From: Antonin Houska <ah@cybertec.at>
To: Srinath Reddy Sadipiralla <srinath2133@gmail.com>
Cc: Tom Lane <tgl@sss.pgh.pa.us>, Andres Freund <andres@anarazel.de>,
    Noah Misch <noah@leadboat.com>,
    Alvaro Herrera <alvherre@alvh.no-ip.org>,
    vignesh C <vignesh21@gmail.com>,
    Amit Kapila <amit.kapila16@gmail.com>,
    Mihail Nikalayeu <mihailnikalayeu@gmail.com>,
    Matthias van de Meent <boekewurm+postgres@gmail.com>,
    Pg Hackers <pgsql-hackers@lists.postgresql.org>,
    Robert Treat <rob@xzilla.net>
Subject: Re: Adding REPACK [concurrently]
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Comments: In-reply-to Antonin Houska <ah@cybertec.at>
   message dated "Tue, 07 Apr 2026 10:57:46 +0200."
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Date: Tue, 07 Apr 2026 11:29:17 +0200
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Antonin Houska <ah@cybertec.at> wrote:

> Srinath Reddy Sadipiralla <srinath2133@gmail.com> wrote:
> =

> > i looked into this , it seems like valgrind catches the uninitialised =
padding bytes, which
> > repack worker is writing using BufFileWrite, it seems this fix solved =
the problem.
> > =

> > diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time=
/snapmgr.c
> > index 2e6197f5f35..f5682b87626 100644
> > --- a/src/backend/utils/time/snapmgr.c
> > +++ b/src/backend/utils/time/snapmgr.c
> > @@ -1739,6 +1739,8 @@ SerializeSnapshot(Snapshot snapshot, char *start=
_address)
> >  =

> >   Assert(snapshot->subxcnt >=3D 0);
> >  =

> > + MemSet(&serialized_snapshot, 0, sizeof(SerializedSnapshotData));
> > +
> >   /* Copy all required fields */
> >   serialized_snapshot.xmin =3D snapshot->xmin;
> >   serialized_snapshot.xmax =3D snapshot->xmax;
> > =

> > thoughts?
> =

> Could you reproduce the failure in your environment?
> =

> I haven't thought of this explanation because BufFileWrite() only copies=
 the
> data to a buffer in the BufFile structure and BufFileDumpBuffer() writes=
 the
> buffer. Maybe valgrind is able to track the copying?

Given this message, you may be right:

=3D=3D1617044=3D=3D  Address 0x12d745e2 is 106 bytes inside a block of siz=
e 8,272 client-defined

In my environment, the 'buffer' field starts at offset 80 into the BufFile
structure. We first write 8 bytes into it

	BufFileWrite(file, &snap_size, sizeof(snap_size));

followed by the snapshot. Since sizeof(SerializedSnapshotData) is 24, the
offset 106 should be the padding following the 'takenDuringRecovery' field=
.

-- =

Antonin Houska
Web: https://www.cybertec-postgresql.com