Horton

Re: [HACKERS] make async slave to wait for lsn to be replayed

Alexander Korotkov <aekorotkov@gmail.com>

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From: Alexander Korotkov <aekorotkov@gmail.com>
To: Kartyshov Ivan <i.kartyshov@postgrespro.ru>
Cc: Kyotaro Horiguchi <horikyota.ntt@gmail.com>, hlinnaka@iki.fi, alvherre@alvh.no-ip.org, pashkin.elfe@gmail.com, bharath.rupireddyforpostgres@gmail.com, euler@eulerto.com, thomas.munro@gmail.com, peter@eisentraut.org, amit.kapila16@gmail.com, dilipbalaut@gmail.com, smithpb2250@gmail.com, pgsql-hackers@lists.postgresql.org
Date: 2024-07-31T16:40:48Z
Lists: pgsql-hackers

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On Mon, Jul 15, 2024 at 2:02 PM Alexander Korotkov <aekorotkov@gmail.com> wrote:
> On Mon, Jul 15, 2024 at 4:24 AM Alexander Korotkov <aekorotkov@gmail.com> wrote:
> > Thanks to Kyotaro for the review.  And thanks to Ivan for the patch
> > revision.  I made another revision of the patch.
>
> I've noticed failures on cfbot.  The attached revision addressed docs
> build failure.  Also it adds some "quits" for background psql sessions
> for tests.  Probably this will address test hangs on windows.

I made the following changes to the patch.

1) I've changed the check for snapshot in pg_wal_replay_wait().  Now
it checks that GetOldestSnapshot() returns NULL.  It happens when both
ActiveSnapshot is NULL and RegisteredSnapshots pairing heap is empty.
This is the same condition when SnapshotResetXmin() sets out xmin to
invalid.  Thus, we are not preventing WAL from replay.  This should be
satisfied when pg_wal_replay_wait() isn't called within a transaction
with an isolation level higher than READ COMMITTED, another procedure,
or a function.  Documented it this way.

2) Explicitly document in the PortalRunUtility() comment that
pg_wal_replay_wait() is another case when active snapshot gets
released.

3) I've removed tests with cascading replication.  It's rather unclear
what problem these tests could potentially spot.

4) Did some improvements to docs, comments and commit message to make
them consistent with the patch contents.

The commit to pg17 was inconsiderate.  But I feel this patch got much
better shape.  Especially, now it's clear when the
pg_wal_replay_wait() procedure can be used.  So, I dare commit this to
pg18 if nobody objects.

------
Regards,
Alexander Korotkov
Supabase

Commits

  1. Ensure standby promotion point in 043_wal_replay_wait.pl

  2. Minor cleanup related to pg_wal_replay_wait() procedure

  3. Adjust pg_wal_replay_wait() procedure behavior on promoted standby

  4. pg_wal_replay_wait(): Fix typo in the doc

  5. Implement pg_wal_replay_wait() stored procedure

  6. Revert: Implement pg_wal_replay_wait() stored procedure

  7. Call WaitLSNCleanup() in AbortTransaction()

  8. Clarify what is protected by WaitLSNLock

  9. Use an LWLock instead of a spinlock in waitlsn.c

  10. Use the pairing heap instead of a flat array for LSN replay waiters

  11. Minor improvements for waitlsn.c

  12. Make the order of the header file includes consistent