before reporting a transaction committed.  Data consistency is still
guaranteed (unlike setting fsync = off), but a crash may lose the effects
of the last few transactions.  Patch by Simon, some editorialization by Tom.

back-stamped for this.

cases where we already hold the desired lock "indirectly", either via
membership in a MultiXact or because the lock was originally taken by a
different subtransaction of the current transaction.  These cases must be
accounted for to avoid needless deadlocks and/or inappropriate replacement of
an exclusive lock with a shared lock.  Per report from Clarence Gardner and
subsequent investigation.

recovery.  In the first place, it doesn't work because slru's
latest_page_number isn't set up yet (this is why we've been hearing reports
of strange "apparent wraparound" log messages during crash recovery, but
only from people who'd managed to advance their next-mxact counters some
considerable distance from 0).  In the second place, it seems a bit unwise
to be throwing away data during crash recovery anwyway.  This latter
consideration convinces me to just disable truncation during recovery,
rather than computing latest_page_number and pushing ahead.

Strip unused include files out unused include files, and add needed
includes to C files.

The next step is to remove unused include files in C files.

when an error occurs during xlog replay.  Also, replace the former risky
'write into a fixed-size buffer with no overflow detection' API for XLOG
record description routines; use an expansible StringInfo instead.  (The
latter accounts for most of the patch bulk.)

Qingqing Zhou

SLRU area.  The number of slots is still a compile-time constant (someday
we might want to change that), but at least it's a different constant for
each SLRU area.  Increase number of subtrans buffers to 32 based on
experimentation with a heavily subtrans-bashing test case, and increase
number of multixact member buffers to 16, since it's obviously silly for
it not to be at least twice the number of multixact offset buffers.

lock, not exclusive, if the desired page is already in memory.  This can
be demonstrated to be a significant win on the pg_subtrans cache when there
is a large window of open transactions.  It should be useful for pg_clog
as well.  I didn't try to make GetMultiXactIdMembers() use the code, as
that would have taken some restructuring, and what with the local cache
for multixact contents it probably wouldn't really make a difference.
Per my recent proposal.

comment line where output as too long, and update typedefs for /lib
directory.  Also fix case where identifiers were used as variable names
in the backend, but as typedefs in ecpg (favor the backend for
indenting).

Backpatch to 8.1.X.

for the SLRU race condition that I posted a few days ago, but we decided
not to use in 8.1 and older branches.

reserving SLRU space for a new MultiXact.  The original coding would have
treated out-of-disk-space as a PANIC condition, which is unnecessary.

multixact's starting offset before the offset has been stored into the
SLRU file.  A simple fix would be to hold the MultiXactGenLock until the
offset has been stored, but that looks like a big concurrency hit.  Instead
rely on knowledge that unset offsets will be zero, and loop when we see
a zero.  This requires a little extra hacking to ensure that zero is never
a valid value for the offset.  Problem reported by Matteo Beccati, fix
ideas from Martijn van Oosterhout, Alvaro Herrera, and Tom Lane.

to 'Size' (that is, size_t), and install overflow detection checks in it.
This allows us to remove the former arbitrary restrictions on NBuffers
etc.  It won't make any difference in a 32-bit machine, but in a 64-bit
machine you could theoretically have terabytes of shared buffers.
(How efficiently we could manage 'em remains to be seen.)  Similarly,
num_temp_buffers, work_mem, and maintenance_work_mem can be set above
2Gb on a 64-bit machine.  Original patch from Koichi Suzuki, additional
work by moi.

Original patch by Hans-Juergen Schoenig, revisions by Karel Zak
and Tom Lane.

transaction IDs, rather than like subtrans; in particular, the information
now survives a database restart.  Per previous discussion, this is
essential for PITR log shipping and for 2PC.

communication structure, and make it its own module with its own lock.
This should reduce contention at least a little, and it definitely makes
the code seem cleaner.  Per my recent proposal.

are creating a new MultiXactId from two regular XIDs.  The original
coding was unnecessarily complicated and didn't save any code anyway.

to eliminate unnecessary deadlocks.  This commit adds SELECT ... FOR SHARE
paralleling SELECT ... FOR UPDATE.  The implementation uses a new SLRU
data structure (managed much like pg_subtrans) to represent multiple-
transaction-ID sets.  When more than one transaction is holding a shared
lock on a particular row, we create a MultiXactId representing that set
of transactions and store its ID in the row's XMAX.  This scheme allows
an effectively unlimited number of row locks, just as we did before,
while not costing any extra overhead except when a shared lock actually
has to be shared.   Still TODO: use the regular lock manager to control
the grant order when multiple backends are waiting for a row lock.

Alvaro Herrera and Tom Lane.