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Re: Implement waiting for wal lsn replay: reloaded

Alexander Korotkov <aekorotkov@gmail.com>

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From: Alexander Korotkov <aekorotkov@gmail.com>
To: Xuneng Zhou <xunengzhou@gmail.com>
Cc: Heikki Linnakangas <hlinnaka@iki.fi>, Peter Eisentraut <peter@eisentraut.org>, Andres Freund <andres@anarazel.de>, Thomas Munro <thomas.munro@gmail.com>, Álvaro Herrera <alvherre@kurilemu.de>, Chao Li <li.evan.chao@gmail.com>, pgsql-hackers <pgsql-hackers@lists.postgresql.org>, Michael Paquier <michael@paquier.xyz>, jian he <jian.universality@gmail.com>, Tomas Vondra <tomas@vondra.me>, Yura Sokolov <y.sokolov@postgrespro.ru>
Date: 2026-04-05T12:31:46Z
Lists: pgsql-hackers

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On Thu, Jan 29, 2026 at 9:47 AM Alexander Korotkov <aekorotkov@gmail.com> wrote:
> On Tue, Jan 27, 2026 at 3:14 AM Xuneng Zhou <xunengzhou@gmail.com> wrote:
> > Heikki spotted a misplaced wake-up call for replay waiters in
> > PerformWalRecovery. He suggested that the WaitLSNWakeup needs to be
> > invoked immediately after wal record is applied to avoid the potential
> > missed wake-ups when recovery stops/pauses/promotes. It makes sense to
> > me. Please check the attached patch to fix that.
>
> Pushed, thank you!

I've assembled small patches, which I think worth pushing before v19 FF.

1. Avoid syscache lookup in WaitStmtResultDesc().  This is [1] patch,
but I've removed redundant comment in ExecWaitStmt(), which explained
the same as WaitStmtResultDesc() comment.
2. Use WAIT FOR LSN in wait_for_catchup().  I made the following
changes: fallback to polling on not_in_recovery result instead of
croaking, avoid separate pg_is_in_recovery() query, comment why we
may face the recovery conflict and why it is safe to detect this by
the error string.
3. A paragraph to the docs about possible recovery conflicts and their reason.

I'm going to push this on Monday if no objections.

Links.
1. https://www.postgresql.org/message-id/CABPTF7U%2BSUnJX_woQYGe%3D%3DR9Oz%2B-V6X0VO2stBLPGfJmH_LEhw%40mail.gmail.com
2. https://www.postgresql.org/message-id/CABPTF7X0n%3DR50z2fBpj3EbYYz04Ab0-DHJa%2BJfoAEny62QmUdg%40mail.gmail.com

------
Regards,
Alexander Korotkov
Supabase

Commits

Same data as JSON: GET /api/v1/messages/:b64id/commits the thread's linked commits as JSON, with link sources. API reference →

  1. Clean up 019_replslot_limit.pl comments

  2. Stabilize 019_replslot_limit.pl: wait on slot restart_lsn

  3. Fix memory ordering in WAIT FOR LSN wakeup mechanism

  4. Improve WAIT FOR LSN test coverage

  5. Remove redundant WAIT FOR LSN caller-side pre-checks

  6. Use barrier semantics when reading/writing writtenUpto

  7. Use replay position as floor for WAIT FOR LSN standby_(write|flush)

  8. Wake standby_write/standby_flush waiters from the WAL replay loop

  9. Minimal fix for WAIT FOR ... MODE 'standby_flush'

  10. Avoid syscache lookup while building a WAIT FOR tuple descriptor

  11. Document that WAIT FOR may be interrupted by recovery conflicts

  12. Use WAIT FOR LSN in PostgreSQL::Test::Cluster::wait_for_catchup()

  13. Wake LSN waiters before recovery target stop

  14. Remove redundant pg_unreachable() after elog(ERROR) from ExecWaitStmt()

  15. Revert "Use WAIT FOR LSN in PostgreSQL::Test::Cluster::wait_for_catchup()"

  16. Fix variable usage in wakeupWaiters()

  17. Add tab completion for the WAIT FOR LSN MODE option

  18. Add the MODE option to the WAIT FOR LSN command

  19. Extend xlogwait infrastructure with write and flush wait types

  20. Unify error messages

  21. Optimize shared memory usage for WaitLSNProcInfo

  22. Fix WaitLSNWakeup() fast-path check for InvalidXLogRecPtr

  23. Fix incorrect function name in comments

  24. Add infrastructure for efficient LSN waiting

  25. Add pairingheap_initialize() for shared memory usage

  26. Implement WAIT FOR command