I removed the autoconf flag and #ifdefs earlier, but missed these.

* Revert almost all the changes in smgr.c / md.c, to not go through
  the Mirrored* APIs.

* Remove mmxlog stuff. Use upstream "pending relation deletion" code
  instead.

* Get rid of multiple startup passes. Now it's just a single pass like
  in the upstream.

* Revert the way database drop/create are handled to the way it is in
  upstream. Doesn't use PT anymore, but accesses file system directly,
  and WAL-logs a single CREATE/DROP DATABASE WAL record.

* Get rid of MirroredLock

* Remove a few tests that were specific to persistent tables.

* Plus a lot of little removals and reverts to upstream code.

MMXLOG records should only be replayed in standby mode and smgr create
xlog records should not do anything until persistent tables and MMXLOG
records are removed from Greenplum.

Author: Jimmy Yih <jyih@pivotal.io>
Author: Asim Praveen <apraveen@pivotal.io>

cdbappendonlystorage_test.c:
Test that calling appendonly_redo with XLOG_APPENDONLY_INSERT or
XLOG_APPENDONLY_TRUNCATE will call ao_insert_replay or
ao_truncate_replay respectively.

xlog_mm_test.c:
Test that an MMXLOG_REMOVE_FILE AO transaction log record will call
XLogAODropSegmentFile.

cdbmirroredappendonly_test.c:
Test that ao_insert_replay or ao_truncate_replay will call
XLogAOSegmentFile when we cannot find the AO segment file.

xlogutils_test.c:
Test that the invalid_page_tab hash table is properly used when
invalid AO segment files are encountered.

These four pieces combined will test the case when AO XLOG records are
replayed but the corresponding segment files are missing from
filesystem. The replay is entered into the invalid_page_tab hash table
until a corresponding MMXLOG_REMOVE_FILE XLOG record type comes along
to remove the entry.

Currently these unit tests are under --enable-segwalrep configure flag
in the Makefile but can be moved to the actual TARGET if non-segwalrep
tests are added and need to be tested.

Reference:
https://github.com/greenplum-db/gpdb/commit/b659d0479fa57c383e7dbe15043cd2ca9d46f77f

ereport() has one subtle but important difference to elog: it doesn't
evaluate its arguments, if the log level says that the message doesn't
need to be printed. This makes a small but measurable difference in
performance, if the arguments contain more complicated expressions, like
function calls.

While performance testing a workload with very short queries, I saw some
CPU time being used in DtxContextToString. Those calls were coming from the
arguments to elog() statements, and the result was always thrown away,
because the log level was not high enough to actually log anything. Turn
those elog()s into ereport()s, for speed.

The problematic case here was a few elogs containing DtxContextToString
calls, in hot codepaths, but I changed a few surrounding ones too, for
consistency.

Simplify the mock test, to not bother mocking elog(), while we're at it.
The real elog/ereport work just fine in the mock environment.

We also refactor the code out of `CreateCheckPoint()` and make it into
its own function `GetXLogCleanUpTo()`.

Unit test added to verify the new logic works for both master and
segments.

This is hidden under #ifdef USE_SEGWALREP.
Signed-off-by: NAsim R P <apraveen@pivotal.io>

Originally, the reader kept copies of subtransaction information in
two places.  First, it copied SharedLocalSnapshotSlot to share between
writer and reader.  Second, reader kept another copy in subxbuf for
better performance.  Due to lazy xid, subtransaction information can
change in the writer asynchronously with respect to the reader.  This
caused reader's subtransaction information out of date.

This fix removes those copies of subtransaction information in the
reader and adds a reference to the writer's PGPROC to
SharedLocalSnapshotSlot.  Reader should refer to subtransaction
information through writer's PGPROC and pg_subtrans.

Also added is a lwlock per shared snapshot slot.  The lock protects
shared snapshot information between a writer and readers belonging to
the same session.

Fixes github issues #2269 and #2284.
Signed-off-by: NAsim R P <apraveen@pivotal.io>

Instead of creating a mock version of every backend object file, only
create mock files as needed by individual tests. Speeds up the build, and
is nicer anyway.

This inverts the logic of the <testname>_REAL_OBJS mechanism. We now link
with the real version of each object file, *except* those listed in
<testname>_MOCK_OBJS variable.

There are a bunch of backend files that are never mocked, but always linked
into test program as is. List those in Makefile.mock, so that every Makefile
doesn't need to list them. Makes the makefiles simpler, and you need less
boilerplate code when adding a new test program.

Not all of these backend files were listed in the _REAL_OBJS variable of
every test, but in those tests that they were not, they were also not used,
so it doesn't matter if we use the real or mock version.

This way we don't need to link the whole backend into client mock programs.
We've gotten away with it so far, but it would cause clashes if one of the
client programs contained a function or global variable with same name as
in the backend. And of course it's completely unnecessary anyway.

This makes merging and diffing with upstream easier. There were so many
GPDB-added GUCs in guc.c that "git merge" didn't handle it very well, and
you easily got merge conflicts that were error-prone to fix manually.

I kept a few GUCs in guc.c that are not present in REL8_2_0, because they
were backported from later PostgreSQL versions, rather than being
completely GPDB-specific.

Clean up the diff between guc.c and upstream, by removing unnecessary
includes, reordering a few GUCs to match upstream better, and similar
cosmetic changes.

I had to move MAX_KILOBYTES from guc.c to guc_tables.h, because guc_gp.c
needs it too. I also had to export assign_msglvl() function from guc.c, so
that it can be called from other assign_* functions from guc_gp.c. And I
had to add the code to process the ConfigureNames*_gp arrays in addition
to the upstream ones. These things increase the diff against upstream, but
not by too much, IMHO. I also copy-pasted constants like MS_PER_S and
KB_PER_MB, which seems safe as those are natural constants, and should
never change.

Now that we no longer forcibly compile strlcpy.c, and only compile and link
it into strlcpy.o into libpgport when needed, this is no longer needed.
Furthermore, it caused the linking to fail on platforms that don't need
strlcpy.o, like OS X.