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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: 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: 2026-04-21T04:03:30Z
Lists: pgsql-hackers

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On Tue, Apr 21, 2026 at 2:46 AM Alexander Korotkov <aekorotkov@gmail.com> wrote:

> The updated patchset is attached.  It includes improved coverage as
> suggested by Andres upthread.  And documentation that WAIT FOR LSN is
> timeline-blind (per off-list discussion with Xuneng).

I revised the test patch 6 to make the new cases check the intended
WAIT FOR behavior more directly, and to avoid cases where the test
could pass for the wrong reason.

The fresh walreceiver restart test now distinguishes what we can
observe from what is only covered indirectly.
'pg_last_wal_receive_lsn()' reports 'flushedUpto', not 'writtenUpto',
so the test now describes that state accurately and covers
'writtenUpto' through the 'standby_write' result. This seems
appropriate to me since the two positions are seeded in the places and
conditions. Test for flush lsn should also help verify write lsn.

The fencepost tests were split by the actual frontier being tested.
'standby_replay' uses 'pg_last_wal_replay_lsn()', while
'standby_flush' uses 'pg_last_wal_receive_lsn()'. This avoids treating
a replay-derived LSN as if it were also the exact write/flush
boundary. I left 'standby_write' out of the exact fencepost helper
because its frontier is not SQL-visible once walreceiver is stopped.
The async wakeup case now starts the waiter while replay is still
paused, so it must actually sleep before replay and walreceiver are
allowed to advance.

The cascading timeline-switch test now checks the 'WAIT FOR ...
NO_THROW' status from background psql stdout. The previous log-marker
pattern could pass after unexpected returned status, includingn
'timeout', because the following statement would still run. The
'received_tli > 1' check remains, but only as confirmation that the
downstream followed the new timeline; the 'success' status proves the
wait completed as intended.

Please check it.

--
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