* Loop counter */
//     u32 nJRec = 0;     /* Number of Journal Records */
//     u32 dummy;
//     rc = readJournalHdr(pPager, 0, szJ, &nJRec, &dummy);
//     assert( rc!=SQLITE_DONE );

//     /*
//     ** The "pPager->journalHdr+JOURNAL_HDR_SZ(pPager)==pPager->journalOff"
//     ** test is related to ticket #2565.  See the discussion in the
//     ** pager_playback() function for additional information.
//     */
//     if( nJRec==0 
//      && pPager->journalHdr+JOURNAL_HDR_SZ(pPager)==pPager->journalOff
//     ){
//       nJRec = (u32)((szJ - pPager->journalOff)/JOURNAL_PG_SZ(pPager));
//     }
//     for(ii=0; rc==SQLITE_OK && ii<nJRec && pPager->journalOff<szJ; ii++){
//       rc = pager_playback_one_page(pPager, &pPager->journalOff, pDone, 1, 1);
//     }
//     assert( rc!=SQLITE_DONE );
//   }
//   assert( rc!=SQLITE_OK || pPager->journalOff>=szJ );

//   /* Finally,  rollback pages from the sub-journal.  Page that were
//   ** previously rolled back out of the main journal (and are hence in pDone)
//   ** will be skipped.  Out-of-range pages are also skipped.
//   */
//   if( pSavepoint ){
//     u32 ii;            /* Loop counter */
//     i64 offset = (i64)pSavepoint->iSubRec*(4+pPager->pageSize);

//     if( pagerUseWal(pPager) ){
//       rc = sqlite3WalSavepointUndo(pPager->pWal, pSavepoint->aWalData);
//     }
//     for(ii=pSavepoint->iSubRec; rc==SQLITE_OK && ii<pPager->nSubRec; ii++){
//       assert( offset==(i64)ii*(4+pPager->pageSize) );
//       rc = pager_playback_one_page(pPager, &offset, pDone, 0, 1);
//     }
//     assert( rc!=SQLITE_DONE );
//   }

//   sqlite3BitvecDestroy(pDone);
//   if( rc==SQLITE_OK ){
//     pPager->journalOff = szJ;
//   }

//   return rc;
// }

// /*
// ** Change the maximum number of in-memory pages that are allowed
// ** before attempting to recycle clean and unused pages.
// */
// void sqlite3PagerSetCachesize(Pager *pPager, int mxPage){
//   sqlite3PcacheSetCachesize(pPager->pPCache, mxPage);
// }

// /*
// ** Change the maximum number of in-memory pages that are allowed
// ** before attempting to spill pages to journal.
// */
// int sqlite3PagerSetSpillsize(Pager *pPager, int mxPage){
//   return sqlite3PcacheSetSpillsize(pPager->pPCache, mxPage);
// }

// /*
// ** Invoke SQLITE_FCNTL_MMAP_SIZE based on the current value of szMmap.
// */
// static void pagerFixMaplimit(Pager *pPager){
// #if SQLITE_MAX_MMAP_SIZE>0
//   sqlite3_file *fd = pPager->fd;
//   if( isOpen(fd) && fd->pMethods->iVersion>=3 ){
//     sqlite3_int64 sz;
//     sz = pPager->szMmap;
//     pPager->bUseFetch = (sz>0);
//     setGetterMethod(pPager);
//     sqlite3OsFileControlHint(pPager->fd, SQLITE_FCNTL_MMAP_SIZE, &sz);
//   }
// #endif
// }

// /*
// ** Change the maximum size of any memory mapping made of the database file.
// */
// void sqlite3PagerSetMmapLimit(Pager *pPager, sqlite3_int64 szMmap){
//   pPager->szMmap = szMmap;
//   pagerFixMaplimit(pPager);
// }

// /*
// ** Free as much memory as possible from the pager.
// */
// void sqlite3PagerShrink(Pager *pPager){
//   sqlite3PcacheShrink(pPager->pPCache);
// }

// /*
// ** Adjust settings of the pager to those specified in the pgFlags parameter.
// **
// ** The "level" in pgFlags & PAGER_SYNCHRONOUS_MASK sets the robustness
// ** of the database to damage due to OS crashes or power failures by
// ** changing the number of syncs()s when writing the journals.
// ** There are four levels:
// **
// **    OFF       sqlite3OsSync() is never called.  This is the default
// **              for temporary and transient files.
// **
// **    NORMAL    The journal is synced once before writes begin on the
// **              database.  This is normally adequate protection, but
// **              it is theoretically possible, though very unlikely,
// **              that an inopertune power failure could leave the journal
// **              in a state which would cause damage to the database
// **              when it is rolled back.
// **
// **    FULL      The journal is synced twice before writes begin on the
// **              database (with some additional information - the nRec field
// **              of the journal header - being written in between the two
// **              syncs).  If we assume that writing a
// **              single disk sector is atomic, then this mode provides
// **              assurance that the journal will not be corrupted to the
// **              point of causing damage to the database during rollback.
// **
// **    EXTRA     This is like FULL except that is also syncs the directory
// **              that contains the rollback journal after the rollback
// **              journal is unlinked.
// **
// ** The above is for a rollback-journal mode.  For WAL mode, OFF continues
// ** to mean that no syncs ever occur.  NORMAL means that the WAL is synced
// ** prior to the start of checkpoint and that the database file is synced
// ** at the conclusion of the checkpoint if the entire content of the WAL
// ** was written back into the database.  But no sync operations occur for
// ** an ordinary commit in NORMAL mode with WAL.  FULL means that the WAL
// ** file is synced following each commit operation, in addition to the
// ** syncs associated with NORMAL.  There is no difference between FULL
// ** and EXTRA for WAL mode.
// **
// ** Do not confuse synchronous=FULL with SQLITE_SYNC_FULL.  The
// ** SQLITE_SYNC_FULL macro means to use the MacOSX-style full-fsync
// ** using fcntl(F_FULLFSYNC).  SQLITE_SYNC_NORMAL means to do an
// ** ordinary fsync() call.  There is no difference between SQLITE_SYNC_FULL
// ** and SQLITE_SYNC_NORMAL on platforms other than MacOSX.  But the
// ** synchronous=FULL versus synchronous=NORMAL setting determines when
// ** the xSync primitive is called and is relevant to all platforms.
// **
// ** Numeric values associated with these states are OFF==1, NORMAL=2,
// ** and FULL=3.
// */
// #ifndef SQLITE_OMIT_PAGER_PRAGMAS
// void sqlite3PagerSetFlags(
//   Pager *pPager,        /* The pager to set safety level for */
//   unsigned pgFlags      /* Various flags */
// ){
//   unsigned level = pgFlags & PAGER_SYNCHRONOUS_MASK;
//   if( pPager->tempFile ){
//     pPager->noSync = 1;
//     pPager->fullSync = 0;
//     pPager->extraSync = 0;
//   }else{
//     pPager->noSync =  level==PAGER_SYNCHRONOUS_OFF ?1:0;
//     pPager->fullSync = level>=PAGER_SYNCHRONOUS_FULL ?1:0;
//     pPager->extraSync = level==PAGER_SYNCHRONOUS_EXTRA ?1:0;
//   }
//   if( pPager->noSync ){
//     pPager->syncFlags = 0;
//   }else if( pgFlags & PAGER_FULLFSYNC ){
//     pPager->syncFlags = SQLITE_SYNC_FULL;
//   }else{
//     pPager->syncFlags = SQLITE_SYNC_NORMAL;
//   }
//   pPager->walSyncFlags = (pPager->syncFlags<<2);
//   if( pPager->fullSync ){
//     pPager->walSyncFlags |= pPager->syncFlags;
//   }
//   if( (pgFlags & PAGER_CKPT_FULLFSYNC) && !pPager->noSync ){
//     pPager->walSyncFlags |= (SQLITE_SYNC_FULL<<2);
//   }
//   if( pgFlags & PAGER_CACHESPILL ){
//     pPager->doNotSpill &= ~SPILLFLAG_OFF;
//   }else{
//     pPager->doNotSpill |= SPILLFLAG_OFF;
//   }
// }
// #endif

// /*
// ** The following global variable is incremented whenever the library
// ** attempts to open a temporary file.  This information is used for
// ** testing and analysis only.  
// */
// #ifdef SQLITE_TEST
// int sqlite3_opentemp_count = 0;
// #endif

// /*
// ** Open a temporary file.
// **
// ** Write the file descriptor into *pFile. Return SQLITE_OK on success 
// ** or some other error code if we fail. The OS will automatically 
// ** delete the temporary file when it is closed.
// **
// ** The flags passed to the VFS layer xOpen() call are those specified
// ** by parameter vfsFlags ORed with the following:
// **
// **     SQLITE_OPEN_READWRITE
// **     SQLITE_OPEN_CREATE
// **     SQLITE_OPEN_EXCLUSIVE
// **     SQLITE_OPEN_DELETEONCLOSE
// */
// static int pagerOpentemp(
//   Pager *pPager,        /* The pager object */
//   sqlite3_file *pFile,  /* Write the file descriptor here */
//   int vfsFlags          /* Flags passed through to the VFS */
// ){
//   int rc;               /* Return code */

// #ifdef SQLITE_TEST
//   sqlite3_opentemp_count++;  /* Used for testing and analysis only */
// #endif

//   vfsFlags |=  SQLITE_OPEN_READWRITE | SQLITE_OPEN_CREATE |
//             SQLITE_OPEN_EXCLUSIVE | SQLITE_OPEN_DELETEONCLOSE;
//   rc = sqlite3OsOpen(pPager->pVfs, 0, pFile, vfsFlags, 0);
//   assert( rc!=SQLITE_OK || isOpen(pFile) );
//   return rc;
// }

// /*
// ** Set the busy handler function.
// **
// ** The pager invokes the busy-handler if sqlite3OsLock() returns 
// ** SQLITE_BUSY when trying to upgrade from no-lock to a SHARED lock,
// ** or when trying to upgrade from a RESERVED lock to an EXCLUSIVE 
// ** lock. It does *not* invoke the busy handler when upgrading from
// ** SHARED to RESERVED, or when upgrading from SHARED to EXCLUSIVE
// ** (which occurs during hot-journal rollback). Summary:
// **
// **   Transition                        | Invokes xBusyHandler
// **   --------------------------------------------------------
// **   NO_LOCK       -> SHARED_LOCK      | Yes
// **   SHARED_LOCK   -> RESERVED_LOCK    | No
// **   SHARED_LOCK   -> EXCLUSIVE_LOCK   | No
// **   RESERVED_LOCK -> EXCLUSIVE_LOCK   | Yes
// **
// ** If the busy-handler callback returns non-zero, the lock is 
// ** retried. If it returns zero, then the SQLITE_BUSY error is
// ** returned to the caller of the pager API function.
// */
// void sqlite3PagerSetBusyHandler(
//   Pager *pPager,                       /* Pager object */
//   int (*xBusyHandler)(void *),         /* Pointer to busy-handler function */
//   void *pBusyHandlerArg                /* Argument to pass to xBusyHandler */
// ){
//   void **ap;
//   pPager->xBusyHandler = xBusyHandler;
//   pPager->pBusyHandlerArg = pBusyHandlerArg;
//   ap = (void **)&pPager->xBusyHandler;
//   assert( ((int(*)(void *))(ap[0]))==xBusyHandler );
//   assert( ap[1]==pBusyHandlerArg );
//   sqlite3OsFileControlHint(pPager->fd, SQLITE_FCNTL_BUSYHANDLER, (void *)ap);
// }

// /*
// ** Change the page size used by the Pager object. The new page size 
// ** is passed in *pPageSize.
// **
// ** If the pager is in the error state when this function is called, it
// ** is a no-op. The value returned is the error state error code (i.e. 
// ** one of SQLITE_IOERR, an SQLITE_IOERR_xxx sub-code or SQLITE_FULL).
// **
// ** Otherwise, if all of the following are true:
// **
// **   * the new page size (value of *pPageSize) is valid (a power 
// **     of two between 512 and SQLITE_MAX_PAGE_SIZE, inclusive), and
// **
// **   * there are no outstanding page references, and
// **
// **   * the database is either not an in-memory database or it is
// **     an in-memory database that currently consists of zero pages.
// **
// ** then the pager object page size is set to *pPageSize.
// **
// ** If the page size is changed, then this function uses sqlite3PagerMalloc() 
// ** to obtain a new Pager.pTmpSpace buffer. If this allocation attempt 
// ** fails, SQLITE_NOMEM is returned and the page size remains unchanged. 
// ** In all other cases, SQLITE_OK is returned.
// **
// ** If the page size is not changed, either because one of the enumerated
// ** conditions above is not true, the pager was in error state when this
// ** function was called, or because the memory allocation attempt failed, 
// ** then *pPageSize is set to the old, retained page size before returning.
// */
// int sqlite3PagerSetPagesize(Pager *pPager, u32 *pPageSize, int nReserve){
//   int rc = SQLITE_OK;

//   /* It is not possible to do a full assert_pager_state() here, as this
//   ** function may be called from within PagerOpen(), before the state
//   ** of the Pager object is internally consistent.
//   **
//   ** At one point this function returned an error if the pager was in 
//   ** PAGER_ERROR state. But since PAGER_ERROR state guarantees that
//   ** there is at least one outstanding page reference, this function
//   ** is a no-op for that case anyhow.
//   */

//   u32 pageSize = *pPageSize;
//   assert( pageSize==0 || (pageSize>=512 && pageSize<=SQLITE_MAX_PAGE_SIZE) );
//   if( (pPager->memDb==0 || pPager->dbSize==0)
//    && sqlite3PcacheRefCount(pPager->pPCache)==0 
//    && pageSize && pageSize!=(u32)pPager->pageSize 
//   ){
//     char *pNew = NULL;             /* New temp space */
//     i64 nByte = 0;

//     if( pPager->eState>PAGER_OPEN && isOpen(pPager->fd) ){
//       rc = sqlite3OsFileSize(pPager->fd, &nByte);
//     }
//     if( rc==SQLITE_OK ){
//       /* 8 bytes of zeroed overrun space is sufficient so that the b-tree
//       * cell header parser will never run off the end of the allocation */
//       pNew = (char *)sqlite3PageMalloc(pageSize+8);
//       if( !pNew ){
//         rc = SQLITE_NOMEM;
//       }else{
//         memset(pNew+pageSize, 0, 8);
//       }
//     }

//     if( rc==SQLITE_OK ){
//       pager_reset(pPager);
//       rc = sqlite3PcacheSetPageSize(pPager->pPCache, pageSize);
//     }
//     if( rc==SQLITE_OK ){
//       sqlite3PageFree(pPager->pTmpSpace);
//       pPager->pTmpSpace = pNew;
//       pPager->dbSize = (Pgno)((nByte+pageSize-1)/pageSize);
//       pPager->pageSize = pageSize;
//     }else{
//       sqlite3PageFree(pNew);
//     }
//   }

//   *pPageSize = pPager->pageSize;
//   if( rc==SQLITE_OK ){
//     if( nReserve<0 ) nReserve = pPager->nReserve;
//     assert( nReserve>=0 && nReserve<1000 );
//     pPager->nReserve = (i16)nReserve;
//     pagerFixMaplimit(pPager);
//   }
//   return rc;
// }

// /*
// ** Return a pointer to the "temporary page" buffer held internally
// ** by the pager.  This is a buffer that is big enough to hold the
// ** entire content of a database page.  This buffer is used internally
// ** during rollback and will be overwritten whenever a rollback
// ** occurs.  But other modules are free to use it too, as long as
// ** no rollbacks are happening.
// */
// void *sqlite3PagerTempSpace(Pager *pPager){
//   return pPager->pTmpSpace;
// }

// /*
// ** Attempt to set the maximum database page count if mxPage is positive. 
// ** Make no changes if mxPage is zero or negative.  And never reduce the
// ** maximum page count below the current size of the database.
// **
// ** Regardless of mxPage, return the current maximum page count.
// */
// Pgno sqlite3PagerMaxPageCount(Pager *pPager, Pgno mxPage){
//   if( mxPage>0 ){
//     pPager->mxPgno = mxPage;
//   }
//   assert( pPager->eState!=PAGER_OPEN );      /* Called only by OP_MaxPgcnt */
//   /* assert( pPager->mxPgno>=pPager->dbSize ); */
//   /* OP_MaxPgcnt ensures that the parameter passed to this function is not
//   ** less than the total number of valid pages in the database. But this
//   ** may be less than Pager.dbSize, and so the assert() above is not valid */
//   return pPager->mxPgno;
// }

// /*
// ** The following set of routines are used to disable the simulated
// ** I/O error mechanism.  These routines are used to avoid simulated
// ** errors in places where we do not care about errors.
// **
// ** Unless -DSQLITE_TEST=1 is used, these routines are all no-ops
// ** and generate no code.
// */
// #ifdef SQLITE_TEST
// extern int sqlite3_io_error_pending;
// extern int sqlite3_io_error_hit;
// static int saved_cnt;
// void disable_simulated_io_errors(void){
//   saved_cnt = sqlite3_io_error_pending;
//   sqlite3_io_error_pending = -1;
// }
// void enable_simulated_io_errors(void){
//   sqlite3_io_error_pending = saved_cnt;
// }
// #else
// # define disable_simulated_io_errors()
// # define enable_simulated_io_errors()
// #endif

// /*
// ** Read the first N bytes from the beginning of the file into memory
// ** that pDest points to. 
// **
// ** If the pager was opened on a transient file (zFilename==""), or
// ** opened on a file less than N bytes in size, the output buffer is
// ** zeroed and SQLITE_OK returned. The rationale for this is that this 
// ** function is used to read database headers, and a new transient or
// ** zero sized database has a header than consists entirely of zeroes.
// **
// ** If any IO error apart from SQLITE_IOERR_SHORT_READ is encountered,
// ** the error code is returned to the caller and the contents of the
// ** output buffer undefined.
// */
// int sqlite3PagerReadFileheader(Pager *pPager, int N, unsigned char *pDest){
//   int rc = SQLITE_OK;
//   memset(pDest, 0, N);
//   assert( isOpen(pPager->fd) || pPager->tempFile );

//   /* This routine is only called by btree immediately after creating
//   ** the Pager object.  There has not been an opportunity to transition
//   ** to WAL mode yet.
//   */
//   assert( !pagerUseWal(pPager) );

//   if( isOpen(pPager->fd) ){
//     IOTRACE(("DBHDR %p 0 %d\n", pPager, N))
//     rc = sqlite3OsRead(pPager->fd, pDest, N, 0);
//     if( rc==SQLITE_IOERR_SHORT_READ ){
//       rc = SQLITE_OK;
//     }
//   }
//   return rc;
// }

// /*
// ** This function may only be called when a read-transaction is open on
// ** the pager. It returns the total number of pages in the database.
// **
// ** However, if the file is between 1 and <page-size> bytes in size, then 
// ** this is considered a 1 page file.
// */
// void sqlite3PagerPagecount(Pager *pPager, int *pnPage){
//   assert( pPager->eState>=PAGER_READER );
//   assert( pPager->eState!=PAGER_WRITER_FINISHED );
//   *pnPage = (int)pPager->dbSize;
// }


// /*
// ** Try to obtain a lock of type locktype on the database file. If
// ** a similar or greater lock is already held, this function is a no-op
// ** (returning SQLITE_OK immediately).
// **
// ** Otherwise, attempt to obtain the lock using sqlite3OsLock(). Invoke 
// ** the busy callback if the lock is currently not available. Repeat 
// ** until the busy callback returns false or until the attempt to 
// ** obtain the lock succeeds.
// **
// ** Return SQLITE_OK on success and an error code if we cannot obtain
// ** the lock. If the lock is obtained successfully, set the Pager.state 
// ** variable to locktype before returning.
// */
// static int pager_wait_on_lock(Pager *pPager, int locktype){
//   int rc;                              /* Return code */

//   /* Check that this is either a no-op (because the requested lock is 
//   ** already held), or one of the transitions that the busy-handler
//   ** may be invoked during, according to the comment above
//   ** sqlite3PagerSetBusyhandler().
//   */
//   assert( (pPager->eLock>=locktype)
//        || (pPager->eLock==NO_LOCK && locktype==SHARED_LOCK)
//        || (pPager->eLock==RESERVED_LOCK && locktype==EXCLUSIVE_LOCK)
//   );

//   do {
//     rc = pagerLockDb(pPager, locktype);
//   }while( rc==SQLITE_BUSY && pPager->xBusyHandler(pPager->pBusyHandlerArg) );
//   return rc;
// }

// /*
// ** Function assertTruncateConstraint(pPager) checks that one of the 
// ** following is true for all dirty pages currently in the page-cache:
// **
// **   a) The page number is less than or equal to the size of the 
// **      current database image, in pages, OR
// **
// **   b) if the page content were written at this time, it would not
// **      be necessary to write the current content out to the sub-journal
// **      (as determined by function subjRequiresPage()).
// **
// ** If the condition asserted by this function were not true, and the
// ** dirty page were to be discarded from the cache via the pagerStress()
// ** routine, pagerStress() would not write the current page content to
// ** the database file. If a savepoint transaction were rolled back after
// ** this happened, the correct behavior would be to restore the current
// ** content of the page. However, since this content is not present in either
// ** the database file or the portion of the rollback journal and 
// ** sub-journal rolled back the content could not be restored and the
// ** database image would become corrupt. It is therefore fortunate that 
// ** this circumstance cannot arise.
// */
// #if defined(SQLITE_DEBUG)
// static void assertTruncateConstraintCb(PgHdr *pPg){
//   assert( pPg->flags&PGHDR_DIRTY );
//   assert( !subjRequiresPage(pPg) || pPg->pgno<=pPg->pPager->dbSize );
// }
// static void assertTruncateConstraint(Pager *pPager){
//   sqlite3PcacheIterateDirty(pPager->pPCache, assertTruncateConstraintCb);
// }
// #else
// # define assertTruncateConstraint(pPager)
// #endif

// /*
// ** Truncate the in-memory database file image to nPage pages. This 
// ** function does not actually modify the database file on disk. It 
// ** just sets the internal state of the pager object so that the 
// ** truncation will be done when the current transaction is committed.
// **
// ** This function is only called right before committing a transaction.
// ** Once this function has been called, the transaction must either be
// ** rolled back or committed. It is not safe to call this function and
// ** then continue writing to the database.
// */
// void sqlite3PagerTruncateImage(Pager *pPager, Pgno nPage){
//   assert( pPager->dbSize>=nPage || CORRUPT_DB );
//   testcase( pPager->dbSize<nPage );
//   assert( pPager->eState>=PAGER_WRITER_CACHEMOD );
//   pPager->dbSize = nPage;

//   /* At one point the code here called assertTruncateConstraint() to
//   ** ensure that all pages being truncated away by this operation are,
//   ** if one or more savepoints are open, present in the savepoint 
//   ** journal so that they can be restored if the savepoint is rolled
//   ** back. This is no longer necessary as this function is now only
//   ** called right before committing a transaction. So although the 
//   ** Pager object may still have open savepoints (Pager.nSavepoint!=0), 
//   ** they cannot be rolled back. So the assertTruncateConstraint() call
//   ** is no longer correct. */
// }


// /*
// ** This function is called before attempting a hot-journal rollback. It
// ** syncs the journal file to disk, then sets pPager->journalHdr to the
// ** size of the journal file so that the pager_playback() routine knows
// ** that the entire journal file has been synced.
// **
// ** Syncing a hot-journal to disk before attempting to roll it back ensures 
// ** that if a power-failure occurs during the rollback, the process that
// ** attempts rollback following system recovery sees the same journal
// ** content as this process.
// **
// ** If everything goes as planned, SQLITE_OK is returned. Otherwise, 
// ** an SQLite error code.
// */
// static int pagerSyncHotJournal(Pager *pPager){
//   int rc = SQLITE_OK;
//   if( !pPager->noSync ){
//     rc = sqlite3OsSync(pPager->jfd, SQLITE_SYNC_NORMAL);
//   }
//   if( rc==SQLITE_OK ){
//     rc = sqlite3OsFileSize(pPager->jfd, &pPager->journalHdr);
//   }
//   return rc;
// }

// #if SQLITE_MAX_MMAP_SIZE>0
// /*
// ** Obtain a reference to a memory mapped page object for page number pgno. 
// ** The new object will use the pointer pData, obtained from xFetch().
// ** If successful, set *ppPage to point to the new page reference
// ** and return SQLITE_OK. Otherwise, return an SQLite error code and set
// ** *ppPage to zero.
// **
// ** Page references obtained by calling this function should be released
// ** by calling pagerReleaseMapPage().
// */
// static int pagerAcquireMapPage(
//   Pager *pPager,                  /* Pager object */
//   Pgno pgno,                      /* Page number */
//   void *pData,                    /* xFetch()'d data for this page */
//   PgHdr **ppPage                  /* OUT: Acquired page object */
// ){
//   PgHdr *p;                       /* Memory mapped page to return */
  
//   if( pPager->pMmapFreelist ){
//     *ppPage = p = pPager->pMmapFreelist;
//     pPager->pMmapFreelist = p->pDirty;
//     p->pDirty = 0;
//     assert( pPager->nExtra>=8 );
//     memset(p->pExtra, 0, 8);
//   }else{
//     *ppPage = p = (PgHdr *)sqlite3MallocZero(sizeof(PgHdr) + pPager->nExtra);
//     if( p==0 ){
//       sqlite3OsUnfetch(pPager->fd, (i64)(pgno-1) * pPager->pageSize, pData);
//       return SQLITE_NOMEM;
//     }
//     p->pExtra = (void *)&p[1];
//     p->flags = PGHDR_MMAP;
//     p->nRef = 1;
//     p->pPager = pPager;
//   }

//   assert( p->pExtra==(void *)&p[1] );
//   assert( p->pPage==0 );
//   assert( p->flags==PGHDR_MMAP );
//   assert( p->pPager==pPager );
//   assert( p->nRef==1 );

//   p->pgno = pgno;
//   p->pData = pData;
//   pPager->nMmapOut++;

//   return SQLITE_OK;
// }
// #endif

// /*
// ** Release a reference to page pPg. pPg must have been returned by an 
// ** earlier call to pagerAcquireMapPage().
// */
// static void pagerReleaseMapPage(PgHdr *pPg){
//   Pager *pPager = pPg->pPager;
//   pPager->nMmapOut--;
//   pPg->pDirty = pPager->pMmapFreelist;
//   pPager->pMmapFreelist = pPg;

//   assert( pPager->fd->pMethods->iVersion>=3 );
//   sqlite3OsUnfetch(pPager->fd, (i64)(pPg->pgno-1)*pPager->pageSize, pPg->pData);
// }

// /*
// ** Free all PgHdr objects stored in the Pager.pMmapFreelist list.
// */
// static void pagerFreeMapHdrs(Pager *pPager){
//   PgHdr *p;
//   PgHdr *pNext;
//   for(p=pPager->pMmapFreelist; p; p=pNext){
//     pNext = p->pDirty;
//     sqlite3_free(p);
//   }
// }

// /* Verify that the database file has not be deleted or renamed out from
// ** under the pager.  Return SQLITE_OK if the database is still where it ought
// ** to be on disk.  Return non-zero (SQLITE_READONLY_DBMOVED or some other error
// ** code from sqlite3OsAccess()) if the database has gone missing.
// */
// static int databaseIsUnmoved(Pager *pPager){
//   int bHasMoved = 0;
//   int rc;

//   if( pPager->tempFile ) return SQLITE_OK;
//   if( pPager->dbSize==0 ) return SQLITE_OK;
//   assert( pPager->zFilename && pPager->zFilename[0] );
//   rc = sqlite3OsFileControl(pPager->fd, SQLITE_FCNTL_HAS_MOVED, &bHasMoved);
//   if( rc==SQLITE_NOTFOUND ){
//     /* If the HAS_MOVED file-control is unimplemented, assume that the file
//     ** has not been moved.  That is the historical behavior of SQLite: prior to
//     ** version 3.8.3, it never checked */
//     rc = SQLITE_OK;
//   }else if( rc==SQLITE_OK && bHasMoved ){
//     rc = SQLITE_READONLY_DBMOVED;
//   }
//   return rc;
// }


// /*
// ** Shutdown the page cache.  Free all memory and close all files.
// **
// ** If a transaction was in progress when this routine is called, that
// ** transaction is rolled back.  All outstanding pages are invalidated
// ** and their memory is freed.  Any attempt to use a page associated
// ** with this page cache after this function returns will likely
// ** result in a coredump.
// **
// ** This function always succeeds. If a transaction is active an attempt
// ** is made to roll it back. If an error occurs during the rollback 
// ** a hot journal may be left in the filesystem but no error is returned
// ** to the caller.
// */
// int sqlite3PagerClose(Pager *pPager, sqlite3 *db){
//   u8 *pTmp = (u8*)pPager->pTmpSpace;
//   assert( db || pagerUseWal(pPager)==0 );
//   assert( assert_pager_state(pPager) );
//   disable_simulated_io_errors();
//   sqlite3BeginBenignMalloc();
//   pagerFreeMapHdrs(pPager);
//   /* pPager->errCode = 0; */
//   pPager->exclusiveMode = 0;
// #ifndef SQLITE_OMIT_WAL
//   {
//     u8 *a = 0;
//     assert( db || pPager->pWal==0 );
//     if( db && 0==(db->flags & SQLITE_NoCkptOnClose) 
//      && SQLITE_OK==databaseIsUnmoved(pPager)
//     ){
//       a = pTmp;
//     }
//     sqlite3WalClose(pPager->pWal, db, pPager->walSyncFlags, pPager->pageSize,a);
//     pPager->pWal = 0;
//   }
// #endif
//   pager_reset(pPager);
//   if( MEMDB ){
//     pager_unlock(pPager);
//   }else{
//     /* If it is open, sync the journal file before calling UnlockAndRollback.
//     ** If this is not done, then an unsynced portion of the open journal 
//     ** file may be played back into the database. If a power failure occurs 
//     ** while this is happening, the database could become corrupt.
//     **
//     ** If an error occurs while trying to sync the journal, shift the pager
//     ** into the ERROR state. This causes UnlockAndRollback to unlock the
//     ** database and close the journal file without attempting to roll it
//     ** back or finalize it. The next database user will have to do hot-journal
//     ** rollback before accessing the database file.
//     */
//     if( isOpen(pPager->jfd) ){
//       pager_error(pPager, pagerSyncHotJournal(pPager));
//     }
//     pagerUnlockAndRollback(pPager);
//   }
//   sqlite3EndBenignMalloc();
//   enable_simulated_io_errors();
//   PAGERTRACE(("CLOSE %d\n", PAGERID(pPager)));
//   IOTRACE(("CLOSE %p\n", pPager))
//   sqlite3OsClose(pPager->jfd);
//   sqlite3OsClose(pPager->fd);
//   sqlite3PageFree(pTmp);
//   sqlite3PcacheClose(pPager->pPCache);
//   assert( !pPager->aSavepoint && !pPager->pInJournal );
//   assert( !isOpen(pPager->jfd) && !isOpen(pPager->sjfd) );

//   sqlite3_free(pPager);
//   return SQLITE_OK;
// }

// #if !defined(NDEBUG) || defined(SQLITE_TEST)
// /*
// ** Return the page number for page pPg.
// */
// Pgno sqlite3PagerPagenumber(DbPage *pPg){
//   return pPg->pgno;
// }
// #endif

// /*
// ** Increment the reference count for page pPg.
// */
// void sqlite3PagerRef(DbPage *pPg){
//   sqlite3PcacheRef(pPg);
// }

// /*
// ** Sync the journal. In other words, make sure all the pages that have
// ** been written to the journal have actually reached the surface of the
// ** disk and can be restored in the event of a hot-journal rollback.
// **
// ** If the Pager.noSync flag is set, then this function is a no-op.
// ** Otherwise, the actions required depend on the journal-mode and the 
// ** device characteristics of the file-system, as follows:
// **
// **   * If the journal file is an in-memory journal file, no action need
// **     be taken.
// **
// **   * Otherwise, if the device does not support the SAFE_APPEND property,
// **     then the nRec field of the most recently written journal header
// **     is updated to contain the number of journal records that have
// **     been written following it. If the pager is operating in full-sync
// **     mode, then the journal file is synced before this field is updated.
// **
// **   * If the device does not support the SEQUENTIAL property, then 
// **     journal file is synced.
// **
// ** Or, in pseudo-code:
// **
// **   if( NOT <in-memory journal> ){
// **     if( NOT SAFE_APPEND ){
// **       if( <full-sync mode> ) xSync(<journal file>);
// **       <update nRec field>
// **     } 
// **     if( NOT SEQUENTIAL ) xSync(<journal file>);
// **   }
// **
// ** If successful, this routine clears the PGHDR_NEED_SYNC flag of every 
// ** page currently held in memory before returning SQLITE_OK. If an IO
// ** error is encountered, then the IO error code is returned to the caller.
// */
// static int syncJournal(Pager *pPager, int newHdr){
//   int rc;                         /* Return code */

//   assert( pPager->eState==PAGER_WRITER_CACHEMOD
//        || pPager->eState==PAGER_WRITER_DBMOD
//   );
//   assert( assert_pager_state(pPager) );
//   assert( !pagerUseWal(pPager) );

//   rc = sqlite3PagerExclusiveLock(pPager);
//   if( rc!=SQLITE_OK ) return rc;

//   if( !pPager->noSync ){
//     assert( !pPager->tempFile );
//     if( isOpen(pPager->jfd) && pPager->journalMode!=PAGER_JOURNALMODE_MEMORY ){
//       const int iDc = sqlite3OsDeviceCharacteristics(pPager->fd);
//       assert( isOpen(pPager->jfd) );

//       if( 0==(iDc&SQLITE_IOCAP_SAFE_APPEND) ){
//         /* This block deals with an obscure problem. If the last connection
//         ** that wrote to this database was operating in persistent-journal
//         ** mode, then the journal file may at this point actually be larger
//         ** than Pager.journalOff bytes. If the next thing in the journal
//         ** file happens to be a journal-header (written as part of the
//         ** previous connection's transaction), and a crash or power-failure 
//         ** occurs after nRec is updated but before this connection writes 
//         ** anything else to the journal file (or commits/rolls back its 
//         ** transaction), then SQLite may become confused when doing the 
//         ** hot-journal rollback following recovery. It may roll back all
//         ** of this connections data, then proceed to rolling back the old,
//         ** out-of-date data that follows it. Database corruption.
//         **
//         ** To work around this, if the journal file does appear to contain
//         ** a valid header following Pager.journalOff, then write a 0x00
//         ** byte to the start of it to prevent it from being recognized.
//         **
//         ** Variable iNextHdrOffset is set to the offset at which this
//         ** problematic header will occur, if it exists. aMagic is used 
//         ** as a temporary buffer to inspect the first couple of bytes of
//         ** the potential journal header.
//         */
//         i64 iNextHdrOffset;
//         u8 aMagic[8];
//         u8 zHeader[sizeof(aJournalMagic)+4];

//         memcpy(zHeader, aJournalMagic, sizeof(aJournalMagic));
//         put32bits(&zHeader[sizeof(aJournalMagic)], pPager->nRec);

//         iNextHdrOffset = journalHdrOffset(pPager);
//         rc = sqlite3OsRead(pPager->jfd, aMagic, 8, iNextHdrOffset);
//         if( rc==SQLITE_OK && 0==memcmp(aMagic, aJournalMagic, 8) ){
//           static const u8 zerobyte = 0;
//           rc = sqlite3OsWrite(pPager->jfd, &zerobyte, 1, iNextHdrOffset);
//         }
//         if( rc!=SQLITE_OK && rc!=SQLITE_IOERR_SHORT_READ ){
//           return rc;
//         }

//         /* Write the nRec value into the journal file header. If in
//         ** full-synchronous mode, sync the journal first. This ensures that
//         ** all data has really hit the disk before nRec is updated to mark
//         ** it as a candidate for rollback.
//         **
//         ** This is not required if the persistent media supports the
//         ** SAFE_APPEND property. Because in this case it is not possible 
//         ** for garbage data to be appended to the file, the nRec field
//         ** is populated with 0xFFFFFFFF when the journal header is written
//         ** and never needs to be updated.
//         */
//         if( pPager->fullSync && 0==(iDc&SQLITE_IOCAP_SEQUENTIAL) ){
//           PAGERTRACE(("SYNC journal of %d\n", PAGERID(pPager)));
//           IOTRACE(("JSYNC %p\n", pPager))
//           rc = sqlite3OsSync(pPager->jfd, pPager->syncFlags);
//           if( rc!=SQLITE_OK ) return rc;
//         }
//         IOTRACE(("JHDR %p %lld\n", pPager, pPager->journalHdr));
//         rc = sqlite3OsWrite(
//             pPager->jfd, zHeader, sizeof(zHeader), pPager->journalHdr
//         );
//         if( rc!=SQLITE_OK ) return rc;
//       }
//       if( 0==(iDc&SQLITE_IOCAP_SEQUENTIAL) ){
//         PAGERTRACE(("SYNC journal of %d\n", PAGERID(pPager)));
//         IOTRACE(("JSYNC %p\n", pPager))
//         rc = sqlite3OsSync(pPager->jfd, pPager->syncFlags| 
//           (pPager->syncFlags==SQLITE_SYNC_FULL?SQLITE_SYNC_DATAONLY:0)
//         );
//         if( rc!=SQLITE_OK ) return rc;
//       }

//       pPager->journalHdr = pPager->journalOff;
//       if( newHdr && 0==(iDc&SQLITE_IOCAP_SAFE_APPEND) ){
//         pPager->nRec = 0;
//         rc = writeJournalHdr(pPager);
//         if( rc!=SQLITE_OK ) return rc;
//       }
//     }else{
//       pPager->journalHdr = pPager->journalOff;
//     }
//   }

//   /* Unless the pager is in noSync mode, the journal file was just 
//   ** successfully synced. Either way, clear the PGHDR_NEED_SYNC flag on 
//   ** all pages.
//   */
//   sqlite3PcacheClearSyncFlags(pPager->pPCache);
//   pPager->eState = PAGER_WRITER_DBMOD;
//   assert( assert_pager_state(pPager) );
//   return SQLITE_OK;
// }

// /*
// ** The argument is the first in a linked list of dirty pages connected
// ** by the PgHdr.pDirty pointer. This function writes each one of the
// ** in-memory pages in the list to the database file. The argument may
// ** be NULL, representing an empty list. In this case this function is
// ** a no-op.
// **
// ** The pager must hold at least a RESERVED lock when this function
// ** is called. Before writing anything to the database file, this lock
// ** is upgraded to an EXCLUSIVE lock. If the lock cannot be obtained,
// ** SQLITE_BUSY is returned and no data is written to the database file.
// ** 
// ** If the pager is a temp-file pager and the actual file-system file
// ** is not yet open, it is created and opened before any data is 
// ** written out.
// **
// ** Once the lock has been upgraded and, if necessary, the file opened,
// ** the pages are written out to the database file in list order. Writing
// ** a page is skipped if it meets either of the following criteria:
// **
// **   * The page number is greater than Pager.dbSize, or
// **   * The PGHDR_DONT_WRITE flag is set on the page.
// **
// ** If writing out a page causes the database file to grow, Pager.dbFileSize
// ** is updated accordingly. If page 1 is written out, then the value cached
// ** in Pager.dbFileVers[] is updated to match the new value stored in
// ** the database file.
// **
// ** If everything is successful, SQLITE_OK is returned. If an IO error 
// ** occurs, an IO error code is returned. Or, if the EXCLUSIVE lock cannot
// ** be obtained, SQLITE_BUSY is returned.
// */
// static int pager_write_pagelist(Pager *pPager, PgHdr *pList){
//   int rc = SQLITE_OK;                  /* Return code */

//   /* This function is only called for rollback pagers in WRITER_DBMOD state. */
//   assert( !pagerUseWal(pPager) );
//   assert( pPager->tempFile || pPager->eState==PAGER_WRITER_DBMOD );
//   assert( pPager->eLock==EXCLUSIVE_LOCK );
//   assert( isOpen(pPager->fd) || pList->pDirty==0 );

//   /* If the file is a temp-file has not yet been opened, open it now. It
//   ** is not possible for rc to be other than SQLITE_OK if this branch
//   ** is taken, as pager_wait_on_lock() is a no-op for temp-files.
//   */
//   if( !isOpen(pPager->fd) ){
//     assert( pPager->tempFile && rc==SQLITE_OK );
//     rc = pagerOpentemp(pPager, pPager->fd, pPager->vfsFlags);
//   }

//   /* Before the first write, give the VFS a hint of what the final
//   ** file size will be.
//   */
//   assert( rc!=SQLITE_OK || isOpen(pPager->fd) );
//   if( rc==SQLITE_OK 
//    && pPager->dbHintSize<pPager->dbSize
//    && (pList->pDirty || pList->pgno>pPager->dbHintSize)
//   ){
//     sqlite3_int64 szFile = pPager->pageSize * (sqlite3_int64)pPager->dbSize;
//     sqlite3OsFileControlHint(pPager->fd, SQLITE_FCNTL_SIZE_HINT, &szFile);
//     pPager->dbHintSize = pPager->dbSize;
//   }

//   while( rc==SQLITE_OK && pList ){
//     Pgno pgno = pList->pgno;

//     /* If there are dirty pages in the page cache with page numbers greater
//     ** than Pager.dbSize, this means sqlite3PagerTruncateImage() was called to
//     ** make the file smaller (presumably by auto-vacuum code). Do not write
//     ** any such pages to the file.
//     **
//     ** Also, do not write out any page that has the PGHDR_DONT_WRITE flag
//     ** set (set by sqlite3PagerDontWrite()).
//     */
//     if( pgno<=pPager->dbSize && 0==(pList->flags&PGHDR_DONT_WRITE) ){
//       i64 offset = (pgno-1)*(i64)pPager->pageSize;   /* Offset to write */
//       char *pData;                                   /* Data to write */    

//       assert( (pList->flags&PGHDR_NEED_SYNC)==0 );
//       if( pList->pgno==1 ) pager_write_changecounter(pList);

//       pData = pList->pData;

//       /* Write out the page data. */
//       rc = sqlite3OsWrite(pPager->fd, pData, pPager->pageSize, offset);

//       /* If page 1 was just written, update Pager.dbFileVers to match
//       ** the value now stored in the database file. If writing this 
//       ** page caused the database file to grow, update dbFileSize. 
//       */
//       if( pgno==1 ){
//         memcpy(&pPager->dbFileVers, &pData[24], sizeof(pPager->dbFileVers));
//       }
//       if( pgno>pPager->dbFileSize ){
//         pPager->dbFileSize = pgno;
//       }
//       pPager->aStat[PAGER_STAT_WRITE]++;

//       /* Update any backup objects copying the contents of this pager. */
//       sqlite3BackupUpdate(pPager->pBackup, pgno, (u8*)pList->pData);

//       PAGERTRACE(("STORE %d page %d hash(%08x)\n",
//                    PAGERID(pPager), pgno, pager_pagehash(pList)));
//       IOTRACE(("PGOUT %p %d\n", pPager, pgno));
//       PAGER_INCR(sqlite3_pager_writedb_count);
//     }else{
//       PAGERTRACE(("NOSTORE %d page %d\n", PAGERID(pPager), pgno));
//     }
//     pager_set_pagehash(pList);
//     pList = pList->pDirty;
//   }

//   return rc;
// }

// /*
// ** Ensure that the sub-journal file is open. If it is already open, this 
// ** function is a no-op.
// **
// ** SQLITE_OK is returned if everything goes according to plan. An 
// ** SQLITE_IOERR_XXX error code is returned if a call to sqlite3OsOpen() 
// ** fails.
// */
// static int openSubJournal(Pager *pPager){
//   int rc = SQLITE_OK;
//   if( !isOpen(pPager->sjfd) ){
//     const int flags =  SQLITE_OPEN_SUBJOURNAL | SQLITE_OPEN_READWRITE 
//       | SQLITE_OPEN_CREATE | SQLITE_OPEN_EXCLUSIVE 
//       | SQLITE_OPEN_DELETEONCLOSE;
//     int nStmtSpill = sqlite3Config.nStmtSpill;
//     if( pPager->journalMode==PAGER_JOURNALMODE_MEMORY || pPager->subjInMemory ){
//       nStmtSpill = -1;
//     }
//     rc = sqlite3JournalOpen(pPager->pVfs, 0, pPager->sjfd, flags, nStmtSpill);
//   }
//   return rc;
// }

// /*
// ** Append a record of the current state of page pPg to the sub-journal. 
// **
// ** If successful, set the bit corresponding to pPg->pgno in the bitvecs
// ** for all open savepoints before returning.
// **
// ** This function returns SQLITE_OK if everything is successful, an IO
// ** error code if the attempt to write to the sub-journal fails, or 
// ** SQLITE_NOMEM if a malloc fails while setting a bit in a savepoint
// ** bitvec.
// */
// static int subjournalPage(PgHdr *pPg){
//   int rc = SQLITE_OK;
//   Pager *pPager = pPg->pPager;
//   if( pPager->journalMode!=PAGER_JOURNALMODE_OFF ){

//     /* Open the sub-journal, if it has not already been opened */
//     assert( pPager->useJournal );
//     assert( isOpen(pPager->jfd) || pagerUseWal(pPager) );
//     assert( isOpen(pPager->sjfd) || pPager->nSubRec==0 );
//     assert( pagerUseWal(pPager) 
//          || pageInJournal(pPager, pPg) 
//          || pPg->pgno>pPager->dbOrigSize 
//     );
//     rc = openSubJournal(pPager);

//     /* If the sub-journal was opened successfully (or was already open),
//     ** write the journal record into the file.  */
//     if( rc==SQLITE_OK ){
//       void *pData = pPg->pData;
//       i64 offset = (i64)pPager->nSubRec*(4+pPager->pageSize);
//       char *pData2;
//       pData2 = pData;
//       PAGERTRACE(("STMT-JOURNAL %d page %d\n", PAGERID(pPager), pPg->pgno));
//       rc = write32bits(pPager->sjfd, offset, pPg->pgno);
//       if( rc==SQLITE_OK ){
//         rc = sqlite3OsWrite(pPager->sjfd, pData2, pPager->pageSize, offset+4);
//       }
//     }
//   }
//   if( rc==SQLITE_OK ){
//     pPager->nSubRec++;
//     assert( pPager->nSavepoint>0 );
//     rc = addToSavepointBitvecs(pPager, pPg->pgno);
//   }
//   return rc;
// }
// static int subjournalPageIfRequired(PgHdr *pPg){
//   if( subjRequiresPage(pPg) ){
//     return subjournalPage(pPg);
//   }else{
//     return SQLITE_OK;
//   }
// }

// /*
// ** This function is called by the pcache layer when it has reached some
// ** soft memory limit. The first argument is a pointer to a Pager object
// ** (cast as a void*). The pager is always 'purgeable' (not an in-memory
// ** database). The second argument is a reference to a page that is 
// ** currently dirty but has no outstanding references. The page
// ** is always associated with the Pager object passed as the first 
// ** argument.
// **
// ** The job of this function is to make pPg clean by writing its contents
// ** out to the database file, if possible. This may involve syncing the
// ** journal file. 
// **
// ** If successful, sqlite3PcacheMakeClean() is called on the page and
// ** SQLITE_OK returned. If an IO error occurs while trying to make the
// ** page clean, the IO error code is returned. If the page cannot be
// ** made clean for some other reason, but no error occurs, then SQLITE_OK
// ** is returned by sqlite3PcacheMakeClean() is not called.
// */
// static int pagerStress(void *p, PgHdr *pPg){
//   Pager *pPager = (Pager *)p;
//   int rc = SQLITE_OK;

//   assert( pPg->pPager==pPager );
//   assert( pPg->flags&PGHDR_DIRTY );

//   /* The doNotSpill NOSYNC bit is set during times when doing a sync of
//   ** journal (and adding a new header) is not allowed.  This occurs
//   ** during calls to sqlite3PagerWrite() while trying to journal multiple
//   ** pages belonging to the same sector.
//   **
//   ** The doNotSpill ROLLBACK and OFF bits inhibits all cache spilling
//   ** regardless of whether or not a sync is required.  This is set during
//   ** a rollback or by user request, respectively.
//   **
//   ** Spilling is also prohibited when in an error state since that could
//   ** lead to database corruption.   In the current implementation it 
//   ** is impossible for sqlite3PcacheFetch() to be called with createFlag==3
//   ** while in the error state, hence it is impossible for this routine to
//   ** be called in the error state.  Nevertheless, we include a NEVER()
//   ** test for the error state as a safeguard against future changes.
//   */
//   if( NEVER(pPager->errCode) ) return SQLITE_OK;
//   testcase( pPager->doNotSpill & SPILLFLAG_ROLLBACK );
//   testcase( pPager->doNotSpill & SPILLFLAG_OFF );
//   testcase( pPager->doNotSpill & SPILLFLAG_NOSYNC );
//   if( pPager->doNotSpill
//    && ((pPager->doNotSpill & (SPILLFLAG_ROLLBACK|SPILLFLAG_OFF))!=0
//       || (pPg->flags & PGHDR_NEED_SYNC)!=0)
//   ){
//     return SQLITE_OK;
//   }

//   pPager->aStat[PAGER_STAT_SPILL]++;
//   pPg->pDirty = 0;
//   if( pagerUseWal(pPager) ){
//     /* Write a single frame for this page to the log. */
//     rc = subjournalPageIfRequired(pPg); 
//     if( rc==SQLITE_OK ){
//       rc = pagerWalFrames(pPager, pPg, 0, 0);
//     }
//   }else{
    
// #ifdef SQLITE_ENABLE_BATCH_ATOMIC_WRITE
//     if( pPager->tempFile==0 ){
//       rc = sqlite3JournalCreate(pPager->jfd);
//       if( rc!=SQLITE_OK ) return pager_error(pPager, rc);
//     }
// #endif
  
//     /* Sync the journal file if required. */
//     if( pPg->flags&PGHDR_NEED_SYNC 
//      || pPager->eState==PAGER_WRITER_CACHEMOD
//     ){
//       rc = syncJournal(pPager, 1);
//     }
  
//     /* Write the contents of the page out to the database file. */
//     if( rc==SQLITE_OK ){
//       assert( (pPg->flags&PGHDR_NEED_SYNC)==0 );
//       rc = pager_write_pagelist(pPager, pPg);
//     }
//   }

//   /* Mark the page as clean. */
//   if( rc==SQLITE_OK ){
//     PAGERTRACE(("STRESS %d page %d\n", PAGERID(pPager), pPg->pgno));
//     sqlite3PcacheMakeClean(pPg);
//   }

//   return pager_error(pPager, rc); 
// }

// /*
// ** Flush all unreferenced dirty pages to disk.
// */
// int sqlite3PagerFlush(Pager *pPager){
//   int rc = pPager->errCode;
//   if( !MEMDB ){
//     PgHdr *pList = sqlite3PcacheDirtyList(pPager->pPCache);
//     assert( assert_pager_state(pPager) );
//     while( rc==SQLITE_OK && pList ){
//       PgHdr *pNext = pList->pDirty;
//       if( pList->nRef==0 ){
//         rc = pagerStress((void*)pPager, pList);
//       }
//       pList = pNext;
//     }
//   }

//   return rc;
// }

// /*
// ** Allocate and initialize a new Pager object and put a pointer to it
// ** in *ppPager. The pager should eventually be freed by passing it
// ** to sqlite3PagerClose().
// **
// ** The zFilename argument is the path to the database file to open.
// ** If zFilename is NULL then a randomly-named temporary file is created
// ** and used as the file to be cached. Temporary files are be deleted
// ** automatically when they are closed. If zFilename is ":memory:" then 
// ** all information is held in cache. It is never written to disk. 
// ** This can be used to implement an in-memory database.
// **
// ** The nExtra parameter specifies the number of bytes of space allocated
// ** along with each page reference. This space is available to the user
// ** via the sqlite3PagerGetExtra() API.  When a new page is allocated, the
// ** first 8 bytes of this space are zeroed but the remainder is uninitialized.
// ** (The extra space is used by btree as the MemPage object.)
// **
// ** The flags argument is used to specify properties that affect the
// ** operation of the pager. It should be passed some bitwise combination
// ** of the PAGER_* flags.
// **
// ** The vfsFlags parameter is a bitmask to pass to the flags parameter
// ** of the xOpen() method of the supplied VFS when opening files. 
// **
// ** If the pager object is allocated and the specified file opened 
// ** successfully, SQLITE_OK is returned and *ppPager set to point to
// ** the new pager object. If an error occurs, *ppPager is set to NULL
// ** and error code returned. This function may return SQLITE_NOMEM
// ** (sqlite3Malloc() is used to allocate memory), SQLITE_CANTOPEN or 
// ** various SQLITE_IO_XXX errors.
// */
// int sqlite3PagerOpen(
//   sqlite3_vfs *pVfs,       /* The virtual file system to use */
//   Pager **ppPager,         /* OUT: Return the Pager structure here */
//   const char *zFilename,   /* Name of the database file to open */
//   int nExtra,              /* Extra bytes append to each in-memory page */
//   int flags,               /* flags controlling this file */
//   int vfsFlags,            /* flags passed through to sqlite3_vfs.xOpen() */
//   void (*xReinit)(DbPage*) /* Function to reinitialize pages */
// ){
//   u8 *pPtr;
//   Pager *pPager = 0;       /* Pager object to allocate and return */
//   int rc = SQLITE_OK;      /* Return code */
//   int tempFile = 0;        /* True for temp files (incl. in-memory files) */
//   int memDb = 0;           /* True if this is an in-memory file */
// #ifndef SQLITE_OMIT_DESERIALIZE
//   int memJM = 0;           /* Memory journal mode */
// #else
// # define memJM 0
// #endif
//   int readOnly = 0;        /* True if this is a read-only file */
//   int journalFileSize;     /* Bytes to allocate for each journal fd */
//   char *zPathname = 0;     /* Full path to database file */
//   int nPathname = 0;       /* Number of bytes in zPathname */
//   int useJournal = (flags & PAGER_OMIT_JOURNAL)==0; /* False to omit journal */
//   int pcacheSize = sqlite3PcacheSize();       /* Bytes to allocate for PCache */
//   u32 szPageDflt = SQLITE_DEFAULT_PAGE_SIZE;  /* Default page size */
//   const char *zUri = 0;    /* URI args to copy */
//   int nUriByte = 1;        /* Number of bytes of URI args at *zUri */
//   int nUri = 0;            /* Number of URI parameters */

//   /* Figure out how much space is required for each journal file-handle
//   ** (there are two of them, the main journal and the sub-journal).  */
//   journalFileSize = ROUND8(sqlite3JournalSize(pVfs));

//   /* Set the output variable to NULL in case an error occurs. */
//   *ppPager = 0;

// #ifndef SQLITE_OMIT_MEMORYDB
//   if( flags & PAGER_MEMORY ){
//     memDb = 1;
//     if( zFilename && zFilename[0] ){
//       zPathname = sqlite3DbStrDup(0, zFilename);
//       if( zPathname==0  ) return SQLITE_NOMEM;
//       nPathname = sqlite3Strlen30(zPathname);
//       zFilename = 0;
//     }
//   }
// #endif

//   /* Compute and store the full pathname in an allocated buffer pointed
//   ** to by zPathname, length nPathname. Or, if this is a temporary file,
//   ** leave both nPathname and zPathname set to 0.
//   */
//   if( zFilename && zFilename[0] ){
//     const char *z;
//     nPathname = pVfs->mxPathname+1;
//     zPathname = sqlite3DbMallocRaw(0, nPathname*2);
//     if( zPathname==0 ){
//       return SQLITE_NOMEM;
//     }
//     zPathname[0] = 0; /* Make sure initialized even if FullPathname() fails */
//     rc = sqlite3OsFullPathname(pVfs, zFilename, nPathname, zPathname);
//     if( rc!=SQLITE_OK ){
//       if( rc==SQLITE_OK_SYMLINK ){
//         if( vfsFlags & SQLITE_OPEN_NOFOLLOW ){
//           rc = SQLITE_CANTOPEN_SYMLINK;
//         }else{
//           rc = SQLITE_OK;
//         }
//       }
//     }
//     nPathname = sqlite3Strlen30(zPathname);
//     z = zUri = &zFilename[sqlite3Strlen30(zFilename)+1];
//     while( *z ){
//       z += strlen(z)+1;
//       z += strlen(z)+1;
//       nUri++;
//     }
//     nUriByte = (int)(&z[1] - zUri);
//     assert( nUriByte>=1 );
//     if( rc==SQLITE_OK && nPathname+8>pVfs->mxPathname ){
//       /* This branch is taken when the journal path required by
//       ** the database being opened will be more than pVfs->mxPathname
//       ** bytes in length. This means the database cannot be opened,
//       ** as it will not be possible to open the journal file or even
//       ** check for a hot-journal before reading.
//       */
//       rc = SQLITE_CANTOPEN_BKPT;
//     }
//     if( rc!=SQLITE_OK ){
//       sqlite3DbFree(0, zPathname);
//       return rc;
//     }
//   }

//   /* Allocate memory for the Pager structure, PCache object, the
//   ** three file descriptors, the database file name and the journal 
//   ** file name. The layout in memory is as follows:
//   **
//   **     Pager object                    (sizeof(Pager) bytes)
//   **     PCache object                   (sqlite3PcacheSize() bytes)
//   **     Database file handle            (pVfs->szOsFile bytes)
//   **     Sub-journal file handle         (journalFileSize bytes)
//   **     Main journal file handle        (journalFileSize bytes)
//   **     Ptr back to the Pager           (sizeof(Pager*) bytes)
//   **     \0\0\0\0 database prefix        (4 bytes)
//   **     Database file name              (nPathname+1 bytes)
//   **     URI query parameters            (nUriByte bytes)
//   **     Journal filename                (nPathname+8+1 bytes)
//   **     WAL filename                    (nPathname+4+1 bytes)
//   **     \0\0\0 terminator               (3 bytes)
//   **
//   ** Some 3rd-party software, over which we have no control, depends on
//   ** the specific order of the filenames and the \0 separators between them
//   ** so that it can (for example) find the database filename given the WAL
//   ** filename without using the sqlite3_filename_database() API.  This is a
//   ** misuse of SQLite and a bug in the 3rd-party software, but the 3rd-party
//   ** software is in widespread use, so we try to avoid changing the filename
//   ** order and formatting if possible.  In particular, the details of the
//   ** filename format expected by 3rd-party software should be as follows:
//   **
//   **   - Main Database Path
//   **   - \0
//   **   - Multiple URI components consisting of:
//   **     - Key
//   **     - \0
//   **     - Value
//   **     - \0
//   **   - \0
//   **   - Journal Path
//   **   - \0
//   **   - WAL Path (zWALName)
//   **   - \0
//   **
//   ** The sqlite3_create_filename() interface and the databaseFilename() utility
//   ** that is used by sqlite3_filename_database() and kin also depend on the
//   ** specific formatting and order of the various filenames, so if the format
//   ** changes here, be sure to change it there as well.
//   */
//   pPtr = (u8 *)sqlite3MallocZero(
//     ROUND8(sizeof(*pPager)) +            /* Pager structure */
//     ROUND8(pcacheSize) +                 /* PCache object */
//     ROUND8(pVfs->szOsFile) +             /* The main db file */
//     journalFileSize * 2 +                /* The two journal files */
//     sizeof(pPager) +                     /* Space to hold a pointer */
//     4 +                                  /* Database prefix */
//     nPathname + 1 +                      /* database filename */
//     nUriByte +                           /* query parameters */
//     nPathname + 8 + 1 +                  /* Journal filename */
// #ifndef SQLITE_OMIT_WAL
//     nPathname + 4 + 1 +                  /* WAL filename */
// #endif
//     3                                    /* Terminator */
//   );
//   assert( EIGHT_BYTE_ALIGNMENT(SQLITE_INT_TO_PTR(journalFileSize)) );
//   if( !pPtr ){
//     sqlite3DbFree(0, zPathname);
//     return SQLITE_NOMEM;
//   }
//   pPager = (Pager*)pPtr;                  pPtr += ROUND8(sizeof(*pPager));
//   pPager->pPCache = (PCache*)pPtr;        pPtr += ROUND8(pcacheSize);
//   pPager->fd = (sqlite3_file*)pPtr;       pPtr += ROUND8(pVfs->szOsFile);
//   pPager->sjfd = (sqlite3_file*)pPtr;     pPtr += journalFileSize;
//   pPager->jfd =  (sqlite3_file*)pPtr;     pPtr += journalFileSize;
//   assert( EIGHT_BYTE_ALIGNMENT(pPager->jfd) );
//   memcpy(pPtr, &pPager, sizeof(pPager));  pPtr += sizeof(pPager);

//   /* Fill in the Pager.zFilename and pPager.zQueryParam fields */
//                                           pPtr += 4;  /* Skip zero prefix */
//   pPager->zFilename = (char*)pPtr;
//   if( nPathname>0 ){
//     memcpy(pPtr, zPathname, nPathname);   pPtr += nPathname + 1;
//     if( zUri ){
//       memcpy(pPtr, zUri, nUriByte);       pPtr += nUriByte;
//     }else{
//                                           pPtr++;
//     }
//   }


//   /* Fill in Pager.zJournal */
//   if( nPathname>0 ){
//     pPager->zJournal = (char*)pPtr;
//     memcpy(pPtr, zPathname, nPathname);   pPtr += nPathname;
//     memcpy(pPtr, "-journal",8);           pPtr += 8 + 1;
// #ifdef SQLITE_ENABLE_8_3_NAMES
//     sqlite3FileSuffix3(zFilename,pPager->zJournal);
//     pPtr = (u8*)(pPager->zJournal + sqlite3Strlen30(pPager->zJournal)+1);
// #endif
//   }else{
//     pPager->zJournal = 0;
//   }

// #ifndef SQLITE_OMIT_WAL
//   /* Fill in Pager.zWal */
//   if( nPathname>0 ){
//     pPager->zWal = (char*)pPtr;
//     memcpy(pPtr, zPathname, nPathname);   pPtr += nPathname;
//     memcpy(pPtr, "-wal", 4);              pPtr += 4 + 1;
// #ifdef SQLITE_ENABLE_8_3_NAMES
//     sqlite3FileSuffix3(zFilename, pPager->zWal);
//     pPtr = (u8*)(pPager->zWal + sqlite3Strlen30(pPager->zWal)+1);
// #endif
//   }else{
//     pPager->zWal = 0;
//   }
// #endif
//   (void)pPtr;  /* Suppress warning about unused pPtr value */

//   if( nPathname ) sqlite3DbFree(0, zPathname);
//   pPager->pVfs = pVfs;
//   pPager->vfsFlags = vfsFlags;

//   /* Open the pager file.
//   */
//   if( zFilename && zFilename[0] ){
//     int fout = 0;                    /* VFS flags returned by xOpen() */
//     rc = sqlite3OsOpen(pVfs, pPager->zFilename, pPager->fd, vfsFlags, &fout);
//     assert( !memDb );
// #ifndef SQLITE_OMIT_DESERIALIZE
//     pPager->memVfs = memJM = (fout&SQLITE_OPEN_MEMORY)!=0;
// #endif
//     readOnly = (fout&SQLITE_OPEN_READONLY)!=0;

//     /* If the file was successfully opened for read/write access,
//     ** choose a default page size in case we have to create the
//     ** database file. The default page size is the maximum of:
//     **
//     **    + SQLITE_DEFAULT_PAGE_SIZE,
//     **    + The value returned by sqlite3OsSectorSize()
//     **    + The largest page size that can be written atomically.
//     */
//     if( rc==SQLITE_OK ){
//       int iDc = sqlite3OsDeviceCharacteristics(pPager->fd);
//       if( !readOnly ){
//         setSectorSize(pPager);
//         assert(SQLITE_DEFAULT_PAGE_SIZE<=SQLITE_MAX_DEFAULT_PAGE_SIZE);
//         if( szPageDflt<pPager->sectorSize ){
//           if( pPager->sectorSize>SQLITE_MAX_DEFAULT_PAGE_SIZE ){
//             szPageDflt = SQLITE_MAX_DEFAULT_PAGE_SIZE;
//           }else{
//             szPageDflt = (u32)pPager->sectorSize;
//           }
//         }
// #ifdef SQLITE_ENABLE_ATOMIC_WRITE
//         {
//           int ii;
//           assert(SQLITE_IOCAP_ATOMIC512==(512>>8));
//           assert(SQLITE_IOCAP_ATOMIC64K==(65536>>8));
//           assert(SQLITE_MAX_DEFAULT_PAGE_SIZE<=65536);
//           for(ii=szPageDflt; ii<=SQLITE_MAX_DEFAULT_PAGE_SIZE; ii=ii*2){
//             if( iDc&(SQLITE_IOCAP_ATOMIC|(ii>>8)) ){
//               szPageDflt = ii;
//             }
//           }
//         }
// #endif
//       }
//       pPager->noLock = sqlite3_uri_boolean(pPager->zFilename, "nolock", 0);
//       if( (iDc & SQLITE_IOCAP_IMMUTABLE)!=0
//        || sqlite3_uri_boolean(pPager->zFilename, "immutable", 0) ){
//           vfsFlags |= SQLITE_OPEN_READONLY;
//           goto act_like_temp_file;
//       }
//     }
//   }else{
//     /* If a temporary file is requested, it is not opened immediately.
//     ** In this case we accept the default page size and delay actually
//     ** opening the file until the first call to OsWrite().
//     **
//     ** This branch is also run for an in-memory database. An in-memory
//     ** database is the same as a temp-file that is never written out to
//     ** disk and uses an in-memory rollback journal.
//     **
//     ** This branch also runs for files marked as immutable.
//     */ 
// act_like_temp_file:
//     tempFile = 1;
//     pPager->eState = PAGER_READER;     /* Pretend we already have a lock */
//     pPager->eLock = EXCLUSIVE_LOCK;    /* Pretend we are in EXCLUSIVE mode */
//     pPager->noLock = 1;                /* Do no locking */
//     readOnly = (vfsFlags&SQLITE_OPEN_READONLY);
//   }

//   /* The following call to PagerSetPagesize() serves to set the value of 
//   ** Pager.pageSize and to allocate the Pager.pTmpSpace buffer.
//   */
//   if( rc==SQLITE_OK ){
//     assert( pPager->memDb==0 );
//     rc = sqlite3PagerSetPagesize(pPager, &szPageDflt, -1);
//     testcase( rc!=SQLITE_OK );
//   }

//   /* Initialize the PCache object. */
//   if( rc==SQLITE_OK ){
//     nExtra = ROUND8(nExtra);
//     assert( nExtra>=8 && nExtra<1000 );
//     rc = sqlite3PcacheOpen(szPageDflt, nExtra, !memDb,
//                        !memDb?pagerStress:0, (void *)pPager, pPager->pPCache);
//   }

//   /* If an error occurred above, free the  Pager structure and close the file.
//   */
//   if( rc!=SQLITE_OK ){
//     sqlite3OsClose(pPager->fd);
//     sqlite3PageFree(pPager->pTmpSpace);
//     sqlite3_free(pPager);
//     return rc;
//   }

//   PAGERTRACE(("OPEN %d %s\n", FILEHANDLEID(pPager->fd), pPager->zFilename));
//   IOTRACE(("OPEN %p %s\n", pPager, pPager->zFilename))

//   pPager->useJournal = (u8)useJournal;
//   /* pPager->stmtOpen = 0; */
//   /* pPager->stmtInUse = 0; */
//   /* pPager->nRef = 0; */
//   /* pPager->stmtSize = 0; */
//   /* pPager->stmtJSize = 0; */
//   /* pPager->nPage = 0; */
//   pPager->mxPgno = SQLITE_MAX_PAGE_COUNT;
//   /* pPager->state = PAGER_UNLOCK; */
//   /* pPager->errMask = 0; */
//   pPager->tempFile = (u8)tempFile;
//   assert( tempFile==PAGER_LOCKINGMODE_NORMAL 
//           || tempFile==PAGER_LOCKINGMODE_EXCLUSIVE );
//   assert( PAGER_LOCKINGMODE_EXCLUSIVE==1 );
//   pPager->exclusiveMode = (u8)tempFile; 
//   pPager->changeCountDone = pPager->tempFile;
//   pPager->memDb = (u8)memDb;
//   pPager->readOnly = (u8)readOnly;
//   assert( useJournal || pPager->tempFile );
//   pPager->noSync = pPager->tempFile;
//   if( pPager->noSync ){
//     assert( pPager->fullSync==0 );
//     assert( pPager->extraSync==0 );
//     assert( pPager->syncFlags==0 );
//     assert( pPager->walSyncFlags==0 );
//   }else{
//     pPager->fullSync = 1;
//     pPager->extraSync = 0;
//     pPager->syncFlags = SQLITE_SYNC_NORMAL;
//     pPager->walSyncFlags = SQLITE_SYNC_NORMAL | (SQLITE_SYNC_NORMAL<<2);
//   }
//   /* pPager->pFirst = 0; */
//   /* pPager->pFirstSynced = 0; */
//   /* pPager->pLast = 0; */
//   pPager->nExtra = (u16)nExtra;
//   pPager->journalSizeLimit = SQLITE_DEFAULT_JOURNAL_SIZE_LIMIT;
//   assert( isOpen(pPager->fd) || tempFile );
//   setSectorSize(pPager);
//   if( !useJournal ){
//     pPager->journalMode = PAGER_JOURNALMODE_OFF;
//   }else if( memDb || memJM ){
//     pPager->journalMode = PAGER_JOURNALMODE_MEMORY;
//   }
//   /* pPager->xBusyHandler = 0; */
//   /* pPager->pBusyHandlerArg = 0; */
//   pPager->xReiniter = xReinit;
//   setGetterMethod(pPager);
//   /* memset(pPager->aHash, 0, sizeof(pPager->aHash)); */
//   /* pPager->szMmap = SQLITE_DEFAULT_MMAP_SIZE // will be set by btree.c */

//   *ppPager = pPager;
//   return SQLITE_OK;
// }

// /*
// ** Return the sqlite3_file for the main database given the name
// ** of the corresonding WAL or Journal name as passed into
// ** xOpen.
// */
// sqlite3_file *sqlite3_database_file_object(const char *zName){
//   Pager *pPager;
//   while( zName[-1]!=0 || zName[-2]!=0 || zName[-3]!=0 || zName[-4]!=0 ){
//     zName--;
//   }
//   pPager = *(Pager**)(zName - 4 - sizeof(Pager*));
//   return pPager->fd;
// }


// /*
// ** This function is called after transitioning from PAGER_UNLOCK to
// ** PAGER_SHARED state. It tests if there is a hot journal present in
// ** the file-system for the given pager. A hot journal is one that 
// ** needs to be played back. According to this function, a hot-journal
// ** file exists if the following criteria are met:
// **
// **   * The journal file exists in the file system, and
// **   * No process holds a RESERVED or greater lock on the database file, and
// **   * The database file itself is greater than 0 bytes in size, and
// **   * The first byte of the journal file exists and is not 0x00.
// **
// ** If the current size of the database file is 0 but a journal file
// ** exists, that is probably an old journal left over from a prior
// ** database with the same name. In this case the journal file is
// ** just deleted using OsDelete, *pExists is set to 0 and SQLITE_OK
// ** is returned.
// **
// ** This routine does not check if there is a super-journal filename
// ** at the end of the file. If there is, and that super-journal file
// ** does not exist, then the journal file is not really hot. In this
// ** case this routine will return a false-positive. The pager_playback()
// ** routine will discover that the journal file is not really hot and 
// ** will not roll it back. 
// **
// ** If a hot-journal file is found to exist, *pExists is set to 1 and 
// ** SQLITE_OK returned. If no hot-journal file is present, *pExists is
// ** set to 0 and SQLITE_OK returned. If an IO error occurs while trying
// ** to determine whether or not a hot-journal file exists, the IO error
// ** code is returned and the value of *pExists is undefined.
// */
// static int hasHotJournal(Pager *pPager, int *pExists){
//   sqlite3_vfs * const pVfs = pPager->pVfs;
//   int rc = SQLITE_OK;           /* Return code */
//   int exists = 1;               /* True if a journal file is present */
//   int jrnlOpen = !!isOpen(pPager->jfd);

//   assert( pPager->useJournal );
//   assert( isOpen(pPager->fd) );
//   assert( pPager->eState==PAGER_OPEN );

//   assert( jrnlOpen==0 || ( sqlite3OsDeviceCharacteristics(pPager->jfd) &
//     SQLITE_IOCAP_UNDELETABLE_WHEN_OPEN
//   ));

//   *pExists = 0;
//   if( !jrnlOpen ){
//     rc = sqlite3OsAccess(pVfs, pPager->zJournal, SQLITE_ACCESS_EXISTS, &exists);
//   }
//   if( rc==SQLITE_OK && exists ){
//     int locked = 0;             /* True if some process holds a RESERVED lock */

//     /* Race condition here:  Another process might have been holding the
//     ** the RESERVED lock and have a journal open at the sqlite3OsAccess() 
//     ** call above, but then delete the journal and drop the lock before
//     ** we get to the following sqlite3OsCheckReservedLock() call.  If that
//     ** is the case, this routine might think there is a hot journal when
//     ** in fact there is none.  This results in a false-positive which will
//     ** be dealt with by the playback routine.  Ticket #3883.
//     */
//     rc = sqlite3OsCheckReservedLock(pPager->fd, &locked);
//     if( rc==SQLITE_OK && !locked ){
//       Pgno nPage;                 /* Number of pages in database file */

//       assert( pPager->tempFile==0 );
//       rc = pagerPagecount(pPager, &nPage);
//       if( rc==SQLITE_OK ){
//         /* If the database is zero pages in size, that means that either (1) the
//         ** journal is a remnant from a prior database with the same name where
//         ** the database file but not the journal was deleted, or (2) the initial
//         ** transaction that populates a new database is being rolled back.
//         ** In either case, the journal file can be deleted.  However, take care
//         ** not to delete the journal file if it is already open due to
//         ** journal_mode=PERSIST.
//         */
//         if( nPage==0 && !jrnlOpen ){
//           sqlite3BeginBenignMalloc();
//           if( pagerLockDb(pPager, RESERVED_LOCK)==SQLITE_OK ){
//             sqlite3OsDelete(pVfs, pPager->zJournal, 0);
//             if( !pPager->exclusiveMode ) pagerUnlockDb(pPager, SHARED_LOCK);
//           }
//           sqlite3EndBenignMalloc();
//         }else{
//           /* The journal file exists and no other connection has a reserved
//           ** or greater lock on the database file. Now check that there is
//           ** at least one non-zero bytes at the start of the journal file.
//           ** If there is, then we consider this journal to be hot. If not, 
//           ** it can be ignored.
//           */
//           if( !jrnlOpen ){
//             int f = SQLITE_OPEN_READONLY|SQLITE_OPEN_MAIN_JOURNAL;
//             rc = sqlite3OsOpen(pVfs, pPager->zJournal, pPager->jfd, f, &f);
//           }
//           if( rc==SQLITE_OK ){
//             u8 first = 0;
//             rc = sqlite3OsRead(pPager->jfd, (void *)&first, 1, 0);
//             if( rc==SQLITE_IOERR_SHORT_READ ){
//               rc = SQLITE_OK;
//             }
//             if( !jrnlOpen ){
//               sqlite3OsClose(pPager->jfd);
//             }
//             *pExists = (first!=0);
//           }else if( rc==SQLITE_CANTOPEN ){
//             /* If we cannot open the rollback journal file in order to see if
//             ** it has a zero header, that might be due to an I/O error, or
//             ** it might be due to the race condition described above and in
//             ** ticket #3883.  Either way, assume that the journal is hot.
//             ** This might be a false positive.  But if it is, then the
//             ** automatic journal playback and recovery mechanism will deal
//             ** with it under an EXCLUSIVE lock where we do not need to
//             ** worry so much with race conditions.
//             */
//             *pExists = 1;
//             rc = SQLITE_OK;
//           }
//         }
//       }
//     }
//   }

//   return rc;
// }

// /*
// ** This function is called to obtain a shared lock on the database file.
// ** It is illegal to call sqlite3PagerGet() until after this function
// ** has been successfully called. If a shared-lock is already held when
// ** this function is called, it is a no-op.
// **
// ** The following operations are also performed by this function.
// **
// **   1) If the pager is currently in PAGER_OPEN state (no lock held
// **      on the database file), then an attempt is made to obtain a
// **      SHARED lock on the database file. Immediately after obtaining
// **      the SHARED lock, the file-system is checked for a hot-journal,
// **      which is played back if present. Following any hot-journal 
// **      rollback, the contents of the cache are validated by checking
// **      the 'change-counter' field of the database file header and
// **      discarded if they are found to be invalid.
// **
// **   2) If the pager is running in exclusive-mode, and there are currently
// **      no outstanding references to any pages, and is in the error state,
// **      then an attempt is made to clear the error state by discarding
// **      the contents of the page cache and rolling back any open journal
// **      file.
// **
// ** If everything is successful, SQLITE_OK is returned. If an IO error 
// ** occurs while locking the database, checking for a hot-journal file or 
// ** rolling back a journal file, the IO error code is returned.
// */
// int sqlite3PagerSharedLock(Pager *pPager){
//   int rc = SQLITE_OK;                /* Return code */

//   /* This routine is only called from b-tree and only when there are no
//   ** outstanding pages. This implies that the pager state should either
//   ** be OPEN or READER. READER is only possible if the pager is or was in 
//   ** exclusive access mode.  */
//   assert( sqlite3PcacheRefCount(pPager->pPCache)==0 );
//   assert( assert_pager_state(pPager) );
//   assert( pPager->eState==PAGER_OPEN || pPager->eState==PAGER_READER );
//   assert( pPager->errCode==SQLITE_OK );

//   if( !pagerUseWal(pPager) && pPager->eState==PAGER_OPEN ){
//     int bHotJournal = 1;          /* True if there exists a hot journal-file */

//     assert( !MEMDB );
//     assert( pPager->tempFile==0 || pPager->eLock==EXCLUSIVE_LOCK );

//     rc = pager_wait_on_lock(pPager, SHARED_LOCK);
//     if( rc!=SQLITE_OK ){
//       assert( pPager->eLock==NO_LOCK || pPager->eLock==UNKNOWN_LOCK );
//       goto failed;
//     }

//     /* If a journal file exists, and there is no RESERVED lock on the
//     ** database file, then it either needs to be played back or deleted.
//     */
//     if( pPager->eLock<=SHARED_LOCK ){
//       rc = hasHotJournal(pPager, &bHotJournal);
//     }
//     if( rc!=SQLITE_OK ){
//       goto failed;
//     }
//     if( bHotJournal ){
//       if( pPager->readOnly ){
//         rc = SQLITE_READONLY_ROLLBACK;
//         goto failed;
//       }

//       /* Get an EXCLUSIVE lock on the database file. At this point it is
//       ** important that a RESERVED lock is not obtained on the way to the
//       ** EXCLUSIVE lock. If it were, another process might open the
//       ** database file, detect the RESERVED lock, and conclude that the
//       ** database is safe to read while this process is still rolling the 
//       ** hot-journal back.
//       ** 
//       ** Because the intermediate RESERVED lock is not requested, any
//       ** other process attempting to access the database file will get to 
//       ** this point in the code and fail to obtain its own EXCLUSIVE lock 
//       ** on the database file.
//       **
//       ** Unless the pager is in locking_mode=exclusive mode, the lock is
//       ** downgraded to SHARED_LOCK before this function returns.
//       */
//       rc = pagerLockDb(pPager, EXCLUSIVE_LOCK);
//       if( rc!=SQLITE_OK ){
//         goto failed;
//       }
 
//       /* If it is not already open and the file exists on disk, open the 
//       ** journal for read/write access. Write access is required because 
//       ** in exclusive-access mode the file descriptor will be kept open 
//       ** and possibly used for a transaction later on. Also, write-access 
//       ** is usually required to finalize the journal in journal_mode=persist 
//       ** mode (and also for journal_mode=truncate on some systems).
//       **
//       ** If the journal does not exist, it usually means that some 
//       ** other connection managed to get in and roll it back before 
//       ** this connection obtained the exclusive lock above. Or, it 
//       ** may mean that the pager was in the error-state when this
//       ** function was called and the journal file does not exist.
//       */
//       if( !isOpen(pPager->jfd) ){
//         sqlite3_vfs * const pVfs = pPager->pVfs;
//         int bExists;              /* True if journal file exists */
//         rc = sqlite3OsAccess(
//             pVfs, pPager->zJournal, SQLITE_ACCESS_EXISTS, &bExists);
//         if( rc==SQLITE_OK && bExists ){
//           int fout = 0;
//           int f = SQLITE_OPEN_READWRITE|SQLITE_OPEN_MAIN_JOURNAL;
//           assert( !pPager->tempFile );
//           rc = sqlite3OsOpen(pVfs, pPager->zJournal, pPager->jfd, f, &fout);
//           assert( rc!=SQLITE_OK || isOpen(pPager->jfd) );
//           if( rc==SQLITE_OK && fout&SQLITE_OPEN_READONLY ){
//             rc = SQLITE_CANTOPEN_BKPT;
//             sqlite3OsClose(pPager->jfd);
//           }
//         }
//       }
 
//       /* Playback and delete the journal.  Drop the database write
//       ** lock and reacquire the read lock. Purge the cache before
//       ** playing back the hot-journal so that we don't end up with
//       ** an inconsistent cache.  Sync the hot journal before playing
//       ** it back since the process that crashed and left the hot journal
//       ** probably did not sync it and we are required to always sync
//       ** the journal before playing it back.
//       */
//       if( isOpen(pPager->jfd) ){
//         assert( rc==SQLITE_OK );
//         rc = pagerSyncHotJournal(pPager);
//         if( rc==SQLITE_OK ){
//           rc = pager_playback(pPager, !pPager->tempFile);
//           pPager->eState = PAGER_OPEN;
//         }
//       }else if( !pPager->exclusiveMode ){
//         pagerUnlockDb(pPager, SHARED_LOCK);
//       }

//       if( rc!=SQLITE_OK ){
//         /* This branch is taken if an error occurs while trying to open
//         ** or roll back a hot-journal while holding an EXCLUSIVE lock. The
//         ** pager_unlock() routine will be called before returning to unlock
//         ** the file. If the unlock attempt fails, then Pager.eLock must be
//         ** set to UNKNOWN_LOCK (see the comment above the #define for 
//         ** UNKNOWN_LOCK above for an explanation). 
//         **
//         ** In order to get pager_unlock() to do this, set Pager.eState to
//         ** PAGER_ERROR now. This is not actually counted as a transition
//         ** to ERROR state in the state diagram at the top of this file,
//         ** since we know that the same call to pager_unlock() will very
//         ** shortly transition the pager object to the OPEN state. Calling
//         ** assert_pager_state() would fail now, as it should not be possible
//         ** to be in ERROR state when there are zero outstanding page 
//         ** references.
//         */
//         pager_error(pPager, rc);
//         goto failed;
//       }

//       assert( pPager->eState==PAGER_OPEN );
//       assert( (pPager->eLock==SHARED_LOCK)
//            || (pPager->exclusiveMode && pPager->eLock>SHARED_LOCK)
//       );
//     }

//     if( !pPager->tempFile && pPager->hasHeldSharedLock ){
//       /* The shared-lock has just been acquired then check to
//       ** see if the database has been modified.  If the database has changed,
//       ** flush the cache.  The hasHeldSharedLock flag prevents this from
//       ** occurring on the very first access to a file, in order to save a
//       ** single unnecessary sqlite3OsRead() call at the start-up.
//       **
//       ** Database changes are detected by looking at 15 bytes beginning
//       ** at offset 24 into the file.  The first 4 of these 16 bytes are
//       ** a 32-bit counter that is incremented with each change.  The
//       ** other bytes change randomly with each file change when
//       ** a codec is in use.
//       ** 
//       ** There is a vanishingly small chance that a change will not be 
//       ** detected.  The chance of an undetected change is so small that
//       ** it can be neglected.
//       */
//       char dbFileVers[sizeof(pPager->dbFileVers)];

//       IOTRACE(("CKVERS %p %d\n", pPager, sizeof(dbFileVers)));
//       rc = sqlite3OsRead(pPager->fd, &dbFileVers, sizeof(dbFileVers), 24);
//       if( rc!=SQLITE_OK ){
//         if( rc!=SQLITE_IOERR_SHORT_READ ){
//           goto failed;
//         }
//         memset(dbFileVers, 0, sizeof(dbFileVers));
//       }

//       if( memcmp(pPager->dbFileVers, dbFileVers, sizeof(dbFileVers))!=0 ){
//         pager_reset(pPager);

//         /* Unmap the database file. It is possible that external processes
//         ** may have truncated the database file and then extended it back
//         ** to its original size while this process was not holding a lock.
//         ** In this case there may exist a Pager.pMap mapping that appears
//         ** to be the right size but is not actually valid. Avoid this
//         ** possibility by unmapping the db here. */
//         if( USEFETCH(pPager) ){
//           sqlite3OsUnfetch(pPager->fd, 0, 0);
//         }
//       }
//     }

//     /* If there is a WAL file in the file-system, open this database in WAL
//     ** mode. Otherwise, the following function call is a no-op.
//     */
//     rc = pagerOpenWalIfPresent(pPager);
// #ifndef SQLITE_OMIT_WAL
//     assert( pPager->pWal==0 || rc==SQLITE_OK );
// #endif
//   }

//   if( pagerUseWal(pPager) ){
//     assert( rc==SQLITE_OK );
//     rc = pagerBeginReadTransaction(pPager);
//   }

//   if( pPager->tempFile==0 && pPager->eState==PAGER_OPEN && rc==SQLITE_OK ){
//     rc = pagerPagecount(pPager, &pPager->dbSize);
//   }

//  failed:
//   if( rc!=SQLITE_OK ){
//     assert( !MEMDB );
//     pager_unlock(pPager);
//     assert( pPager->eState==PAGER_OPEN );
//   }else{
//     pPager->eState = PAGER_READER;
//     pPager->hasHeldSharedLock = 1;
//   }
//   return rc;
// }

// /*
// ** If the reference count has reached zero, rollback any active
// ** transaction and unlock the pager.
// **
// ** Except, in locking_mode=EXCLUSIVE when there is nothing to in
// ** the rollback journal, the unlock is not performed and there is
// ** nothing to rollback, so this routine is a no-op.
// */ 
// static void pagerUnlockIfUnused(Pager *pPager){
//   if( sqlite3PcacheRefCount(pPager->pPCache)==0 ){
//     assert( pPager->nMmapOut==0 ); /* because page1 is never memory mapped */
//     pagerUnlockAndRollback(pPager);
//   }
// }

// /*
// ** The page getter methods each try to acquire a reference to a
// ** page with page number pgno. If the requested reference is 
// ** successfully obtained, it is copied to *ppPage and SQLITE_OK returned.
// **
// ** There are different implementations of the getter method depending
// ** on the current state of the pager.
// **
// **     getPageNormal()         --  The normal getter
// **     getPageError()          --  Used if the pager is in an error state
// **     getPageMmap()           --  Used if memory-mapped I/O is enabled
// **
// ** If the requested page is already in the cache, it is returned. 
// ** Otherwise, a new page object is allocated and populated with data
// ** read from the database file. In some cases, the pcache module may
// ** choose not to allocate a new page object and may reuse an existing
// ** object with no outstanding references.
// **
// ** The extra data appended to a page is always initialized to zeros the 
// ** first time a page is loaded into memory. If the page requested is 
// ** already in the cache when this function is called, then the extra
// ** data is left as it was when the page object was last used.
// **
// ** If the database image is smaller than the requested page or if 
// ** the flags parameter contains the PAGER_GET_NOCONTENT bit and the 
// ** requested page is not already stored in the cache, then no 
// ** actual disk read occurs. In this case the memory image of the 
// ** page is initialized to all zeros. 
// **
// ** If PAGER_GET_NOCONTENT is true, it means that we do not care about
// ** the contents of the page. This occurs in two scenarios:
// **
// **   a) When reading a free-list leaf page from the database, and
// **
// **   b) When a savepoint is being rolled back and we need to load
// **      a new page into the cache to be filled with the data read
// **      from the savepoint journal.
// **
// ** If PAGER_GET_NOCONTENT is true, then the data returned is zeroed instead
// ** of being read from the database. Additionally, the bits corresponding
// ** to pgno in Pager.pInJournal (bitvec of pages already written to the
// ** journal file) and the PagerSavepoint.pInSavepoint bitvecs of any open
// ** savepoints are set. This means if the page is made writable at any
// ** point in the future, using a call to sqlite3PagerWrite(), its contents
// ** will not be journaled. This saves IO.
// **
// ** The acquisition might fail for several reasons.  In all cases,
// ** an appropriate error code is returned and *ppPage is set to NULL.
// **
// ** See also sqlite3PagerLookup().  Both this routine and Lookup() attempt
// ** to find a page in the in-memory cache first.  If the page is not already
// ** in memory, this routine goes to disk to read it in whereas Lookup()
// ** just returns 0.  This routine acquires a read-lock the first time it
// ** has to go to disk, and could also playback an old journal if necessary.
// ** Since Lookup() never goes to disk, it never has to deal with locks
// ** or journal files.
// */
// static int getPageNormal(
//   Pager *pPager,      /* The pager open on the database file */
//   Pgno pgno,          /* Page number to fetch */
//   DbPage **ppPage,    /* Write a pointer to the page here */
//   int flags           /* PAGER_GET_XXX flags */
// ){
//   int rc = SQLITE_OK;
//   PgHdr *pPg;
//   u8 noContent;                   /* True if PAGER_GET_NOCONTENT is set */
//   sqlite3_pcache_page *pBase;

//   assert( pPager->errCode==SQLITE_OK );
//   assert( pPager->eState>=PAGER_READER );
//   assert( assert_pager_state(pPager) );
//   assert( pPager->hasHeldSharedLock==1 );

//   if( pgno==0 ) return SQLITE_CORRUPT_BKPT;
//   pBase = sqlite3PcacheFetch(pPager->pPCache, pgno, 3);
//   if( pBase==0 ){
//     pPg = 0;
//     rc = sqlite3PcacheFetchStress(pPager->pPCache, pgno, &pBase);
//     if( rc!=SQLITE_OK ) goto pager_acquire_err;
//     if( pBase==0 ){
//       rc = SQLITE_NOMEM;
//       goto pager_acquire_err;
//     }
//   }
//   pPg = *ppPage = sqlite3PcacheFetchFinish(pPager->pPCache, pgno, pBase);
//   assert( pPg==(*ppPage) );
//   assert( pPg->pgno==pgno );
//   assert( pPg->pPager==pPager || pPg->pPager==0 );

//   noContent = (flags & PAGER_GET_NOCONTENT)!=0;
//   if( pPg->pPager && !noContent ){
//     /* In this case the pcache already contains an initialized copy of
//     ** the page. Return without further ado.  */
//     assert( pgno!=PAGER_MJ_PGNO(pPager) );
//     pPager->aStat[PAGER_STAT_HIT]++;
//     return SQLITE_OK;

//   }else{
//     /* The pager cache has created a new page. Its content needs to 
//     ** be initialized. But first some error checks:
//     **
//     ** (*) obsolete.  Was: maximum page number is 2^31
//     ** (2) Never try to fetch the locking page
//     */
//     if( pgno==PAGER_MJ_PGNO(pPager) ){
//       rc = SQLITE_CORRUPT_BKPT;
//       goto pager_acquire_err;
//     }

//     pPg->pPager = pPager;

//     assert( !isOpen(pPager->fd) || !MEMDB );
//     if( !isOpen(pPager->fd) || pPager->dbSize<pgno || noContent ){
//       if( pgno>pPager->mxPgno ){
//         rc = SQLITE_FULL;
//         goto pager_acquire_err;
//       }
//       if( noContent ){
//         /* Failure to set the bits in the InJournal bit-vectors is benign.
//         ** It merely means that we might do some extra work to journal a 
//         ** page that does not need to be journaled.  Nevertheless, be sure 
//         ** to test the case where a malloc error occurs while trying to set 
//         ** a bit in a bit vector.
//         */
//         sqlite3BeginBenignMalloc();
//         if( pgno<=pPager->dbOrigSize ){
//           TESTONLY( rc = ) sqlite3BitvecSet(pPager->pInJournal, pgno);
//           testcase( rc==SQLITE_NOMEM );
//         }
//         TESTONLY( rc = ) addToSavepointBitvecs(pPager, pgno);
//         testcase( rc==SQLITE_NOMEM );
//         sqlite3EndBenignMalloc();
//       }
//       memset(pPg->pData, 0, pPager->pageSize);
//       IOTRACE(("ZERO %p %d\n", pPager, pgno));
//     }else{
//       assert( pPg->pPager==pPager );
//       pPager->aStat[PAGER_STAT_MISS]++;
//       rc = readDbPage(pPg);
//       if( rc!=SQLITE_OK ){
//         goto pager_acquire_err;
//       }
//     }
//     pager_set_pagehash(pPg);
//   }
//   return SQLITE_OK;

// pager_acquire_err:
//   assert( rc!=SQLITE_OK );
//   if( pPg ){
//     sqlite3PcacheDrop(pPg);
//   }
//   pagerUnlockIfUnused(pPager);
//   *ppPage = 0;
//   return rc;
// }

// #if SQLITE_MAX_MMAP_SIZE>0
// /* The page getter for when memory-mapped I/O is enabled */
// static int getPageMMap(
//   Pager *pPager,      /* The pager open on the database file */
//   Pgno pgno,          /* Page number to fetch */
//   DbPage **ppPage,    /* Write a pointer to the page here */
//   int flags           /* PAGER_GET_XXX flags */
// ){
//   int rc = SQLITE_OK;
//   PgHdr *pPg = 0;
//   u32 iFrame = 0;                 /* Frame to read from WAL file */

//   /* It is acceptable to use a read-only (mmap) page for any page except
//   ** page 1 if there is no write-transaction open or the ACQUIRE_READONLY
//   ** flag was specified by the caller. And so long as the db is not a 
//   ** temporary or in-memory database.  */
//   const int bMmapOk = (pgno>1
//    && (pPager->eState==PAGER_READER || (flags & PAGER_GET_READONLY))
//   );

//   assert( USEFETCH(pPager) );

//   /* Optimization note:  Adding the "pgno<=1" term before "pgno==0" here
//   ** allows the compiler optimizer to reuse the results of the "pgno>1"
//   ** test in the previous statement, and avoid testing pgno==0 in the
//   ** common case where pgno is large. */
//   if( pgno<=1 && pgno==0 ){
//     return SQLITE_CORRUPT_BKPT;
//   }
//   assert( pPager->eState>=PAGER_READER );
//   assert( assert_pager_state(pPager) );
//   assert( pPager->hasHeldSharedLock==1 );
//   assert( pPager->errCode==SQLITE_OK );

//   if( bMmapOk && pagerUseWal(pPager) ){
//     rc = sqlite3WalFindFrame(pPager->pWal, pgno, &iFrame);
//     if( rc!=SQLITE_OK ){
//       *ppPage = 0;
//       return rc;
//     }
//   }
//   if( bMmapOk && iFrame==0 ){
//     void *pData = 0;
//     rc = sqlite3OsFetch(pPager->fd, 
//         (i64)(pgno-1) * pPager->pageSize, pPager->pageSize, &pData
//     );
//     if( rc==SQLITE_OK && pData ){
//       if( pPager->eState>PAGER_READER || pPager->tempFile ){
//         pPg = sqlite3PagerLookup(pPager, pgno);
//       }
//       if( pPg==0 ){
//         rc = pagerAcquireMapPage(pPager, pgno, pData, &pPg);
//       }else{
//         sqlite3OsUnfetch(pPager->fd, (i64)(pgno-1)*pPager->pageSize, pData);
//       }
//       if( pPg ){
//         assert( rc==SQLITE_OK );
//         *ppPage = pPg;
//         return SQLITE_OK;
//       }
//     }
//     if( rc!=SQLITE_OK ){
//       *ppPage = 0;
//       return rc;
//     }
//   }
//   return getPageNormal(pPager, pgno, ppPage, flags);
// }
// #endif /* SQLITE_MAX_MMAP_SIZE>0 */

// /* The page getter method for when the pager is an error state */
// static int getPageError(
//   Pager *pPager,      /* The pager open on the database file */
//   Pgno pgno,          /* Page number to fetch */
//   DbPage **ppPage,    /* Write a pointer to the page here */
//   int flags           /* PAGER_GET_XXX flags */
// ){
//   UNUSED_PARAMETER(pgno);
//   UNUSED_PARAMETER(flags);
//   assert( pPager->errCode!=SQLITE_OK );
//   *ppPage = 0;
//   return pPager->errCode;
// }


// /* Dispatch all page fetch requests to the appropriate getter method.
// */
// int sqlite3PagerGet(
//   Pager *pPager,      /* The pager open on the database file */
//   Pgno pgno,          /* Page number to fetch */
//   DbPage **ppPage,    /* Write a pointer to the page here */
//   int flags           /* PAGER_GET_XXX flags */
// ){
//   return pPager->xGet(pPager, pgno, ppPage, flags);
// }

// /*
// ** Acquire a page if it is already in the in-memory cache.  Do
// ** not read the page from disk.  Return a pointer to the page,
// ** or 0 if the page is not in cache. 
// **
// ** See also sqlite3PagerGet().  The difference between this routine
// ** and sqlite3PagerGet() is that _get() will go to the disk and read
// ** in the page if the page is not already in cache.  This routine
// ** returns NULL if the page is not in cache or if a disk I/O error 
// ** has ever happened.
// */
// DbPage *sqlite3PagerLookup(Pager *pPager, Pgno pgno){
//   sqlite3_pcache_page *pPage;
//   assert( pPager!=0 );
//   assert( pgno!=0 );
//   assert( pPager->pPCache!=0 );
//   pPage = sqlite3PcacheFetch(pPager->pPCache, pgno, 0);
//   assert( pPage==0 || pPager->hasHeldSharedLock );
//   if( pPage==0 ) return 0;
//   return sqlite3PcacheFetchFinish(pPager->pPCache, pgno, pPage);
// }

// /*
// ** Release a page reference.
// **
// ** The sqlite3PagerUnref() and sqlite3PagerUnrefNotNull() may only be
// ** used if we know that the page being released is not the last page.
// ** The btree layer always holds page1 open until the end, so these first
// ** to routines can be used to release any page other than BtShared.pPage1.
// **
// ** Use sqlite3PagerUnrefPageOne() to release page1.  This latter routine
// ** checks the total number of outstanding pages and if the number of
// ** pages reaches zero it drops the database lock.
// */
// void sqlite3PagerUnrefNotNull(DbPage *pPg){
//   TESTONLY( Pager *pPager = pPg->pPager; )
//   assert( pPg!=0 );
//   if( pPg->flags & PGHDR_MMAP ){
//     assert( pPg->pgno!=1 );  /* Page1 is never memory mapped */
//     pagerReleaseMapPage(pPg);
//   }else{
//     sqlite3PcacheRelease(pPg);
//   }
//   /* Do not use this routine to release the last reference to page1 */
//   assert( sqlite3PcacheRefCount(pPager->pPCache)>0 );
// }
// void sqlite3PagerUnref(DbPage *pPg){
//   if( pPg ) sqlite3PagerUnrefNotNull(pPg);
// }
// void sqlite3PagerUnrefPageOne(DbPage *pPg){
//   Pager *pPager;
//   assert( pPg!=0 );
//   assert( pPg->pgno==1 );
//   assert( (pPg->flags & PGHDR_MMAP)==0 ); /* Page1 is never memory mapped */
//   pPager = pPg->pPager;
//   sqlite3PcacheRelease(pPg);
//   pagerUnlockIfUnused(pPager);
// }

// /*
// ** This function is called at the start of every write transaction.
// ** There must already be a RESERVED or EXCLUSIVE lock on the database 
// ** file when this routine is called.
// **
// ** Open the journal file for pager pPager and write a journal header
// ** to the start of it. If there are active savepoints, open the sub-journal
// ** as well. This function is only used when the journal file is being 
// ** opened to write a rollback log for a transaction. It is not used 
// ** when opening a hot journal file to roll it back.
// **
// ** If the journal file is already open (as it may be in exclusive mode),
// ** then this function just writes a journal header to the start of the
// ** already open file. 
// **
// ** Whether or not the journal file is opened by this function, the
// ** Pager.pInJournal bitvec structure is allocated.
// **
// ** Return SQLITE_OK if everything is successful. Otherwise, return 
// ** SQLITE_NOMEM if the attempt to allocate Pager.pInJournal fails, or 
// ** an IO error code if opening or writing the journal file fails.
// */
// static int pager_open_journal(Pager *pPager){
//   int rc = SQLITE_OK;                        /* Return code */
//   sqlite3_vfs * const pVfs = pPager->pVfs;   /* Local cache of vfs pointer */

//   assert( pPager->eState==PAGER_WRITER_LOCKED );
//   assert( assert_pager_state(pPager) );
//   assert( pPager->pInJournal==0 );
  
//   /* If already in the error state, this function is a no-op.  But on
//   ** the other hand, this routine is never called if we are already in
//   ** an error state. */
//   if( NEVER(pPager->errCode) ) return pPager->errCode;

//   if( !pagerUseWal(pPager) && pPager->journalMode!=PAGER_JOURNALMODE_OFF ){
//     pPager->pInJournal = sqlite3BitvecCreate(pPager->dbSize);
//     if( pPager->pInJournal==0 ){
//       return SQLITE_NOMEM;
//     }
  
//     /* Open the journal file if it is not already open. */
//     if( !isOpen(pPager->jfd) ){
//       if( pPager->journalMode==PAGER_JOURNALMODE_MEMORY ){
//         sqlite3MemJournalOpen(pPager->jfd);
//       }else{
//         int flags = SQLITE_OPEN_READWRITE|SQLITE_OPEN_CREATE;
//         int nSpill;

//         if( pPager->tempFile ){
//           flags |= (SQLITE_OPEN_DELETEONCLOSE|SQLITE_OPEN_TEMP_JOURNAL);
//           nSpill = sqlite3Config.nStmtSpill;
//         }else{
//           flags |= SQLITE_OPEN_MAIN_JOURNAL;
//           nSpill = jrnlBufferSize(pPager);
//         }
          
//         /* Verify that the database still has the same name as it did when
//         ** it was originally opened. */
//         rc = databaseIsUnmoved(pPager);
//         if( rc==SQLITE_OK ){
//           rc = sqlite3JournalOpen (
//               pVfs, pPager->zJournal, pPager->jfd, flags, nSpill
//           );
//         }
//       }
//       assert( rc!=SQLITE_OK || isOpen(pPager->jfd) );
//     }
  
  
//     /* Write the first journal header to the journal file and open 
//     ** the sub-journal if necessary.
//     */
//     if( rc==SQLITE_OK ){
//       /* TODO: Check if all of these are really required. */
//       pPager->nRec = 0;
//       pPager->journalOff = 0;
//       pPager->setSuper = 0;
//       pPager->journalHdr = 0;
//       rc = writeJournalHdr(pPager);
//     }
//   }

//   if( rc!=SQLITE_OK ){
//     sqlite3BitvecDestroy(pPager->pInJournal);
//     pPager->pInJournal = 0;
//   }else{
//     assert( pPager->eState==PAGER_WRITER_LOCKED );
//     pPager->eState = PAGER_WRITER_CACHEMOD;
//   }

//   return rc;
// }

// /*
// ** Begin a write-transaction on the specified pager object. If a 
// ** write-transaction has already been opened, this function is a no-op.
// **
// ** If the exFlag argument is false, then acquire at least a RESERVED
// ** lock on the database file. If exFlag is true, then acquire at least
// ** an EXCLUSIVE lock. If such a lock is already held, no locking 
// ** functions need be called.
// **
// ** If the subjInMemory argument is non-zero, then any sub-journal opened
// ** within this transaction will be opened as an in-memory file. This
// ** has no effect if the sub-journal is already opened (as it may be when
// ** running in exclusive mode) or if the transaction does not require a
// ** sub-journal. If the subjInMemory argument is zero, then any required
// ** sub-journal is implemented in-memory if pPager is an in-memory database, 
// ** or using a temporary file otherwise.
// */
// int sqlite3PagerBegin(Pager *pPager, int exFlag, int subjInMemory){
//   int rc = SQLITE_OK;

//   if( pPager->errCode ) return pPager->errCode;
//   assert( pPager->eState>=PAGER_READER && pPager->eState<PAGER_ERROR );
//   pPager->subjInMemory = (u8)subjInMemory;

//   if( pPager->eState==PAGER_READER ){
//     assert( pPager->pInJournal==0 );

//     if( pagerUseWal(pPager) ){
//       /* If the pager is configured to use locking_mode=exclusive, and an
//       ** exclusive lock on the database is not already held, obtain it now.
//       */
//       if( pPager->exclusiveMode && sqlite3WalExclusiveMode(pPager->pWal, -1) ){
//         rc = pagerLockDb(pPager, EXCLUSIVE_LOCK);
//         if( rc!=SQLITE_OK ){
//           return rc;
//         }
//         (void)sqlite3WalExclusiveMode(pPager->pWal, 1);
//       }

//       /* Grab the write lock on the log file. If successful, upgrade to
//       ** PAGER_RESERVED state. Otherwise, return an error code to the caller.
//       ** The busy-handler is not invoked if another connection already
//       ** holds the write-lock. If possible, the upper layer will call it.
//       */
//       rc = sqlite3WalBeginWriteTransaction(pPager->pWal);
//     }else{
//       /* Obtain a RESERVED lock on the database file. If the exFlag parameter
//       ** is true, then immediately upgrade this to an EXCLUSIVE lock. The
//       ** busy-handler callback can be used when upgrading to the EXCLUSIVE
//       ** lock, but not when obtaining the RESERVED lock.
//       */
//       rc = pagerLockDb(pPager, RESERVED_LOCK);
//       if( rc==SQLITE_OK && exFlag ){
//         rc = pager_wait_on_lock(pPager, EXCLUSIVE_LOCK);
//       }
//     }

//     if( rc==SQLITE_OK ){
//       /* Change to WRITER_LOCKED state.
//       **
//       ** WAL mode sets Pager.eState to PAGER_WRITER_LOCKED or CACHEMOD
//       ** when it has an open transaction, but never to DBMOD or FINISHED.
//       ** This is because in those states the code to roll back savepoint 
//       ** transactions may copy data from the sub-journal into the database 
//       ** file as well as into the page cache. Which would be incorrect in 
//       ** WAL mode.
//       */
//       pPager->eState = PAGER_WRITER_LOCKED;
//       pPager->dbHintSize = pPager->dbSize;
//       pPager->dbFileSize = pPager->dbSize;
//       pPager->dbOrigSize = pPager->dbSize;
//       pPager->journalOff = 0;
//     }

//     assert( rc==SQLITE_OK || pPager->eState==PAGER_READER );
//     assert( rc!=SQLITE_OK || pPager->eState==PAGER_WRITER_LOCKED );
//     assert( assert_pager_state(pPager) );
//   }

//   PAGERTRACE(("TRANSACTION %d\n", PAGERID(pPager)));
//   return rc;
// }

// /*
// ** Write page pPg onto the end of the rollback journal.
// */
// static SQLITE_NOINLINE int pagerAddPageToRollbackJournal(PgHdr *pPg){
//   Pager *pPager = pPg->pPager;
//   int rc;
//   u32 cksum;
//   char *pData2;
//   i64 iOff = pPager->journalOff;

//   /* We should never write to the journal file the page that
//   ** contains the database locks.  The following assert verifies
//   ** that we do not. */
//   assert( pPg->pgno!=PAGER_MJ_PGNO(pPager) );

//   assert( pPager->journalHdr<=pPager->journalOff );
//   pData2 = pPg->pData;
//   cksum = pager_cksum(pPager, (u8*)pData2);

//   /* Even if an IO or diskfull error occurs while journalling the
//   ** page in the block above, set the need-sync flag for the page.
//   ** Otherwise, when the transaction is rolled back, the logic in
//   ** playback_one_page() will think that the page needs to be restored
//   ** in the database file. And if an IO error occurs while doing so,
//   ** then corruption may follow.
//   */
//   pPg->flags |= PGHDR_NEED_SYNC;

//   rc = write32bits(pPager->jfd, iOff, pPg->pgno);
//   if( rc!=SQLITE_OK ) return rc;
//   rc = sqlite3OsWrite(pPager->jfd, pData2, pPager->pageSize, iOff+4);
//   if( rc!=SQLITE_OK ) return rc;
//   rc = write32bits(pPager->jfd, iOff+pPager->pageSize+4, cksum);
//   if( rc!=SQLITE_OK ) return rc;

//   IOTRACE(("JOUT %p %d %lld %d\n", pPager, pPg->pgno, 
//            pPager->journalOff, pPager->pageSize));
//   PAGER_INCR(sqlite3_pager_writej_count);
//   PAGERTRACE(("JOURNAL %d page %d needSync=%d hash(%08x)\n",
//        PAGERID(pPager), pPg->pgno, 
//        ((pPg->flags&PGHDR_NEED_SYNC)?1:0), pager_pagehash(pPg)));

//   pPager->journalOff += 8 + pPager->pageSize;
//   pPager->nRec++;
//   assert( pPager->pInJournal!=0 );
//   rc = sqlite3BitvecSet(pPager->pInJournal, pPg->pgno);
//   testcase( rc==SQLITE_NOMEM );
//   assert( rc==SQLITE_OK || rc==SQLITE_NOMEM );
//   rc |= addToSavepointBitvecs(pPager, pPg->pgno);
//   assert( rc==SQLITE_OK || rc==SQLITE_NOMEM );
//   return rc;
// }

// /*
// ** Mark a single data page as writeable. The page is written into the 
// ** main journal or sub-journal as required. If the page is written into
// ** one of the journals, the corresponding bit is set in the 
// ** Pager.pInJournal bitvec and the PagerSavepoint.pInSavepoint bitvecs
// ** of any open savepoints as appropriate.
// */
// static int pager_write(PgHdr *pPg){
//   Pager *pPager = pPg->pPager;
//   int rc = SQLITE_OK;

//   /* This routine is not called unless a write-transaction has already 
//   ** been started. The journal file may or may not be open at this point.
//   ** It is never called in the ERROR state.
//   */
//   assert( pPager->eState==PAGER_WRITER_LOCKED
//        || pPager->eState==PAGER_WRITER_CACHEMOD
//        || pPager->eState==PAGER_WRITER_DBMOD
//   );
//   assert( assert_pager_state(pPager) );
//   assert( pPager->errCode==0 );
//   assert( pPager->readOnly==0 );
//   CHECK_PAGE(pPg);

//   /* The journal file needs to be opened. Higher level routines have already
//   ** obtained the necessary locks to begin the write-transaction, but the
//   ** rollback journal might not yet be open. Open it now if this is the case.
//   **
//   ** This is done before calling sqlite3PcacheMakeDirty() on the page. 
//   ** Otherwise, if it were done after calling sqlite3PcacheMakeDirty(), then
//   ** an error might occur and the pager would end up in WRITER_LOCKED state
//   ** with pages marked as dirty in the cache.
//   */
//   if( pPager->eState==PAGER_WRITER_LOCKED ){
//     rc = pager_open_journal(pPager);
//     if( rc!=SQLITE_OK ) return rc;
//   }
//   assert( pPager->eState>=PAGER_WRITER_CACHEMOD );
//   assert( assert_pager_state(pPager) );

//   /* Mark the page that is about to be modified as dirty. */
//   sqlite3PcacheMakeDirty(pPg);

//   /* If a rollback journal is in use, them make sure the page that is about
//   ** to change is in the rollback journal, or if the page is a new page off
//   ** then end of the file, make sure it is marked as PGHDR_NEED_SYNC.
//   */
//   assert( (pPager->pInJournal!=0) == isOpen(pPager->jfd) );
//   if( pPager->pInJournal!=0
//    && sqlite3BitvecTestNotNull(pPager->pInJournal, pPg->pgno)==0
//   ){
//     assert( pagerUseWal(pPager)==0 );
//     if( pPg->pgno<=pPager->dbOrigSize ){
//       rc = pagerAddPageToRollbackJournal(pPg);
//       if( rc!=SQLITE_OK ){
//         return rc;
//       }
//     }else{
//       if( pPager->eState!=PAGER_WRITER_DBMOD ){
//         pPg->flags |= PGHDR_NEED_SYNC;
//       }
//       PAGERTRACE(("APPEND %d page %d needSync=%d\n",
//               PAGERID(pPager), pPg->pgno,
//              ((pPg->flags&PGHDR_NEED_SYNC)?1:0)));
//     }
//   }

//   /* The PGHDR_DIRTY bit is set above when the page was added to the dirty-list
//   ** and before writing the page into the rollback journal.  Wait until now,
//   ** after the page has been successfully journalled, before setting the
//   ** PGHDR_WRITEABLE bit that indicates that the page can be safely modified.
//   */
//   pPg->flags |= PGHDR_WRITEABLE;
  
//   /* If the statement journal is open and the page is not in it,
//   ** then write the page into the statement journal.
//   */
//   if( pPager->nSavepoint>0 ){
//     rc = subjournalPageIfRequired(pPg);
//   }

//   /* Update the database size and return. */
//   if( pPager->dbSize<pPg->pgno ){
//     pPager->dbSize = pPg->pgno;
//   }
//   return rc;
// }

// /*
// ** This is a variant of sqlite3PagerWrite() that runs when the sector size
// ** is larger than the page size.  SQLite makes the (reasonable) assumption that
// ** all bytes of a sector are written together by hardware.  Hence, all bytes of
// ** a sector need to be journalled in case of a power loss in the middle of
// ** a write.
// **
// ** Usually, the sector size is less than or equal to the page size, in which
// ** case pages can be individually written.  This routine only runs in the
// ** exceptional case where the page size is smaller than the sector size.
// */
// static SQLITE_NOINLINE int pagerWriteLargeSector(PgHdr *pPg){
//   int rc = SQLITE_OK;          /* Return code */
//   Pgno nPageCount;             /* Total number of pages in database file */
//   Pgno pg1;                    /* First page of the sector pPg is located on. */
//   int nPage = 0;               /* Number of pages starting at pg1 to journal */
//   int ii;                      /* Loop counter */
//   int needSync = 0;            /* True if any page has PGHDR_NEED_SYNC */
//   Pager *pPager = pPg->pPager; /* The pager that owns pPg */
//   Pgno nPagePerSector = (pPager->sectorSize/pPager->pageSize);

//   /* Set the doNotSpill NOSYNC bit to 1. This is because we cannot allow
//   ** a journal header to be written between the pages journaled by
//   ** this function.
//   */
//   assert( !MEMDB );
//   assert( (pPager->doNotSpill & SPILLFLAG_NOSYNC)==0 );
//   pPager->doNotSpill |= SPILLFLAG_NOSYNC;

//   /* This trick assumes that both the page-size and sector-size are
//   ** an integer power of 2. It sets variable pg1 to the identifier
//   ** of the first page of the sector pPg is located on.
//   */
//   pg1 = ((pPg->pgno-1) & ~(nPagePerSector-1)) + 1;

//   nPageCount = pPager->dbSize;
//   if( pPg->pgno>nPageCount ){
//     nPage = (pPg->pgno - pg1)+1;
//   }else if( (pg1+nPagePerSector-1)>nPageCount ){
//     nPage = nPageCount+1-pg1;
//   }else{
//     nPage = nPagePerSector;
//   }
//   assert(nPage>0);
//   assert(pg1<=pPg->pgno);
//   assert((pg1+nPage)>pPg->pgno);

//   for(ii=0; ii<nPage && rc==SQLITE_OK; ii++){
//     Pgno pg = pg1+ii;
//     PgHdr *pPage;
//     if( pg==pPg->pgno || !sqlite3BitvecTest(pPager->pInJournal, pg) ){
//       if( pg!=PAGER_MJ_PGNO(pPager) ){
//         rc = sqlite3PagerGet(pPager, pg, &pPage, 0);
//         if( rc==SQLITE_OK ){
//           rc = pager_write(pPage);
//           if( pPage->flags&PGHDR_NEED_SYNC ){
//             needSync = 1;
//           }
//           sqlite3PagerUnrefNotNull(pPage);
//         }
//       }
//     }else if( (pPage = sqlite3PagerLookup(pPager, pg))!=0 ){
//       if( pPage->flags&PGHDR_NEED_SYNC ){
//         needSync = 1;
//       }
//       sqlite3PagerUnrefNotNull(pPage);
//     }
//   }

//   /* If the PGHDR_NEED_SYNC flag is set for any of the nPage pages 
//   ** starting at pg1, then it needs to be set for all of them. Because
//   ** writing to any of these nPage pages may damage the others, the
//   ** journal file must contain sync()ed copies of all of them
//   ** before any of them can be written out to the database file.
//   */
//   if( rc==SQLITE_OK && needSync ){
//     assert( !MEMDB );
//     for(ii=0; ii<nPage; ii++){
//       PgHdr *pPage = sqlite3PagerLookup(pPager, pg1+ii);
//       if( pPage ){
//         pPage->flags |= PGHDR_NEED_SYNC;
//         sqlite3PagerUnrefNotNull(pPage);
//       }
//     }
//   }

//   assert( (pPager->doNotSpill & SPILLFLAG_NOSYNC)!=0 );
//   pPager->doNotSpill &= ~SPILLFLAG_NOSYNC;
//   return rc;
// }

// /*
// ** Mark a data page as writeable. This routine must be called before 
// ** making changes to a page. The caller must check the return value 
// ** of this function and be careful not to change any page data unless 
// ** this routine returns SQLITE_OK.
// **
// ** The difference between this function and pager_write() is that this
// ** function also deals with the special case where 2 or more pages
// ** fit on a single disk sector. In this case all co-resident pages
// ** must have been written to the journal file before returning.
// **
// ** If an error occurs, SQLITE_NOMEM or an IO error code is returned
// ** as appropriate. Otherwise, SQLITE_OK.
// */
// int sqlite3PagerWrite(PgHdr *pPg){
//   Pager *pPager = pPg->pPager;
//   assert( (pPg->flags & PGHDR_MMAP)==0 );
//   assert( pPager->eState>=PAGER_WRITER_LOCKED );
//   assert( assert_pager_state(pPager) );
//   if( (pPg->flags & PGHDR_WRITEABLE)!=0 && pPager->dbSize>=pPg->pgno ){
//     if( pPager->nSavepoint ) return subjournalPageIfRequired(pPg);
//     return SQLITE_OK;
//   }else if( pPager->errCode ){
//     return pPager->errCode;
//   }else if( pPager->sectorSize > (u32)pPager->pageSize ){
//     assert( pPager->tempFile==0 );
//     return pagerWriteLargeSector(pPg);
//   }else{
//     return pager_write(pPg);
//   }
// }

// /*
// ** Return TRUE if the page given in the argument was previously passed
// ** to sqlite3PagerWrite().  In other words, return TRUE if it is ok
// ** to change the content of the page.
// */
// #ifndef NDEBUG
// int sqlite3PagerIswriteable(DbPage *pPg){
//   return pPg->flags & PGHDR_WRITEABLE;
// }
// #endif

// /*
// ** A call to this routine tells the pager that it is not necessary to
// ** write the information on page pPg back to the disk, even though
// ** that page might be marked as dirty.  This happens, for example, when
// ** the page has been added as a leaf of the freelist and so its
// ** content no longer matters.
// **
// ** The overlying software layer calls this routine when all of the data
// ** on the given page is unused. The pager marks the page as clean so
// ** that it does not get written to disk.
// **
// ** Tests show that this optimization can quadruple the speed of large 
// ** DELETE operations.
// **
// ** This optimization cannot be used with a temp-file, as the page may
// ** have been dirty at the start of the transaction. In that case, if
// ** memory pressure forces page pPg out of the cache, the data does need 
// ** to be written out to disk so that it may be read back in if the 
// ** current transaction is rolled back.
// */
// void sqlite3PagerDontWrite(PgHdr *pPg){
//   Pager *pPager = pPg->pPager;
//   if( !pPager->tempFile && (pPg->flags&PGHDR_DIRTY) && pPager->nSavepoint==0 ){
//     PAGERTRACE(("DONT_WRITE page %d of %d\n", pPg->pgno, PAGERID(pPager)));
//     IOTRACE(("CLEAN %p %d\n", pPager, pPg->pgno))
//     pPg->flags |= PGHDR_DONT_WRITE;
//     pPg->flags &= ~PGHDR_WRITEABLE;
//     testcase( pPg->flags & PGHDR_NEED_SYNC );
//     pager_set_pagehash(pPg);
//   }
// }

// /*
// ** This routine is called to increment the value of the database file 
// ** change-counter, stored as a 4-byte big-endian integer starting at 
// ** byte offset 24 of the pager file.  The secondary change counter at
// ** 92 is also updated, as is the SQLite version number at offset 96.
// **
// ** But this only happens if the pPager->changeCountDone flag is false.
// ** To avoid excess churning of page 1, the update only happens once.
// ** See also the pager_write_changecounter() routine that does an 
// ** unconditional update of the change counters.
// **
// ** If the isDirectMode flag is zero, then this is done by calling 
// ** sqlite3PagerWrite() on page 1, then modifying the contents of the
// ** page data. In this case the file will be updated when the current
// ** transaction is committed.
// **
// ** The isDirectMode flag may only be non-zero if the library was compiled
// ** with the SQLITE_ENABLE_ATOMIC_WRITE macro defined. In this case,
// ** if isDirect is non-zero, then the database file is updated directly
// ** by writing an updated version of page 1 using a call to the 
// ** sqlite3OsWrite() function.
// */
// static int pager_incr_changecounter(Pager *pPager, int isDirectMode){
//   int rc = SQLITE_OK;

//   assert( pPager->eState==PAGER_WRITER_CACHEMOD
//        || pPager->eState==PAGER_WRITER_DBMOD
//   );
//   assert( assert_pager_state(pPager) );

//   /* Declare and initialize constant integer 'isDirect'. If the
//   ** atomic-write optimization is enabled in this build, then isDirect
//   ** is initialized to the value passed as the isDirectMode parameter
//   ** to this function. Otherwise, it is always set to zero.
//   **
//   ** The idea is that if the atomic-write optimization is not
//   ** enabled at compile time, the compiler can omit the tests of
//   ** 'isDirect' below, as well as the block enclosed in the
//   ** "if( isDirect )" condition.
//   */
// #ifndef SQLITE_ENABLE_ATOMIC_WRITE
// # define DIRECT_MODE 0
//   assert( isDirectMode==0 );
//   UNUSED_PARAMETER(isDirectMode);
// #else
// # define DIRECT_MODE isDirectMode
// #endif

//   if( !pPager->changeCountDone && ALWAYS(pPager->dbSize>0) ){
//     PgHdr *pPgHdr;                /* Reference to page 1 */

//     assert( !pPager->tempFile && isOpen(pPager->fd) );

//     /* Open page 1 of the file for writing. */
//     rc = sqlite3PagerGet(pPager, 1, &pPgHdr, 0);
//     assert( pPgHdr==0 || rc==SQLITE_OK );

//     /* If page one was fetched successfully, and this function is not
//     ** operating in direct-mode, make page 1 writable.  When not in 
//     ** direct mode, page 1 is always held in cache and hence the PagerGet()
//     ** above is always successful - hence the ALWAYS on rc==SQLITE_OK.
//     */
//     if( !DIRECT_MODE && ALWAYS(rc==SQLITE_OK) ){
//       rc = sqlite3PagerWrite(pPgHdr);
//     }

//     if( rc==SQLITE_OK ){
//       /* Actually do the update of the change counter */
//       pager_write_changecounter(pPgHdr);

//       /* If running in direct mode, write the contents of page 1 to the file. */
//       if( DIRECT_MODE ){
//         const void *zBuf;
//         assert( pPager->dbFileSize>0 );
//         zBuf = pPgHdr->pData;
//         if( rc==SQLITE_OK ){
//           rc = sqlite3OsWrite(pPager->fd, zBuf, pPager->pageSize, 0);
//           pPager->aStat[PAGER_STAT_WRITE]++;
//         }
//         if( rc==SQLITE_OK ){
//           /* Update the pager's copy of the change-counter. Otherwise, the
//           ** next time a read transaction is opened the cache will be
//           ** flushed (as the change-counter values will not match).  */
//           const void *pCopy = (const void *)&((const char *)zBuf)[24];
//           memcpy(&pPager->dbFileVers, pCopy, sizeof(pPager->dbFileVers));
//           pPager->changeCountDone = 1;
//         }
//       }else{
//         pPager->changeCountDone = 1;
//       }
//     }

//     /* Release the page reference. */
//     sqlite3PagerUnref(pPgHdr);
//   }
//   return rc;
// }

// /*
// ** Sync the database file to disk. This is a no-op for in-memory databases
// ** or pages with the Pager.noSync flag set.
// **
// ** If successful, or if called on a pager for which it is a no-op, this
// ** function returns SQLITE_OK. Otherwise, an IO error code is returned.
// */
// int sqlite3PagerSync(Pager *pPager, const char *zSuper){
//   int rc = SQLITE_OK;
//   void *pArg = (void*)zSuper;
//   rc = sqlite3OsFileControl(pPager->fd, SQLITE_FCNTL_SYNC, pArg);
//   if( rc==SQLITE_NOTFOUND ) rc = SQLITE_OK;
//   if( rc==SQLITE_OK && !pPager->noSync ){
//     assert( !MEMDB );
//     rc = sqlite3OsSync(pPager->fd, pPager->syncFlags);
//   }
//   return rc;
// }

// /*
// ** This function may only be called while a write-transaction is active in
// ** rollback. If the connection is in WAL mode, this call is a no-op. 
// ** Otherwise, if the connection does not already have an EXCLUSIVE lock on 
// ** the database file, an attempt is made to obtain one.
// **
// ** If the EXCLUSIVE lock is already held or the attempt to obtain it is
// ** successful, or the connection is in WAL mode, SQLITE_OK is returned.
// ** Otherwise, either SQLITE_BUSY or an SQLITE_IOERR_XXX error code is 
// ** returned.
// */
// int sqlite3PagerExclusiveLock(Pager *pPager){
//   int rc = pPager->errCode;
//   assert( assert_pager_state(pPager) );
//   if( rc==SQLITE_OK ){
//     assert( pPager->eState==PAGER_WRITER_CACHEMOD 
//          || pPager->eState==PAGER_WRITER_DBMOD 
//          || pPager->eState==PAGER_WRITER_LOCKED 
//     );
//     assert( assert_pager_state(pPager) );
//     if( 0==pagerUseWal(pPager) ){
//       rc = pager_wait_on_lock(pPager, EXCLUSIVE_LOCK);
//     }
//   }
//   return rc;
// }

// /*
// ** Sync the database file for the pager pPager. zSuper points to the name
// ** of a super-journal file that should be written into the individual
// ** journal file. zSuper may be NULL, which is interpreted as no 
// ** super-journal (a single database transaction).
// **
// ** This routine ensures that:
// **
// **   * The database file change-counter is updated,
// **   * the journal is synced (unless the atomic-write optimization is used),
// **   * all dirty pages are written to the database file, 
// **   * the database file is truncated (if required), and
// **   * the database file synced. 
// **
// ** The only thing that remains to commit the transaction is to finalize 
// ** (delete, truncate or zero the first part of) the journal file (or 
// ** delete the super-journal file if specified).
// **
// ** Note that if zSuper==NULL, this does not overwrite a previous value
// ** passed to an sqlite3PagerCommitPhaseOne() call.
// **
// ** If the final parameter - noSync - is true, then the database file itself
// ** is not synced. The caller must call sqlite3PagerSync() directly to
// ** sync the database file before calling CommitPhaseTwo() to delete the
// ** journal file in this case.
// */
// int sqlite3PagerCommitPhaseOne(
//   Pager *pPager,                  /* Pager object */
//   const char *zSuper,            /* If not NULL, the super-journal name */
//   int noSync                      /* True to omit the xSync on the db file */
// ){
//   int rc = SQLITE_OK;             /* Return code */

//   assert( pPager->eState==PAGER_WRITER_LOCKED
//        || pPager->eState==PAGER_WRITER_CACHEMOD
//        || pPager->eState==PAGER_WRITER_DBMOD
//        || pPager->eState==PAGER_ERROR
//   );
//   assert( assert_pager_state(pPager) );

//   /* If a prior error occurred, report that error again. */
//   if( NEVER(pPager->errCode) ) return pPager->errCode;

//   /* Provide the ability to easily simulate an I/O error during testing */
//   if( sqlite3FaultSim(400) ) return SQLITE_IOERR;

//   PAGERTRACE(("DATABASE SYNC: File=%s zSuper=%s nSize=%d\n", 
//       pPager->zFilename, zSuper, pPager->dbSize));

//   /* If no database changes have been made, return early. */
//   if( pPager->eState<PAGER_WRITER_CACHEMOD ) return SQLITE_OK;

//   assert( MEMDB==0 || pPager->tempFile );
//   assert( isOpen(pPager->fd) || pPager->tempFile );
//   if( 0==pagerFlushOnCommit(pPager, 1) ){
//     /* If this is an in-memory db, or no pages have been written to, or this
//     ** function has already been called, it is mostly a no-op.  However, any
//     ** backup in progress needs to be restarted.  */
//     sqlite3BackupRestart(pPager->pBackup);
//   }else{
//     PgHdr *pList;
//     if( pagerUseWal(pPager) ){
//       PgHdr *pPageOne = 0;
//       pList = sqlite3PcacheDirtyList(pPager->pPCache);
//       if( pList==0 ){
//         /* Must have at least one page for the WAL commit flag.
//         ** Ticket [2d1a5c67dfc2363e44f29d9bbd57f] 2011-05-18 */
//         rc = sqlite3PagerGet(pPager, 1, &pPageOne, 0);
//         pList = pPageOne;
//         pList->pDirty = 0;
//       }
//       assert( rc==SQLITE_OK );
//       if( ALWAYS(pList) ){
//         rc = pagerWalFrames(pPager, pList, pPager->dbSize, 1);
//       }
//       sqlite3PagerUnref(pPageOne);
//       if( rc==SQLITE_OK ){
//         sqlite3PcacheCleanAll(pPager->pPCache);
//       }
//     }else{
//       /* The bBatch boolean is true if the batch-atomic-write commit method
//       ** should be used.  No rollback journal is created if batch-atomic-write
//       ** is enabled.
//       */
// #ifdef SQLITE_ENABLE_BATCH_ATOMIC_WRITE
//       sqlite3_file *fd = pPager->fd;
//       int bBatch = zSuper==0    /* An SQLITE_IOCAP_BATCH_ATOMIC commit */
//         && (sqlite3OsDeviceCharacteristics(fd) & SQLITE_IOCAP_BATCH_ATOMIC)
//         && !pPager->noSync
//         && sqlite3JournalIsInMemory(pPager->jfd);
// #else
// #     define bBatch 0
// #endif

// #ifdef SQLITE_ENABLE_ATOMIC_WRITE
//       /* The following block updates the change-counter. Exactly how it
//       ** does this depends on whether or not the atomic-update optimization
//       ** was enabled at compile time, and if this transaction meets the 
//       ** runtime criteria to use the operation: 
//       **
//       **    * The file-system supports the atomic-write property for
//       **      blocks of size page-size, and 
//       **    * This commit is not part of a multi-file transaction, and
//       **    * Exactly one page has been modified and store in the journal file.
//       **
//       ** If the optimization was not enabled at compile time, then the
//       ** pager_incr_changecounter() function is called to update the change
//       ** counter in 'indirect-mode'. If the optimization is compiled in but
//       ** is not applicable to this transaction, call sqlite3JournalCreate()
//       ** to make sure the journal file has actually been created, then call
//       ** pager_incr_changecounter() to update the change-counter in indirect
//       ** mode. 
//       **
//       ** Otherwise, if the optimization is both enabled and applicable,
//       ** then call pager_incr_changecounter() to update the change-counter
//       ** in 'direct' mode. In this case the journal file will never be
//       ** created for this transaction.
//       */
//       if( bBatch==0 ){
//         PgHdr *pPg;
//         assert( isOpen(pPager->jfd) 
//             || pPager->journalMode==PAGER_JOURNALMODE_OFF 
//             || pPager->journalMode==PAGER_JOURNALMODE_WAL 
//             );
//         if( !zSuper && isOpen(pPager->jfd) 
//          && pPager->journalOff==jrnlBufferSize(pPager) 
//          && pPager->dbSize>=pPager->dbOrigSize
//          && (!(pPg = sqlite3PcacheDirtyList(pPager->pPCache)) || 0==pPg->pDirty)
//         ){
//           /* Update the db file change counter via the direct-write method. The 
//           ** following call will modify the in-memory representation of page 1 
//           ** to include the updated change counter and then write page 1 
//           ** directly to the database file. Because of the atomic-write 
//           ** property of the host file-system, this is safe.
//           */
//           rc = pager_incr_changecounter(pPager, 1);
//         }else{
//           rc = sqlite3JournalCreate(pPager->jfd);
//           if( rc==SQLITE_OK ){
//             rc = pager_incr_changecounter(pPager, 0);
//           }
//         }
//       }
// #else  /* SQLITE_ENABLE_ATOMIC_WRITE */
// #ifdef SQLITE_ENABLE_BATCH_ATOMIC_WRITE
//       if( zSuper ){
//         rc = sqlite3JournalCreate(pPager->jfd);
//         if( rc!=SQLITE_OK ) goto commit_phase_one_exit;
//         assert( bBatch==0 );
//       }
// #endif
//       rc = pager_incr_changecounter(pPager, 0);
// #endif /* !SQLITE_ENABLE_ATOMIC_WRITE */
//       if( rc!=SQLITE_OK ) goto commit_phase_one_exit;
  
//       /* Write the super-journal name into the journal file. If a
//       ** super-journal file name has already been written to the journal file, 
//       ** or if zSuper is NULL (no super-journal), then this call is a no-op.
//       */
//       rc = writeSuperJournal(pPager, zSuper);
//       if( rc!=SQLITE_OK ) goto commit_phase_one_exit;
  
//       /* Sync the journal file and write all dirty pages to the database.
//       ** If the atomic-update optimization is being used, this sync will not 
//       ** create the journal file or perform any real IO.
//       **
//       ** Because the change-counter page was just modified, unless the
//       ** atomic-update optimization is used it is almost certain that the
//       ** journal requires a sync here. However, in locking_mode=exclusive
//       ** on a system under memory pressure it is just possible that this is 
//       ** not the case. In this case it is likely enough that the redundant
//       ** xSync() call will be changed to a no-op by the OS anyhow. 
//       */
//       rc = syncJournal(pPager, 0);
//       if( rc!=SQLITE_OK ) goto commit_phase_one_exit;

//       pList = sqlite3PcacheDirtyList(pPager->pPCache);
// #ifdef SQLITE_ENABLE_BATCH_ATOMIC_WRITE
//       if( bBatch ){
//         rc = sqlite3OsFileControl(fd, SQLITE_FCNTL_BEGIN_ATOMIC_WRITE, 0);
//         if( rc==SQLITE_OK ){
//           rc = pager_write_pagelist(pPager, pList);
//           if( rc==SQLITE_OK ){
//             rc = sqlite3OsFileControl(fd, SQLITE_FCNTL_COMMIT_ATOMIC_WRITE, 0);
//           }
//           if( rc!=SQLITE_OK ){
//             sqlite3OsFileControlHint(fd, SQLITE_FCNTL_ROLLBACK_ATOMIC_WRITE, 0);
//           }
//         }

//         if( (rc&0xFF)==SQLITE_IOERR && rc!=SQLITE_IOERR_NOMEM ){
//           rc = sqlite3JournalCreate(pPager->jfd);
//           if( rc!=SQLITE_OK ){
//             sqlite3OsClose(pPager->jfd);
//             goto commit_phase_one_exit;
//           }
//           bBatch = 0;
//         }else{
//           sqlite3OsClose(pPager->jfd);
//         }
//       }
// #endif /* SQLITE_ENABLE_BATCH_ATOMIC_WRITE */

//       if( bBatch==0 ){
//         rc = pager_write_pagelist(pPager, pList);
//       }
//       if( rc!=SQLITE_OK ){
//         assert( rc!=SQLITE_IOERR_BLOCKED );
//         goto commit_phase_one_exit;
//       }
//       sqlite3PcacheCleanAll(pPager->pPCache);

//       /* If the file on disk is smaller than the database image, use 
//       ** pager_truncate to grow the file here. This can happen if the database
//       ** image was extended as part of the current transaction and then the
//       ** last page in the db image moved to the free-list. In this case the
//       ** last page is never written out to disk, leaving the database file
//       ** undersized. Fix this now if it is the case.  */
//       if( pPager->dbSize>pPager->dbFileSize ){
//         Pgno nNew = pPager->dbSize - (pPager->dbSize==PAGER_MJ_PGNO(pPager));
//         assert( pPager->eState==PAGER_WRITER_DBMOD );
//         rc = pager_truncate(pPager, nNew);
//         if( rc!=SQLITE_OK ) goto commit_phase_one_exit;
//       }
  
//       /* Finally, sync the database file. */
//       if( !noSync ){
//         rc = sqlite3PagerSync(pPager, zSuper);
//       }
//       IOTRACE(("DBSYNC %p\n", pPager))
//     }
//   }

// commit_phase_one_exit:
//   if( rc==SQLITE_OK && !pagerUseWal(pPager) ){
//     pPager->eState = PAGER_WRITER_FINISHED;
//   }
//   return rc;
// }


// /*
// ** When this function is called, the database file has been completely
// ** updated to reflect the changes made by the current transaction and
// ** synced to disk. The journal file still exists in the file-system 
// ** though, and if a failure occurs at this point it will eventually
// ** be used as a hot-journal and the current transaction rolled back.
// **
// ** This function finalizes the journal file, either by deleting, 
// ** truncating or partially zeroing it, so that it cannot be used 
// ** for hot-journal rollback. Once this is done the transaction is
// ** irrevocably committed.
// **
// ** If an error occurs, an IO error code is returned and the pager
// ** moves into the error state. Otherwise, SQLITE_OK is returned.
// */
// int sqlite3PagerCommitPhaseTwo(Pager *pPager){
//   int rc = SQLITE_OK;                  /* Return code */

//   /* This routine should not be called if a prior error has occurred.
//   ** But if (due to a coding error elsewhere in the system) it does get
//   ** called, just return the same error code without doing anything. */
//   if( NEVER(pPager->errCode) ) return pPager->errCode;
//   pPager->iDataVersion++;

//   assert( pPager->eState==PAGER_WRITER_LOCKED
//        || pPager->eState==PAGER_WRITER_FINISHED
//        || (pagerUseWal(pPager) && pPager->eState==PAGER_WRITER_CACHEMOD)
//   );
//   assert( assert_pager_state(pPager) );

//   /* An optimization. If the database was not actually modified during
//   ** this transaction, the pager is running in exclusive-mode and is
//   ** using persistent journals, then this function is a no-op.
//   **
//   ** The start of the journal file currently contains a single journal 
//   ** header with the nRec field set to 0. If such a journal is used as
//   ** a hot-journal during hot-journal rollback, 0 changes will be made
//   ** to the database file. So there is no need to zero the journal 
//   ** header. Since the pager is in exclusive mode, there is no need
//   ** to drop any locks either.
//   */
//   if( pPager->eState==PAGER_WRITER_LOCKED 
//    && pPager->exclusiveMode 
//    && pPager->journalMode==PAGER_JOURNALMODE_PERSIST
//   ){
//     assert( pPager->journalOff==JOURNAL_HDR_SZ(pPager) || !pPager->journalOff );
//     pPager->eState = PAGER_READER;
//     return SQLITE_OK;
//   }

//   PAGERTRACE(("COMMIT %d\n", PAGERID(pPager)));
//   rc = pager_end_transaction(pPager, pPager->setSuper, 1);
//   return pager_error(pPager, rc);
// }

// /*
// ** If a write transaction is open, then all changes made within the 
// ** transaction are reverted and the current write-transaction is closed.
// ** The pager falls back to PAGER_READER state if successful, or PAGER_ERROR
// ** state if an error occurs.
// **
// ** If the pager is already in PAGER_ERROR state when this function is called,
// ** it returns Pager.errCode immediately. No work is performed in this case.
// **
// ** Otherwise, in rollback mode, this function performs two functions:
// **
// **   1) It rolls back the journal file, restoring all database file and 
// **      in-memory cache pages to the state they were in when the transaction
// **      was opened, and
// **
// **   2) It finalizes the journal file, so that it is not used for hot
// **      rollback at any point in the future.
// **
// ** Finalization of the journal file (task 2) is only performed if the 
// ** rollback is successful.
// **
// ** In WAL mode, all cache-entries containing data modified within the
// ** current transaction are either expelled from the cache or reverted to
// ** their pre-transaction state by re-reading data from the database or
// ** WAL files. The WAL transaction is then closed.
// */
// int sqlite3PagerRollback(Pager *pPager){
//   int rc = SQLITE_OK;                  /* Return code */
//   PAGERTRACE(("ROLLBACK %d\n", PAGERID(pPager)));

//   /* PagerRollback() is a no-op if called in READER or OPEN state. If
//   ** the pager is already in the ERROR state, the rollback is not 
//   ** attempted here. Instead, the error code is returned to the caller.
//   */
//   assert( assert_pager_state(pPager) );
//   if( pPager->eState==PAGER_ERROR ) return pPager->errCode;
//   if( pPager->eState<=PAGER_READER ) return SQLITE_OK;

//   if( pagerUseWal(pPager) ){
//     int rc2;
//     rc = sqlite3PagerSavepoint(pPager, SAVEPOINT_ROLLBACK, -1);
//     rc2 = pager_end_transaction(pPager, pPager->setSuper, 0);
//     if( rc==SQLITE_OK ) rc = rc2;
//   }else if( !isOpen(pPager->jfd) || pPager->eState==PAGER_WRITER_LOCKED ){
//     int eState = pPager->eState;
//     rc = pager_end_transaction(pPager, 0, 0);
//     if( !MEMDB && eState>PAGER_WRITER_LOCKED ){
//       /* This can happen using journal_mode=off. Move the pager to the error 
//       ** state to indicate that the contents of the cache may not be trusted.
//       ** Any active readers will get SQLITE_ABORT.
//       */
//       pPager->errCode = SQLITE_ABORT;
//       pPager->eState = PAGER_ERROR;
//       setGetterMethod(pPager);
//       return rc;
//     }
//   }else{
//     rc = pager_playback(pPager, 0);
//   }

//   assert( pPager->eState==PAGER_READER || rc!=SQLITE_OK );
//   assert( rc==SQLITE_OK || rc==SQLITE_FULL || rc==SQLITE_CORRUPT
//           || rc==SQLITE_NOMEM || (rc&0xFF)==SQLITE_IOERR 
//           || rc==SQLITE_CANTOPEN
//   );

//   /* If an error occurs during a ROLLBACK, we can no longer trust the pager
//   ** cache. So call pager_error() on the way out to make any error persistent.
//   */
//   return pager_error(pPager, rc);
// }

// /*
// ** Return TRUE if the database file is opened read-only.  Return FALSE
// ** if the database is (in theory) writable.
// */
// u8 sqlite3PagerIsreadonly(Pager *pPager){
//   return pPager->readOnly;
// }

// #ifdef SQLITE_DEBUG
// /*
// ** Return the sum of the reference counts for all pages held by pPager.
// */
// int sqlite3PagerRefcount(Pager *pPager){
//   return sqlite3PcacheRefCount(pPager->pPCache);
// }
// #endif

// /*
// ** Return the approximate number of bytes of memory currently
// ** used by the pager and its associated cache.
// */
// int sqlite3PagerMemUsed(Pager *pPager){
//   int perPageSize = pPager->pageSize + pPager->nExtra
//     + (int)(sizeof(PgHdr) + 5*sizeof(void*));
//   return perPageSize*sqlite3PcachePagecount(pPager->pPCache)
//            + sqlite3MallocSize(pPager)
//            + pPager->pageSize;
// }

// /*
// ** Return the number of references to the specified page.
// */
// int sqlite3PagerPageRefcount(DbPage *pPage){
//   return sqlite3PcachePageRefcount(pPage);
// }

// #ifdef SQLITE_TEST
// /*
// ** This routine is used for testing and analysis only.
// */
// int *sqlite3PagerStats(Pager *pPager){
//   static int a[11];
//   a[0] = sqlite3PcacheRefCount(pPager->pPCache);
//   a[1] = sqlite3PcachePagecount(pPager->pPCache);
//   a[2] = sqlite3PcacheGetCachesize(pPager->pPCache);
//   a[3] = pPager->eState==PAGER_OPEN ? -1 : (int) pPager->dbSize;
//   a[4] = pPager->eState;
//   a[5] = pPager->errCode;
//   a[6] = pPager->aStat[PAGER_STAT_HIT];
//   a[7] = pPager->aStat[PAGER_STAT_MISS];
//   a[8] = 0;  /* Used to be pPager->nOvfl */
//   a[9] = pPager->nRead;
//   a[10] = pPager->aStat[PAGER_STAT_WRITE];
//   return a;
// }
// #endif

// /*
// ** Parameter eStat must be one of SQLITE_DBSTATUS_CACHE_HIT, _MISS, _WRITE,
// ** or _WRITE+1.  The SQLITE_DBSTATUS_CACHE_WRITE+1 case is a translation
// ** of SQLITE_DBSTATUS_CACHE_SPILL.  The _SPILL case is not contiguous because
// ** it was added later.
// **
// ** Before returning, *pnVal is incremented by the
// ** current cache hit or miss count, according to the value of eStat. If the 
// ** reset parameter is non-zero, the cache hit or miss count is zeroed before 
// ** returning.
// */
// void sqlite3PagerCacheStat(Pager *pPager, int eStat, int reset, int *pnVal){

//   assert( eStat==SQLITE_DBSTATUS_CACHE_HIT
//        || eStat==SQLITE_DBSTATUS_CACHE_MISS
//        || eStat==SQLITE_DBSTATUS_CACHE_WRITE
//        || eStat==SQLITE_DBSTATUS_CACHE_WRITE+1
//   );

//   assert( SQLITE_DBSTATUS_CACHE_HIT+1==SQLITE_DBSTATUS_CACHE_MISS );
//   assert( SQLITE_DBSTATUS_CACHE_HIT+2==SQLITE_DBSTATUS_CACHE_WRITE );
//   assert( PAGER_STAT_HIT==0 && PAGER_STAT_MISS==1
//            && PAGER_STAT_WRITE==2 && PAGER_STAT_SPILL==3 );

//   eStat -= SQLITE_DBSTATUS_CACHE_HIT;
//   *pnVal += pPager->aStat[eStat];
//   if( reset ){
//     pPager->aStat[eStat] = 0;
//   }
// }

// /*
// ** Return true if this is an in-memory or temp-file backed pager.
// */
// int sqlite3PagerIsMemdb(Pager *pPager){
//   return pPager->tempFile || pPager->memVfs;
// }

// /*
// ** Check that there are at least nSavepoint savepoints open. If there are
// ** currently less than nSavepoints open, then open one or more savepoints
// ** to make up the difference. If the number of savepoints is already
// ** equal to nSavepoint, then this function is a no-op.
// **
// ** If a memory allocation fails, SQLITE_NOMEM is returned. If an error 
// ** occurs while opening the sub-journal file, then an IO error code is
// ** returned. Otherwise, SQLITE_OK.
// */
// static SQLITE_NOINLINE int pagerOpenSavepoint(Pager *pPager, int nSavepoint){
//   int rc = SQLITE_OK;                       /* Return code */
//   int nCurrent = pPager->nSavepoint;        /* Current number of savepoints */
//   int ii;                                   /* Iterator variable */
//   PagerSavepoint *aNew;                     /* New Pager.aSavepoint array */

//   assert( pPager->eState>=PAGER_WRITER_LOCKED );
//   assert( assert_pager_state(pPager) );
//   assert( nSavepoint>nCurrent && pPager->useJournal );

//   /* Grow the Pager.aSavepoint array using realloc(). Return SQLITE_NOMEM
//   ** if the allocation fails. Otherwise, zero the new portion in case a 
//   ** malloc failure occurs while populating it in the for(...) loop below.
//   */
//   aNew = (PagerSavepoint *)sqlite3Realloc(
//       pPager->aSavepoint, sizeof(PagerSavepoint)*nSavepoint
//   );
//   if( !aNew ){
//     return SQLITE_NOMEM;
//   }
//   memset(&aNew[nCurrent], 0, (nSavepoint-nCurrent) * sizeof(PagerSavepoint));
//   pPager->aSavepoint = aNew;

//   /* Populate the PagerSavepoint structures just allocated. */
//   for(ii=nCurrent; ii<nSavepoint; ii++){
//     aNew[ii].nOrig = pPager->dbSize;
//     if( isOpen(pPager->jfd) && pPager->journalOff>0 ){
//       aNew[ii].iOffset = pPager->journalOff;
//     }else{
//       aNew[ii].iOffset = JOURNAL_HDR_SZ(pPager);
//     }
//     aNew[ii].iSubRec = pPager->nSubRec;
//     aNew[ii].pInSavepoint = sqlite3BitvecCreate(pPager->dbSize);
//     aNew[ii].bTruncateOnRelease = 1;
//     if( !aNew[ii].pInSavepoint ){
//       return SQLITE_NOMEM;
//     }
//     if( pagerUseWal(pPager) ){
//       sqlite3WalSavepoint(pPager->pWal, aNew[ii].aWalData);
//     }
//     pPager->nSavepoint = ii+1;
//   }
//   assert( pPager->nSavepoint==nSavepoint );
//   assertTruncateConstraint(pPager);
//   return rc;
// }
// int sqlite3PagerOpenSavepoint(Pager *pPager, int nSavepoint){
//   assert( pPager->eState>=PAGER_WRITER_LOCKED );
//   assert( assert_pager_state(pPager) );

//   if( nSavepoint>pPager->nSavepoint && pPager->useJournal ){
//     return pagerOpenSavepoint(pPager, nSavepoint);
//   }else{
//     return SQLITE_OK;
//   }
// }


// /*
// ** This function is called to rollback or release (commit) a savepoint.
// ** The savepoint to release or rollback need not be the most recently 
// ** created savepoint.
// **
// ** Parameter op is always either SAVEPOINT_ROLLBACK or SAVEPOINT_RELEASE.
// ** If it is SAVEPOINT_RELEASE, then release and destroy the savepoint with
// ** index iSavepoint. If it is SAVEPOINT_ROLLBACK, then rollback all changes
// ** that have occurred since the specified savepoint was created.
// **
// ** The savepoint to rollback or release is identified by parameter 
// ** iSavepoint. A value of 0 means to operate on the outermost savepoint
// ** (the first created). A value of (Pager.nSavepoint-1) means operate
// ** on the most recently created savepoint. If iSavepoint is greater than
// ** (Pager.nSavepoint-1), then this function is a no-op.
// **
// ** If a negative value is passed to this function, then the current
// ** transaction is rolled back. This is different to calling 
// ** sqlite3PagerRollback() because this function does not terminate
// ** the transaction or unlock the database, it just restores the 
// ** contents of the database to its original state. 
// **
// ** In any case, all savepoints with an index greater than iSavepoint 
// ** are destroyed. If this is a release operation (op==SAVEPOINT_RELEASE),
// ** then savepoint iSavepoint is also destroyed.
// **
// ** This function may return SQLITE_NOMEM if a memory allocation fails,
// ** or an IO error code if an IO error occurs while rolling back a 
// ** savepoint. If no errors occur, SQLITE_OK is returned.
// */ 
// int sqlite3PagerSavepoint(Pager *pPager, int op, int iSavepoint){
//   int rc = pPager->errCode;
  
// #ifdef SQLITE_ENABLE_ZIPVFS
//   if( op==SAVEPOINT_RELEASE ) rc = SQLITE_OK;
// #endif

//   assert( op==SAVEPOINT_RELEASE || op==SAVEPOINT_ROLLBACK );
//   assert( iSavepoint>=0 || op==SAVEPOINT_ROLLBACK );

//   if( rc==SQLITE_OK && iSavepoint<pPager->nSavepoint ){
//     int ii;            /* Iterator variable */
//     int nNew;          /* Number of remaining savepoints after this op. */

//     /* Figure out how many savepoints will still be active after this
//     ** operation. Store this value in nNew. Then free resources associated 
//     ** with any savepoints that are destroyed by this operation.
//     */
//     nNew = iSavepoint + (( op==SAVEPOINT_RELEASE ) ? 0 : 1);
//     for(ii=nNew; ii<pPager->nSavepoint; ii++){
//       sqlite3BitvecDestroy(pPager->aSavepoint[ii].pInSavepoint);
//     }
//     pPager->nSavepoint = nNew;

//     /* Truncate the sub-journal so that it only includes the parts
//     ** that are still in use. */
//     if( op==SAVEPOINT_RELEASE ){
//       PagerSavepoint *pRel = &pPager->aSavepoint[nNew];
//       if( pRel->bTruncateOnRelease && isOpen(pPager->sjfd) ){
//         /* Only truncate if it is an in-memory sub-journal. */
//         if( sqlite3JournalIsInMemory(pPager->sjfd) ){
//           i64 sz = (pPager->pageSize+4)*(i64)pRel->iSubRec;
//           rc = sqlite3OsTruncate(pPager->sjfd, sz);
//           assert( rc==SQLITE_OK );
//         }
//         pPager->nSubRec = pRel->iSubRec;
//       }
//     }
//     /* Else this is a rollback operation, playback the specified savepoint.
//     ** If this is a temp-file, it is possible that the journal file has
//     ** not yet been opened. In this case there have been no changes to
//     ** the database file, so the playback operation can be skipped.
//     */
//     else if( pagerUseWal(pPager) || isOpen(pPager->jfd) ){
//       PagerSavepoint *pSavepoint = (nNew==0)?0:&pPager->aSavepoint[nNew-1];
//       rc = pagerPlaybackSavepoint(pPager, pSavepoint);
//       assert(rc!=SQLITE_DONE);
//     }
    
// #ifdef SQLITE_ENABLE_ZIPVFS
//     /* If the cache has been modified but the savepoint cannot be rolled 
//     ** back journal_mode=off, put the pager in the error state. This way,
//     ** if the VFS used by this pager includes ZipVFS, the entire transaction
//     ** can be rolled back at the ZipVFS level.  */
//     else if( 
//         pPager->journalMode==PAGER_JOURNALMODE_OFF 
//      && pPager->eState>=PAGER_WRITER_CACHEMOD
//     ){
//       pPager->errCode = SQLITE_ABORT;
//       pPager->eState = PAGER_ERROR;
//       setGetterMethod(pPager);
//     }
// #endif
//   }

//   return rc;
// }

// /*
// ** Return the full pathname of the database file.
// **
// ** Except, if the pager is in-memory only, then return an empty string if
// ** nullIfMemDb is true.  This routine is called with nullIfMemDb==1 when
// ** used to report the filename to the user, for compatibility with legacy
// ** behavior.  But when the Btree needs to know the filename for matching to
// ** shared cache, it uses nullIfMemDb==0 so that in-memory databases can
// ** participate in shared-cache.
// **
// ** The return value to this routine is always safe to use with
// ** sqlite3_uri_parameter() and sqlite3_filename_database() and friends.
// */
// const char *sqlite3PagerFilename(const Pager *pPager, int nullIfMemDb){
//   static const char zFake[8] = { 0, 0, 0, 0, 0, 0, 0, 0 };
//   return (nullIfMemDb && pPager->memDb) ? &zFake[4] : pPager->zFilename;
// }

// /*
// ** Return the VFS structure for the pager.
// */
// sqlite3_vfs *sqlite3PagerVfs(Pager *pPager){
//   return pPager->pVfs;
// }

// /*
// ** Return the file handle for the database file associated
// ** with the pager.  This might return NULL if the file has
// ** not yet been opened.
// */
// sqlite3_file *sqlite3PagerFile(Pager *pPager){
//   return pPager->fd;
// }

// /*
// ** Return the file handle for the journal file (if it exists).
// ** This will be either the rollback journal or the WAL file.
// */
// sqlite3_file *sqlite3PagerJrnlFile(Pager *pPager){
// #if SQLITE_OMIT_WAL
//   return pPager->jfd;
// #else
//   return pPager->pWal ? sqlite3WalFile(pPager->pWal) : pPager->jfd;
// #endif
// }

// /*
// ** Return the full pathname of the journal file.
// */
// const char *sqlite3PagerJournalname(Pager *pPager){
//   return pPager->zJournal;
// }

// #ifndef SQLITE_OMIT_AUTOVACUUM
// /*
// ** Move the page pPg to location pgno in the file.
// **
// ** There must be no references to the page previously located at
// ** pgno (which we call pPgOld) though that page is allowed to be
// ** in cache.  If the page previously located at pgno is not already
// ** in the rollback journal, it is not put there by by this routine.
// **
// ** References to the page pPg remain valid. Updating any
// ** meta-data associated with pPg (i.e. data stored in the nExtra bytes
// ** allocated along with the page) is the responsibility of the caller.
// **
// ** A transaction must be active when this routine is called. It used to be
// ** required that a statement transaction was not active, but this restriction
// ** has been removed (CREATE INDEX needs to move a page when a statement
// ** transaction is active).
// **
// ** If the fourth argument, isCommit, is non-zero, then this page is being
// ** moved as part of a database reorganization just before the transaction 
// ** is being committed. In this case, it is guaranteed that the database page 
// ** pPg refers to will not be written to again within this transaction.
// **
// ** This function may return SQLITE_NOMEM or an IO error code if an error
// ** occurs. Otherwise, it returns SQLITE_OK.
// */
// int sqlite3PagerMovepage(Pager *pPager, DbPage *pPg, Pgno pgno, int isCommit){
//   PgHdr *pPgOld;               /* The page being overwritten. */
//   Pgno needSyncPgno = 0;       /* Old value of pPg->pgno, if sync is required */
//   int rc;                      /* Return code */
//   Pgno origPgno;               /* The original page number */

//   assert( pPg->nRef>0 );
//   assert( pPager->eState==PAGER_WRITER_CACHEMOD
//        || pPager->eState==PAGER_WRITER_DBMOD
//   );
//   assert( assert_pager_state(pPager) );

//   /* In order to be able to rollback, an in-memory database must journal
//   ** the page we are moving from.
//   */
//   assert( pPager->tempFile || !MEMDB );
//   if( pPager->tempFile ){
//     rc = sqlite3PagerWrite(pPg);
//     if( rc ) return rc;
//   }

//   /* If the page being moved is dirty and has not been saved by the latest
//   ** savepoint, then save the current contents of the page into the 
//   ** sub-journal now. This is required to handle the following scenario:
//   **
//   **   BEGIN;
//   **     <journal page X, then modify it in memory>
//   **     SAVEPOINT one;
//   **       <Move page X to location Y>
//   **     ROLLBACK TO one;
//   **
//   ** If page X were not written to the sub-journal here, it would not
//   ** be possible to restore its contents when the "ROLLBACK TO one"
//   ** statement were is processed.
//   **
//   ** subjournalPage() may need to allocate space to store pPg->pgno into
//   ** one or more savepoint bitvecs. This is the reason this function
//   ** may return SQLITE_NOMEM.
//   */
//   if( (pPg->flags & PGHDR_DIRTY)!=0
//    && SQLITE_OK!=(rc = subjournalPageIfRequired(pPg))
//   ){
//     return rc;
//   }

//   PAGERTRACE(("MOVE %d page %d (needSync=%d) moves to %d\n", 
//       PAGERID(pPager), pPg->pgno, (pPg->flags&PGHDR_NEED_SYNC)?1:0, pgno));
//   IOTRACE(("MOVE %p %d %d\n", pPager, pPg->pgno, pgno))

//   /* If the journal needs to be sync()ed before page pPg->pgno can
//   ** be written to, store pPg->pgno in local variable needSyncPgno.
//   **
//   ** If the isCommit flag is set, there is no need to remember that
//   ** the journal needs to be sync()ed before database page pPg->pgno 
//   ** can be written to. The caller has already promised not to write to it.
//   */
//   if( (pPg->flags&PGHDR_NEED_SYNC) && !isCommit ){
//     needSyncPgno = pPg->pgno;
//     assert( pPager->journalMode==PAGER_JOURNALMODE_OFF ||
//             pageInJournal(pPager, pPg) || pPg->pgno>pPager->dbOrigSize );
//     assert( pPg->flags&PGHDR_DIRTY );
//   }

//   /* If the cache contains a page with page-number pgno, remove it
//   ** from its hash chain. Also, if the PGHDR_NEED_SYNC flag was set for 
//   ** page pgno before the 'move' operation, it needs to be retained 
//   ** for the page moved there.
//   */
//   pPg->flags &= ~PGHDR_NEED_SYNC;
//   pPgOld = sqlite3PagerLookup(pPager, pgno);
//   assert( !pPgOld || pPgOld->nRef==1 || CORRUPT_DB );
//   if( pPgOld ){
//     if( NEVER(pPgOld->nRef>1) ){
//       sqlite3PagerUnrefNotNull(pPgOld);
//       return SQLITE_CORRUPT_BKPT;
//     }
//     pPg->flags |= (pPgOld->flags&PGHDR_NEED_SYNC);
//     if( pPager->tempFile ){
//       /* Do not discard pages from an in-memory database since we might
//       ** need to rollback later.  Just move the page out of the way. */
//       sqlite3PcacheMove(pPgOld, pPager->dbSize+1);
//     }else{
//       sqlite3PcacheDrop(pPgOld);
//     }
//   }

//   origPgno = pPg->pgno;
//   sqlite3PcacheMove(pPg, pgno);
//   sqlite3PcacheMakeDirty(pPg);

//   /* For an in-memory database, make sure the original page continues
//   ** to exist, in case the transaction needs to roll back.  Use pPgOld
//   ** as the original page since it has already been allocated.
//   */
//   if( pPager->tempFile && pPgOld ){
//     sqlite3PcacheMove(pPgOld, origPgno);
//     sqlite3PagerUnrefNotNull(pPgOld);
//   }

//   if( needSyncPgno ){
//     /* If needSyncPgno is non-zero, then the journal file needs to be 
//     ** sync()ed before any data is written to database file page needSyncPgno.
//     ** Currently, no such page exists in the page-cache and the 
//     ** "is journaled" bitvec flag has been set. This needs to be remedied by
//     ** loading the page into the pager-cache and setting the PGHDR_NEED_SYNC
//     ** flag.
//     **
//     ** If the attempt to load the page into the page-cache fails, (due
//     ** to a malloc() or IO failure), clear the bit in the pInJournal[]
//     ** array. Otherwise, if the page is loaded and written again in
//     ** this transaction, it may be written to the database file before
//     ** it is synced into the journal file. This way, it may end up in
//     ** the journal file twice, but that is not a problem.
//     */
//     PgHdr *pPgHdr;
//     rc = sqlite3PagerGet(pPager, needSyncPgno, &pPgHdr, 0);
//     if( rc!=SQLITE_OK ){
//       if( needSyncPgno<=pPager->dbOrigSize ){
//         assert( pPager->pTmpSpace!=0 );
//         sqlite3BitvecClear(pPager->pInJournal, needSyncPgno, pPager->pTmpSpace);
//       }
//       return rc;
//     }
//     pPgHdr->flags |= PGHDR_NEED_SYNC;
//     sqlite3PcacheMakeDirty(pPgHdr);
//     sqlite3PagerUnrefNotNull(pPgHdr);
//   }

//   return SQLITE_OK;
// }
// #endif

// /*
// ** The page handle passed as the first argument refers to a dirty page 
// ** with a page number other than iNew. This function changes the page's 
// ** page number to iNew and sets the value of the PgHdr.flags field to 
// ** the value passed as the third parameter.
// */
// void sqlite3PagerRekey(DbPage *pPg, Pgno iNew, u16 flags){
//   assert( pPg->pgno!=iNew );
//   pPg->flags = flags;
//   sqlite3PcacheMove(pPg, iNew);
// }

// /*
// ** Return a pointer to the data for the specified page.
// */
// void *sqlite3PagerGetData(DbPage *pPg){
//   assert( pPg->nRef>0 || pPg->pPager->memDb );
//   return pPg->pData;
// }

// /*
// ** Return a pointer to the Pager.nExtra bytes of "extra" space 
// ** allocated along with the specified page.
// */
// void *sqlite3PagerGetExtra(DbPage *pPg){
//   return pPg->pExtra;
// }

// /*
// ** Get/set the locking-mode for this pager. Parameter eMode must be one
// ** of PAGER_LOCKINGMODE_QUERY, PAGER_LOCKINGMODE_NORMAL or 
// ** PAGER_LOCKINGMODE_EXCLUSIVE. If the parameter is not _QUERY, then
// ** the locking-mode is set to the value specified.
// **
// ** The returned value is either PAGER_LOCKINGMODE_NORMAL or
// ** PAGER_LOCKINGMODE_EXCLUSIVE, indicating the current (possibly updated)
// ** locking-mode.
// */
// int sqlite3PagerLockingMode(Pager *pPager, int eMode){
//   assert( eMode==PAGER_LOCKINGMODE_QUERY
//             || eMode==PAGER_LOCKINGMODE_NORMAL
//             || eMode==PAGER_LOCKINGMODE_EXCLUSIVE );
//   assert( PAGER_LOCKINGMODE_QUERY<0 );
//   assert( PAGER_LOCKINGMODE_NORMAL>=0 && PAGER_LOCKINGMODE_EXCLUSIVE>=0 );
//   assert( pPager->exclusiveMode || 0==sqlite3WalHeapMemory(pPager->pWal) );
//   if( eMode>=0 && !pPager->tempFile && !sqlite3WalHeapMemory(pPager->pWal) ){
//     pPager->exclusiveMode = (u8)eMode;
//   }
//   return (int)pPager->exclusiveMode;
// }

// /*
// ** Set the journal-mode for this pager. Parameter eMode must be one of:
// **
// **    PAGER_JOURNALMODE_DELETE
// **    PAGER_JOURNALMODE_TRUNCATE
// **    PAGER_JOURNALMODE_PERSIST
// **    PAGER_JOURNALMODE_OFF
// **    PAGER_JOURNALMODE_MEMORY
// **    PAGER_JOURNALMODE_WAL
// **
// ** The journalmode is set to the value specified if the change is allowed.
// ** The change may be disallowed for the following reasons:
// **
// **   *  An in-memory database can only have its journal_mode set to _OFF
// **      or _MEMORY.
// **
// **   *  Temporary databases cannot have _WAL journalmode.
// **
// ** The returned indicate the current (possibly updated) journal-mode.
// */
// int sqlite3PagerSetJournalMode(Pager *pPager, int eMode){
//   u8 eOld = pPager->journalMode;    /* Prior journalmode */

//   /* The eMode parameter is always valid */
//   assert(      eMode==PAGER_JOURNALMODE_DELETE
//             || eMode==PAGER_JOURNALMODE_TRUNCATE
//             || eMode==PAGER_JOURNALMODE_PERSIST
//             || eMode==PAGER_JOURNALMODE_OFF 
//             || eMode==PAGER_JOURNALMODE_WAL 
//             || eMode==PAGER_JOURNALMODE_MEMORY );

//   /* This routine is only called from the OP_JournalMode opcode, and
//   ** the logic there will never allow a temporary file to be changed
//   ** to WAL mode.
//   */
//   assert( pPager->tempFile==0 || eMode!=PAGER_JOURNALMODE_WAL );

//   /* Do allow the journalmode of an in-memory database to be set to
//   ** anything other than MEMORY or OFF
//   */
//   if( MEMDB ){
//     assert( eOld==PAGER_JOURNALMODE_MEMORY || eOld==PAGER_JOURNALMODE_OFF );
//     if( eMode!=PAGER_JOURNALMODE_MEMORY && eMode!=PAGER_JOURNALMODE_OFF ){
//       eMode = eOld;
//     }
//   }

//   if( eMode!=eOld ){

//     /* Change the journal mode. */
//     assert( pPager->eState!=PAGER_ERROR );
//     pPager->journalMode = (u8)eMode;

//     /* When transistioning from TRUNCATE or PERSIST to any other journal
//     ** mode except WAL, unless the pager is in locking_mode=exclusive mode,
//     ** delete the journal file.
//     */
//     assert( (PAGER_JOURNALMODE_TRUNCATE & 5)==1 );
//     assert( (PAGER_JOURNALMODE_PERSIST & 5)==1 );
//     assert( (PAGER_JOURNALMODE_DELETE & 5)==0 );
//     assert( (PAGER_JOURNALMODE_MEMORY & 5)==4 );
//     assert( (PAGER_JOURNALMODE_OFF & 5)==0 );
//     assert( (PAGER_JOURNALMODE_WAL & 5)==5 );

//     assert( isOpen(pPager->fd) || pPager->exclusiveMode );
//     if( !pPager->exclusiveMode && (eOld & 5)==1 && (eMode & 1)==0 ){

//       /* In this case we would like to delete the journal file. If it is
//       ** not possible, then that is not a problem. Deleting the journal file
//       ** here is an optimization only.
//       **
//       ** Before deleting the journal file, obtain a RESERVED lock on the
//       ** database file. This ensures that the journal file is not deleted
//       ** while it is in use by some other client.
//       */
//       sqlite3OsClose(pPager->jfd);
//       if( pPager->eLock>=RESERVED_LOCK ){
//         sqlite3OsDelete(pPager->pVfs, pPager->zJournal, 0);
//       }else{
//         int rc = SQLITE_OK;
//         int state = pPager->eState;
//         assert( state==PAGER_OPEN || state==PAGER_READER );
//         if( state==PAGER_OPEN ){
//           rc = sqlite3PagerSharedLock(pPager);
//         }
//         if( pPager->eState==PAGER_READER ){
//           assert( rc==SQLITE_OK );
//           rc = pagerLockDb(pPager, RESERVED_LOCK);
//         }
//         if( rc==SQLITE_OK ){
//           sqlite3OsDelete(pPager->pVfs, pPager->zJournal, 0);
//         }
//         if( rc==SQLITE_OK && state==PAGER_READER ){
//           pagerUnlockDb(pPager, SHARED_LOCK);
//         }else if( state==PAGER_OPEN ){
//           pager_unlock(pPager);
//         }
//         assert( state==pPager->eState );
//       }
//     }else if( eMode==PAGER_JOURNALMODE_OFF ){
//       sqlite3OsClose(pPager->jfd);
//     }
//   }

//   /* Return the new journal mode */
//   return (int)pPager->journalMode;
// }

// /*
// ** Return the current journal mode.
// */
// int sqlite3PagerGetJournalMode(Pager *pPager){
//   return (int)pPager->journalMode;
// }

// /*
// ** Return TRUE if the pager is in a state where it is OK to change the
// ** journalmode.  Journalmode changes can only happen when the database
// ** is unmodified.
// */
// int sqlite3PagerOkToChangeJournalMode(Pager *pPager){
//   assert( assert_pager_state(pPager) );
//   if( pPager->eState>=PAGER_WRITER_CACHEMOD ) return 0;
//   if( NEVER(isOpen(pPager->jfd) && pPager->journalOff>0) ) return 0;
//   return 1;
// }

// /*
// ** Get/set the size-limit used for persistent journal files.
// **
// ** Setting the size limit to -1 means no limit is enforced.
// ** An attempt to set a limit smaller than -1 is a no-op.
// */
// i64 sqlite3PagerJournalSizeLimit(Pager *pPager, i64 iLimit){
//   if( iLimit>=-1 ){
//     pPager->journalSizeLimit = iLimit;
//     sqlite3WalLimit(pPager->pWal, iLimit);
//   }
//   return pPager->journalSizeLimit;
// }

// /*
// ** Return a pointer to the pPager->pBackup variable. The backup module
// ** in backup.c maintains the content of this variable. This module
// ** uses it opaquely as an argument to sqlite3BackupRestart() and
// ** sqlite3BackupUpdate() only.
// */
// sqlite3_backup **sqlite3PagerBackupPtr(Pager *pPager){
//   return &pPager->pBackup;
// }

// #ifndef SQLITE_OMIT_VACUUM
// /*
// ** Unless this is an in-memory or temporary database, clear the pager cache.
// */
// void sqlite3PagerClearCache(Pager *pPager){
//   assert( MEMDB==0 || pPager->tempFile );
//   if( pPager->tempFile==0 ) pager_reset(pPager);
// }
// #endif


// #ifndef SQLITE_OMIT_WAL
// /*
// ** This function is called when the user invokes "PRAGMA wal_checkpoint",
// ** "PRAGMA wal_blocking_checkpoint" or calls the sqlite3_wal_checkpoint()
// ** or wal_blocking_checkpoint() API functions.
// **
// ** Parameter eMode is one of SQLITE_CHECKPOINT_PASSIVE, FULL or RESTART.
// */
// int sqlite3PagerCheckpoint(
//   Pager *pPager,                  /* Checkpoint on this pager */
//   sqlite3 *db,                    /* Db handle used to check for interrupts */
//   int eMode,                      /* Type of checkpoint */
//   int *pnLog,                     /* OUT: Final number of frames in log */
//   int *pnCkpt                     /* OUT: Final number of checkpointed frames */
// ){
//   int rc = SQLITE_OK;
//   if( pPager->pWal ){
//     rc = sqlite3WalCheckpoint(pPager->pWal, db, eMode,
//         (eMode==SQLITE_CHECKPOINT_PASSIVE ? 0 : pPager->xBusyHandler),
//         pPager->pBusyHandlerArg,
//         pPager->walSyncFlags, pPager->pageSize, (u8 *)pPager->pTmpSpace,
//         pnLog, pnCkpt
//     );
//   }
//   return rc;
// }

// int sqlite3PagerWalCallback(Pager *pPager){
//   return sqlite3WalCallback(pPager->pWal);
// }

// /*
// ** Return true if the underlying VFS for the given pager supports the
// ** primitives necessary for write-ahead logging.
// */
// int sqlite3PagerWalSupported(Pager *pPager){
//   const sqlite3_io_methods *pMethods = pPager->fd->pMethods;
//   if( pPager->noLock ) return 0;
//   return pPager->exclusiveMode || (pMethods->iVersion>=2 && pMethods->xShmMap);
// }

// /*
// ** Attempt to take an exclusive lock on the database file. If a PENDING lock
// ** is obtained instead, immediately release it.
// */
// static int pagerExclusiveLock(Pager *pPager){
//   int rc;                         /* Return code */

//   assert( pPager->eLock==SHARED_LOCK || pPager->eLock==EXCLUSIVE_LOCK );
//   rc = pagerLockDb(pPager, EXCLUSIVE_LOCK);
//   if( rc!=SQLITE_OK ){
//     /* If the attempt to grab the exclusive lock failed, release the 
//     ** pending lock that may have been obtained instead.  */
//     pagerUnlockDb(pPager, SHARED_LOCK);
//   }

//   return rc;
// }

// /*
// ** Call sqlite3WalOpen() to open the WAL handle. If the pager is in 
// ** exclusive-locking mode when this function is called, take an EXCLUSIVE
// ** lock on the database file and use heap-memory to store the wal-index
// ** in. Otherwise, use the normal shared-memory.
// */
// static int pagerOpenWal(Pager *pPager){
//   int rc = SQLITE_OK;

//   assert( pPager->pWal==0 && pPager->tempFile==0 );
//   assert( pPager->eLock==SHARED_LOCK || pPager->eLock==EXCLUSIVE_LOCK );

//   /* If the pager is already in exclusive-mode, the WAL module will use 
//   ** heap-memory for the wal-index instead of the VFS shared-memory 
//   ** implementation. Take the exclusive lock now, before opening the WAL
//   ** file, to make sure this is safe.
//   */
//   if( pPager->exclusiveMode ){
//     rc = pagerExclusiveLock(pPager);
//   }

//   /* Open the connection to the log file. If this operation fails, 
//   ** (e.g. due to malloc() failure), return an error code.
//   */
//   if( rc==SQLITE_OK ){
//     rc = sqlite3WalOpen(pPager->pVfs,
//         pPager->fd, pPager->zWal, pPager->exclusiveMode,
//         pPager->journalSizeLimit, &pPager->pWal
//     );
//   }
//   pagerFixMaplimit(pPager);

//   return rc;
// }


// /*
// ** The caller must be holding a SHARED lock on the database file to call
// ** this function.
// **
// ** If the pager passed as the first argument is open on a real database
// ** file (not a temp file or an in-memory database), and the WAL file
// ** is not already open, make an attempt to open it now. If successful,
// ** return SQLITE_OK. If an error occurs or the VFS used by the pager does 
// ** not support the xShmXXX() methods, return an error code. *pbOpen is
// ** not modified in either case.
// **
// ** If the pager is open on a temp-file (or in-memory database), or if
// ** the WAL file is already open, set *pbOpen to 1 and return SQLITE_OK
// ** without doing anything.
// */
// int sqlite3PagerOpenWal(
//   Pager *pPager,                  /* Pager object */
//   int *pbOpen                     /* OUT: Set to true if call is a no-op */
// ){
//   int rc = SQLITE_OK;             /* Return code */

//   assert( assert_pager_state(pPager) );
//   assert( pPager->eState==PAGER_OPEN   || pbOpen );
//   assert( pPager->eState==PAGER_READER || !pbOpen );
//   assert( pbOpen==0 || *pbOpen==0 );
//   assert( pbOpen!=0 || (!pPager->tempFile && !pPager->pWal) );

//   if( !pPager->tempFile && !pPager->pWal ){
//     if( !sqlite3PagerWalSupported(pPager) ) return SQLITE_CANTOPEN;

//     /* Close any rollback journal previously open */
//     sqlite3OsClose(pPager->jfd);

//     rc = pagerOpenWal(pPager);
//     if( rc==SQLITE_OK ){
//       pPager->journalMode = PAGER_JOURNALMODE_WAL;
//       pPager->eState = PAGER_OPEN;
//     }
//   }else{
//     *pbOpen = 1;
//   }

//   return rc;
// }

// /*
// ** This function is called to close the connection to the log file prior
// ** to switching from WAL to rollback mode.
// **
// ** Before closing the log file, this function attempts to take an 
// ** EXCLUSIVE lock on the database file. If this cannot be obtained, an
// ** error (SQLITE_BUSY) is returned and the log connection is not closed.
// ** If successful, the EXCLUSIVE lock is not released before returning.
// */
// int sqlite3PagerCloseWal(Pager *pPager, sqlite3 *db){
//   int rc = SQLITE_OK;

//   assert( pPager->journalMode==PAGER_JOURNALMODE_WAL );

//   /* If the log file is not already open, but does exist in the file-system,
//   ** it may need to be checkpointed before the connection can switch to
//   ** rollback mode. Open it now so this can happen.
//   */
//   if( !pPager->pWal ){
//     int logexists = 0;
//     rc = pagerLockDb(pPager, SHARED_LOCK);
//     if( rc==SQLITE_OK ){
//       rc = sqlite3OsAccess(
//           pPager->pVfs, pPager->zWal, SQLITE_ACCESS_EXISTS, &logexists
//       );
//     }
//     if( rc==SQLITE_OK && logexists ){
//       rc = pagerOpenWal(pPager);
//     }
//   }
    
//   /* Checkpoint and close the log. Because an EXCLUSIVE lock is held on
//   ** the database file, the log and log-summary files will be deleted.
//   */
//   if( rc==SQLITE_OK && pPager->pWal ){
//     rc = pagerExclusiveLock(pPager);
//     if( rc==SQLITE_OK ){
//       rc = sqlite3WalClose(pPager->pWal, db, pPager->walSyncFlags,
//                            pPager->pageSize, (u8*)pPager->pTmpSpace);
//       pPager->pWal = 0;
//       pagerFixMaplimit(pPager);
//       if( rc && !pPager->exclusiveMode ) pagerUnlockDb(pPager, SHARED_LOCK);
//     }
//   }
//   return rc;
// }

// #ifdef SQLITE_ENABLE_SETLK_TIMEOUT
// /*
// ** If pager pPager is a wal-mode database not in exclusive locking mode,
// ** invoke the sqlite3WalWriteLock() function on the associated Wal object 
// ** with the same db and bLock parameters as were passed to this function.
// ** Return an SQLite error code if an error occurs, or SQLITE_OK otherwise.
// */
// int sqlite3PagerWalWriteLock(Pager *pPager, int bLock){
//   int rc = SQLITE_OK;
//   if( pagerUseWal(pPager) && pPager->exclusiveMode==0 ){
//     rc = sqlite3WalWriteLock(pPager->pWal, bLock);
//   }
//   return rc;
// }

// /*
// ** Set the database handle used by the wal layer to determine if 
// ** blocking locks are required.
// */
// void sqlite3PagerWalDb(Pager *pPager, sqlite3 *db){
//   if( pagerUseWal(pPager) ){
//     sqlite3WalDb(pPager->pWal, db);
//   }
// }
// #endif

// #ifdef SQLITE_ENABLE_SNAPSHOT
// /*
// ** If this is a WAL database, obtain a snapshot handle for the snapshot
// ** currently open. Otherwise, return an error.
// */
// int sqlite3PagerSnapshotGet(Pager *pPager, sqlite3_snapshot **ppSnapshot){
//   int rc = SQLITE_ERROR;
//   if( pPager->pWal ){
//     rc = sqlite3WalSnapshotGet(pPager->pWal, ppSnapshot);
//   }
//   return rc;
// }

// /*
// ** If this is a WAL database, store a pointer to pSnapshot. Next time a
// ** read transaction is opened, attempt to read from the snapshot it 
// ** identifies. If this is not a WAL database, return an error.
// */
// int sqlite3PagerSnapshotOpen(
//   Pager *pPager, 
//   sqlite3_snapshot *pSnapshot
// ){
//   int rc = SQLITE_OK;
//   if( pPager->pWal ){
//     sqlite3WalSnapshotOpen(pPager->pWal, pSnapshot);
//   }else{
//     rc = SQLITE_ERROR;
//   }
//   return rc;
// }

// /*
// ** If this is a WAL database, call sqlite3WalSnapshotRecover(). If this 
// ** is not a WAL database, return an error.
// */
// int sqlite3PagerSnapshotRecover(Pager *pPager){
//   int rc;
//   if( pPager->pWal ){
//     rc = sqlite3WalSnapshotRecover(pPager->pWal);
//   }else{
//     rc = SQLITE_ERROR;
//   }
//   return rc;
// }

// /*
// ** The caller currently has a read transaction open on the database.
// ** If this is not a WAL database, SQLITE_ERROR is returned. Otherwise,
// ** this function takes a SHARED lock on the CHECKPOINTER slot and then
// ** checks if the snapshot passed as the second argument is still 
// ** available. If so, SQLITE_OK is returned.
// **
// ** If the snapshot is not available, SQLITE_ERROR is returned. Or, if
// ** the CHECKPOINTER lock cannot be obtained, SQLITE_BUSY. If any error
// ** occurs (any value other than SQLITE_OK is returned), the CHECKPOINTER
// ** lock is released before returning.
// */
// int sqlite3PagerSnapshotCheck(Pager *pPager, sqlite3_snapshot *pSnapshot){
//   int rc;
//   if( pPager->pWal ){
//     rc = sqlite3WalSnapshotCheck(pPager->pWal, pSnapshot);
//   }else{
//     rc = SQLITE_ERROR;
//   }
//   return rc;
// }

// /*
// ** Release a lock obtained by an earlier successful call to
// ** sqlite3PagerSnapshotCheck().
// */
// void sqlite3PagerSnapshotUnlock(Pager *pPager){
//   assert( pPager->pWal );
//   sqlite3WalSnapshotUnlock(pPager->pWal);
// }

// #endif /* SQLITE_ENABLE_SNAPSHOT */
// #endif /* !SQLITE_OMIT_WAL */

// #ifdef SQLITE_ENABLE_ZIPVFS
// /*
// ** A read-lock must be held on the pager when this function is called. If
// ** the pager is in WAL mode and the WAL file currently contains one or more
// ** frames, return the size in bytes of the page images stored within the
// ** WAL frames. Otherwise, if this is not a WAL database or the WAL file
// ** is empty, return 0.
// */
// int sqlite3PagerWalFramesize(Pager *pPager){
//   assert( pPager->eState>=PAGER_READER );
//   return sqlite3WalFramesize(pPager->pWal);
// }
// #endif