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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: Andres Freund <andres@anarazel.de>
Cc: Xuneng Zhou <xunengzhou@gmail.com>, 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: 2026-04-08T07:31:07Z
Lists: pgsql-hackers 
On Wed, Apr 8, 2026 at 3:50 AM Andres Freund <andres@anarazel.de> wrote:
> On 2026-04-08 02:30:44 +0300, Alexander Korotkov wrote:
> > On Tue, Apr 7, 2026 at 6:55 PM Xuneng Zhou <xunengzhou@gmail.com> wrote:
> > I agree to change in WaitLSNWakeup(), memory barrier looks necessary there.
> > Regarding GetCurrentLSNForWaitType(), I don't think barrier is needed
> > here, nor think it makes things clearer.  I think it would be enough
> > to comment that LWLock operations in addLSNWaiter()/deleteLSNWaiter()
> > provide necessary barriers.
>
> That's sufficient for the first iteration, but what guarantees it once you do
> WaitLatch()?  That's likely going to imply a barrier somewhere in the kernel,
> but I don't think there's any actual guarantee.

After WaitLatch(), ResetLatch() contains memory barrier.  And as I
understand, this memory barrier includes guarantees for reading fresh
values after WaitLatch() in typical latch usage scenario.  However, I
see in WaitForLSN() we can exit from WaitLatch() on timeout, and then
potentially exit from loop on timeout without rechecking for the most
fresh LSN.  I suppose we can just do ResetLatch() unconditionally to
fix that.

------
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