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From: Xuneng Zhou <xunengzhou@gmail.com>
Date: Sat, 6 Jun 2026 20:56:10 +0800
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 <CABPTF7UqSh4LCA_Lf2owJg5VBs5jteBfb5q4Jt9BiOdq8bMH8w@mail.gmail.com>
Subject: Re: t/035_standby_logical_decoding.pl might fail on attempt to read
 wrong timeline
To: Bertrand Drouvot <bertranddrouvot.pg@gmail.com>
Cc: Alexander Lakhin <exclusion@gmail.com>,
 pgsql-hackers <pgsql-hackers@postgresql.org>
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Hi Bertrand,

On Sat, Jun 6, 2026 at 7:07=E2=80=AFPM Bertrand Drouvot
<bertranddrouvot.pg@gmail.com> wrote:
>
> Hi Alexander,
>
> On Sat, Jun 06, 2026 at 12:00:00PM +0300, Alexander Lakhin wrote:
> > Hello hackers,
> >
> > That is, walsender requested WAL segment for timeline 1, while in a
> > successful run, it reads WAL for timeline 2.
> >
> > I've managed to reproduce this failure with:
>
> Thanks for the report and the repro!
>
> > As far as I can see, the timeline is chosen in logical_read_xlog_page()
> > depending on the recovery state:
> >         am_cascading_walsender =3D RecoveryInProgress();
> >
> >         if (am_cascading_walsender)
> >                 GetXLogReplayRecPtr(&currTLI);
> >         else
> >                 currTLI =3D GetWALInsertionTimeLine();
>
> Yeah, it looks like there is a race condition here. I think we should che=
ck if
> the insertion timeline has already been set (like the walsummarizer is do=
ing).
>
> I'll work on a fix early next week.

This looks like the right direction to fix. We may want to apply
similar logic to read_local_xlog_page_guts as well. Although the
failure is reported in walsender, SQL logical decoding uses the local
WAL reader and has the same recovery/TLI pattern.

--
Regards,
Xuneng Zhou
HighGo Software Co., Ltd.