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Re: Implement waiting for wal lsn replay: reloaded

Xuneng Zhou <xunengzhou@gmail.com>

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From: Xuneng Zhou <xunengzhou@gmail.com>
To: Alexander Korotkov <aekorotkov@gmail.com>
Cc: 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>, Andres Freund <andres@anarazel.de>, Michael Paquier <michael@paquier.xyz>, jian he <jian.universality@gmail.com>, Tomas Vondra <tomas@vondra.me>, Yura Sokolov <y.sokolov@postgrespro.ru>
Date: 2026-01-06T13:12:41Z
Lists: pgsql-hackers

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Hi,

On Tue, Jan 6, 2026 at 7:54 PM Alexander Korotkov <aekorotkov@gmail.com> wrote:
>
> On Tue, Jan 6, 2026 at 9:29 AM Xuneng Zhou <xunengzhou@gmail.com> wrote:
> > On Tue, Jan 6, 2026 at 1:43 PM Thomas Munro <thomas.munro@gmail.com> wrote:
> > > Could this be causing the recent flapping failures on CI/macOS in
> > > recovery/031_recovery_conflict?  I didn't have time to dig personally
> > > but f30848cb looks relevant:
> > >
> > > Waiting for replication conn standby's replay_lsn to pass 0/03467F58 on primary
> > > error running SQL: 'psql:<stdin>:1: ERROR:  canceling statement due to
> > > conflict with recovery
> > > DETAIL:  User was or might have been using tablespace that must be dropped.'
> > > while running 'psql --no-psqlrc --no-align --tuples-only --quiet
> > > --dbname port=25195
> > > host=/var/folders/g9/7rkt8rt1241bwwhd3_s8ndp40000gn/T/LqcCJnsueI
> > > dbname='postgres' --file - --variable ON_ERROR_STOP=1' with sql 'WAIT
> > > FOR LSN '0/03467F58' WITH (MODE 'standby_replay', timeout '180s',
> > > no_throw);' at /Users/admin/pgsql/src/test/perl/PostgreSQL/Test/Cluster.pm
> > > line 2300.
> > >
> > > https://cirrus-ci.com/task/5771274900733952
> > >
> > > The master branch in time-descending order, macOS tasks only:
> > >
> > >      task_id      | substring |  status
> > > ------------------+-----------+-----------
> > >  6460882231754752 | c970bdc0  | FAILED
> > >  5771274900733952 | 6ca8506e  | FAILED
> > >  6217757068361728 | 63ed3bc7  | FAILED
> > >  5980650261446656 | ae283736  | FAILED
> > >  6585898394976256 | 5f13999a  | COMPLETED
> > >  4527474786172928 | 7f9acc9b  | COMPLETED
> > >  4826100842364928 | e8d4e94a  | COMPLETED
> > >  4540563027918848 | b9ee5f2d  | FAILED
> > >  6358528648019968 | c5af141c  | FAILED
> > >  5998005284765696 | e212a0f8  | COMPLETED
> > >  6488580526178304 | b85d5dc0  | FAILED
> > >  5034091344560128 | 7dc95cc3  | ABORTED
> > >  5688692477526016 | bb048e31  | COMPLETED
> > >  5481187977723904 | d351063e  | COMPLETED
> > >  5101831568752640 | f30848cb  | COMPLETED <-- the change
> > >  6395317408497664 | 3f33b63d  | COMPLETED
> > >  6741325208354816 | 877ae5db  | COMPLETED
> > >  4594007789010944 | de746e0d  | COMPLETED
> > >  6497208998035456 | 461b8cc9  | COMPLETED
> >
> > Thanks for raising this issue. I think it is related to f30848cb after
> > some analysis. I'll prepare a follow-up patch to fix it.
>
> Sorry, I've mistakenly referenced this report from commit [1].  I
> thought it was related, but it appears to be not.  [1] is related to
> the report I've got from Ruikai Peng off-list.
>
> Regarding the present failure, could it happen before ExecWaitStmt()
> calls PopActiveSnapshot() and InvalidateCatalogSnapshot()?  If so, we
> should do preliminary efforts to release these snapshots.
>
> 1. https://git.postgresql.org/pg/commitdiff/bf308639bfcfa38541e24733e074184153a8ab7f
>

I agree that moving PopActiveSnapshot() and
InvalidateCatalogSnapshot() to the very beginning of ExecWaitStmt()
appears to be a sensible optimization. However, in this particular
failure scenario, it may not address the issue.

For tablespace conflicts, recovery conflict resolution uses
GetConflictingVirtualXIDs(InvalidTransactionId, InvalidOid), which
returns all active backends, regardless of their snapshot state. As a
result, even if all snapshots are released at the start of
ExecWaitStmt(), the session would still be canceled during replay of
DROP TABLESPACE.

Given this, I am considering handling this conflict class explicitly:
if the WAIT FOR statement is terminated and the error indicates a
recovery conflict, we fall back to the existing polling-based
approach.

* Ask everybody to cancel their queries immediately so we can ensure no
* temp files remain and we can remove the tablespace. Nuke the entire
* site from orbit, it's the only way to be sure.
*
* XXX: We could work out the pids of active backends using this
* tablespace by examining the temp filenames in the directory. We would
* then convert the pids into VirtualXIDs before attempting to cancel
* them.

I am also wondering whether this optimization would be helpful.

-- 
Best,
Xuneng

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