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From: Amit Kapila <amit.kapila16@gmail.com>
Date: Thu, 19 Jun 2025 15:59:25 +0530
Message-ID: 
 <CAA4eK1+qL9QFOD2Q5kcq0Ff=7OcBLE34QuDBdPcdznUzQwv+eg@mail.gmail.com>
Subject: Re: Slot's restart_lsn may point to removed WAL segment after hard
 restart unexpectedly
To: Alexander Korotkov <aekorotkov@gmail.com>
Cc: Vitaly Davydov <v.davydov@postgrespro.ru>,
 vignesh C <vignesh21@gmail.com>,
	"Hayato Kuroda (Fujitsu)" <kuroda.hayato@fujitsu.com>,
 Tom Lane <tgl@sss.pgh.pa.us>,
	Alexander Lakhin <exclusion@gmail.com>,
 pgsql-hackers <pgsql-hackers@postgresql.org>,
	Masahiko Sawada <sawada.mshk@gmail.com>, "tomas@vondra.me" <tomas@vondra.me>
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On Wed, Jun 18, 2025 at 10:17=E2=80=AFPM Alexander Korotkov
<aekorotkov@gmail.com> wrote:
>
> On Wed, Jun 18, 2025 at 6:50=E2=80=AFPM Vitaly Davydov <v.davydov@postgre=
spro.ru> wrote:
> > > I think, it is a good idea. Once we do not use the generated data, it=
 is ok
> > > just to generate WAL segments using the proposed function. I've teste=
d this
> > > function. The tests worked as expected with and without the fix. The =
attached
> > > patch does the change.
> >
> > Sorry, forgot to attach the patch. It is created on the current master =
branch.
> > It may conflict with your corrections. I hope, it could be useful.
>
> Thank you.  I've integrated this into a patch to improve these tests.
>
> Regarding assertion failure, I've found that assert in
> PhysicalConfirmReceivedLocation() conflicts with restart_lsn
> previously set by ReplicationSlotReserveWal().  As I can see,
> ReplicationSlotReserveWal() just picks fresh XLogCtl->RedoRecPtr lsn.
> So, it doesn't seems there is a guarantee that restart_lsn never goes
> backward.  The commit in ReplicationSlotReserveWal() even states there
> is a "chance that we have to retry".
>

I don't see how this theory can lead to a restart_lsn of a slot going
backwards. The retry mentioned there is just a retry to reserve the
slot's position again if the required WAL is already removed. Such a
retry can only get the position later than the previous restart_lsn.

>  Thus, I propose to remove the
> assertion introduced by ca307d5cec90.
>

If what I said above is correct, then the following part of the commit
message will be incorrect:
"As stated in the ReplicationSlotReserveWal() comment, this is not
always true. Additionally, this issue has been spotted by some
buildfarm
members."

--=20
With Regards,
Amit Kapila.