Horton

Re: logical replication: restart_lsn can go backwards (and more), seems broken since 9.4

Masahiko Sawada <sawada.mshk@gmail.com>

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From: Masahiko Sawada <sawada.mshk@gmail.com>
To: Tomas Vondra <tomas@vondra.me>
Cc: Ashutosh Bapat <ashutosh.bapat.oss@gmail.com>, PostgreSQL Hackers <pgsql-hackers@postgresql.org>
Date: 2024-11-12T06:32:18Z
Lists: pgsql-hackers 
On Mon, Nov 11, 2024 at 2:08 PM Tomas Vondra <tomas@vondra.me> wrote:
>
>
> But neither of those fixes prevents backwards move for confirmed_flush
> LSN, as enforced by asserts in the 0005 patch. I don't know if this
> assert is incorrect or now. It seems natural that once we get a
> confirmation for some LSN, we can't move before that position, but I'm
> not sure about that. Maybe it's too strict.

Hmm, I'm concerned that it might be another problem. I think there are
some cases where a subscriber sends a flush position older than slot's
confirmed_flush as a feedback message. But it seems to be dangerous if
we always accept it as a new confirmed_flush value. It could happen
that confirm_flush could be set to a LSN older than restart_lsn.

Regards,

--
Masahiko Sawada
Amazon Web Services: https://aws.amazon.com

Commits

  1. Fix a possibility of logical replication slot's restart_lsn going backwards.

  2. Introduce logical decoding.