Re: [HACKERS] make async slave to wait for lsn to be replayed

On Tue, Sep 17, 2024 at 9:00 AM Alexander Lakhin <exclusion(at)gmail(dot)com> wrote:
> 16.09.2024 21:55, Alexander Korotkov wrote:
> > Please find two patches attached. The first one does minor cleanup
> > including misuse of words you've pointed. The second one adds missing
> > wait_for_catchup(). That should fix the test failure you've spotted.
> > Please, check if it fixes an issue for you.
>
> Thank you for looking at that!
>
> Unfortunately, the issue is still here — the test failed for me 6 out of
> 10 runs, as below:
> [05:14:02.807](0.135s) ok 8 - got error after standby promote
> error running SQL: 'psql:<stdin>:1: ERROR: recovery is not in progress
> HINT: Waiting for LSN can only be executed during recovery.'
> while running 'psql -XAtq -d port=12734 host=/tmp/04hQ75NuXf dbname='postgres' -f - -v ON_ERROR_STOP=1' with sql 'CALL
> pg_wal_replay_wait('0/300F248');' at .../src/test/recovery/../../../src/test/perl/PostgreSQL/Test/Cluster.pm line 2140.
>
> 043_wal_replay_wait_standby.log:
> 2024-09-17 05:14:02.714 UTC [1817258] 043_wal_replay_wait.pl ERROR: recovery is not in progress
> 2024-09-17 05:14:02.714 UTC [1817258] 043_wal_replay_wait.pl DETAIL: Recovery ended before replaying target LSN
> 2/570CD648; last replay LSN 0/300F210.
> 2024-09-17 05:14:02.714 UTC [1817258] 043_wal_replay_wait.pl STATEMENT: CALL pg_wal_replay_wait('2/570CD648');
> 2024-09-17 05:14:02.714 UTC [1817155] LOG: checkpoint starting: force
> 2024-09-17 05:14:02.714 UTC [1817154] LOG: database system is ready to accept connections
> 2024-09-17 05:14:02.811 UTC [1817270] 043_wal_replay_wait.pl LOG: statement: CALL pg_wal_replay_wait('0/300F248');
> 2024-09-17 05:14:02.811 UTC [1817270] 043_wal_replay_wait.pl ERROR: recovery is not in progress
>
> pg_waldump -p .../t_043_wal_replay_wait_primary_data/pgdata/pg_wal/ 000000010000000000000003
> rmgr: Transaction len (rec/tot): 34/ 34, tx: 748, lsn: 0/0300F1E8, prev 0/0300F1A8, desc: COMMIT
> 2024-09-17 05:14:01.654874 UTC
> rmgr: Standby len (rec/tot): 50/ 50, tx: 0, lsn: 0/0300F210, prev 0/0300F1E8, desc: RUNNING_XACTS
> nextXid 749 latestCompletedXid 748 oldestRunningXid 749
>
> I wonder, can you reproduce this with that bgwriter's modification?

Yes, now I did reproduce. I got that the problem could be that insert
LSN is not yet written at primary, thus wait_for_catchup doesn't wait
for it. I've workarounded that using pg_switch_wal(). The revised
patchset is attached.

> I've also found two more "achievements" coined by 3c5db1d6b:
> doc/src/sgml/func.sgml: It may also happen that target <acronym>lsn</acronym> is not achieved
> src/backend/access/transam/xlog.c- * recovery was ended before achieving the target LSN.

Fixed this at well in 0001.

------
Regards,
Alexander Korotkov
Supabase