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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: Andres Freund <andres@anarazel.de>, Tom Lane <tgl@sss.pgh.pa.us>, Heikki Linnakangas <hlinnaka@iki.fi>, Peter Eisentraut <peter@eisentraut.org>, 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: 2026-04-09T15:21:24Z
Lists: pgsql-hackers

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On Wed, Apr 8, 2026 at 7:59 AM Xuneng Zhou <xunengzhou@gmail.com> wrote:
> > > Patch 0001 looks OK for me.
> > > Regarding patch 0002.  Changes made for GetCurrentLSNForWaitType()
> > > looks reliable for me.  PerformWalRecovery() sets replayed positions
> > > before starting recovery, and in turn before standby can accept
> > > connections.  So, changes to WalReceiverMain() don't look necessary to
> > > me.
> >
> > Yeah, GetCurrentLSNForWaitType seems to be the right place to place
> > the fix. Please see the attached patch 2.
> >
> > I also noticed another relevent problem:
> >
> > During pure archive recovery (no walreceiver), a backend that issues
> > 'WAIT FOR LSN ... MODE 'standby_write' with a target ahead of the
> > current replay position will sleep forever; the startup process
> > replays past the target but only wakes 'STANDBY_REPLAY' waiters.
> >
> > This also affects mixed scenarios: the walreceiver may lag behind
> > replay (e.g., archive restore has delivered WAL faster than
> > streaming), so a 'standby_write' waiter could be waiting on WAL that
> > replay has already consumed.
> >
> > I will write a patch to address this soon.
> >
>
> Here is the patch.

I've assembled all the pending patches together.
0001 adds memory barrier to GetWalRcvWriteRecPtr() as suggested by
Andres off-list.
0002 is basically [1] by Xuneng, but revised given we have a memory
barrier in 0001, and my proposal to do ResetLatch() unconditionally
similar to our other Latch-based loops.
0003 and 0004 are [2] by Xuneng.
0005 is [3] by Xuneng.

I'm going to add them to Commitfest to run CI over them, and have a
closer look over them tomorrow.


Links.
1. https://www.postgresql.org/message-id/CABPTF7Wjk_FbOghyr09Rzu6T2bh-L_KBMqHK%2BzhRXpssU0STyQ%40mail.gmail.com
2. https://www.postgresql.org/message-id/CABPTF7X0iV%3DkGC4gjsTj4NvK_NNEJGM3YTc7Obxs5GOiYoMhEw%40mail.gmail.com
3. https://www.postgresql.org/message-id/CABPTF7UBdEfyxATWntmCfoJrwB6iPrnhkXO7y_Avmqc2bOn27A%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