Re: logical: fix recomputation required LSN on restart_lsn-only advancement

> On Apr 21, 2026, at 15:09, Antonin Houska <ah(at)cybertec(dot)at> wrote:
>
> Chao Li <li(dot)evan(dot)chao(at)gmail(dot)com> wrote:
>
>> While reading logical replication code, I found an issue in LogicalConfirmReceivedLocation().
>>
>> In LogicalConfirmReceivedLocation(), updated_restart is tracked independently from updated_xmin, and the slot is marked dirty and saved when either one changed. But after that, ReplicationSlotsComputeRequiredLSN() is still only called inside "if (updated_xmin)”.
>>
>> So for the restart-only case:
>>
>> * updated_restart = true
>> * updated_xmin = false
>> * ReplicationSlotSave() runs
>> * ReplicationSlotsComputeRequiredLSN() does not run because updated_xmin is false
>>
>> That means the global retention point managed by XLogSetReplicationSlotMinimumLSN() can stay stale until some later unrelated event recomputes it. Since ReplicationSlotsComputeRequiredLSN() derives the global minimum from slot restat_lsn, skipping it after a restart-only advance can retain excess WAL and may lead to WAL bloat.
>>
>> This patch fixes the problem by moving ReplicationSlotsComputeRequiredLSN() under “if (updated_restart)”.
>
> FYI, this overlaps with another post in the REPACK thread [1].
>
>> Looks like this issue has been there for a long time, so if this analysis is correct, it may also be worth back-patching.
>
> As REPACK in PG 19 does not let xmin advance (that should be fixed in the
> future), I think makes sense to apply [1] to v19. However, during logical
> replication, xmin (IMO) gets updated rather often, so the problem should not
> be that severe in earlier versions.
>
> [1] https://www.postgresql.org/message-id/TYRPR01MB14195633567DA00ABD42570B794592%40TYRPR01MB14195.jpnprd01.prod.outlook.com
>
> --
> Antonin Houska
> Web: https://www.cybertec-postgresql.com

Thanks for pointing out that. I will review that patch.

Best regards,
--
Chao Li (Evan)
HighGo Software Co., Ltd.
https://www.highgo.com/