Re: Synchronizing slots from primary to standby

On Mon, Apr 8, 2024 at 7:01 PM Zhijie Hou (Fujitsu)
<houzj.fnst@fujitsu.com> wrote:
>
> Thanks for pushing.
>
> I checked the BF status, and noticed one BF failure, which I think is related to
> a miss in the test code.
> https://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=adder&dt=2024-04-08%2012%3A04%3A27
>
> From the following log, I can see the sync failed because the standby is
> lagging behind of the failover slot.
>
> -----
> # No postmaster PID for node "cascading_standby"
> error running SQL: 'psql:<stdin>:1: ERROR:  skipping slot synchronization as the received slot sync LSN 0/4000148 for
slot"snap_test_slot" is ahead of the standby position 0/4000114' 
> while running 'psql -XAtq -d port=50074 host=/tmp/t4HQFlrDmI dbname='postgres' -f - -v ON_ERROR_STOP=1' with sql
'SELECTpg_sync_replication_slots();' at /home/bf/bf-build/adder/HEAD/pgsql/src/test/perl/PostgreSQL/Test/Cluster.pm
line2042. 
> # Postmaster PID for node "publisher" is 3715298
> -----
>
> I think it's because we missed to call wait_for_replay_catchup before syncing
> slots.
>
> -----
> $primary->safe_psql('postgres',
>         "SELECT pg_create_logical_replication_slot('snap_test_slot', 'test_decoding', false, false, true);"
> );
> # ? missed to wait here
> $standby1->safe_psql('postgres', "SELECT pg_sync_replication_slots();");
> -----
>
> While testing, I noticed another place where we were calling
> wait_for_replay_catchup before doing pg_replication_slot_advance, which also has
> a small possibility to cause the failover slot to be ahead of the standby if
> some logs are written in between these two steps. So, I adjusted them together.
>
> Here is a small patch to improve the test.
>

LGTM. I'll push this tomorrow morning unless there are any more
comments or suggestions.

--
With Regards,
Amit Kapila.