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Re: Adding REPACK [concurrently]

Antonin Houska <ah@cybertec.at>

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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>
Date: 2026-04-07T09:29:17Z
Lists: pgsql-hackers 
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 >= 0);
> >  
> > + MemSet(&serialized_snapshot, 0, sizeof(SerializedSnapshotData));
> > +
> >   /* Copy all required fields */
> >   serialized_snapshot.xmin = snapshot->xmin;
> >   serialized_snapshot.xmax = 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:

==1617044==  Address 0x12d745e2 is 106 bytes inside a block of size 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

Commits

Same data as JSON: GET /api/v1/messages/:b64id/commits the thread's linked commits as JSON, with link sources. API reference →

  1. Allow old WAL recycling during REPACK CONCURRENTLY

  2. Advance restart_lsn more eagerly in LogicalConfirmReceivedLocation

  3. Remove unnecessary signal handler change

  4. Improve REPACK (CONCURRENTLY) error messages some more

  5. Revert "Allow logical replication snapshots to be database-specific"

  6. Move REPACK (CONCURRENTLY) test out of stock regression tests

  7. REPACK: do not require REPLICATION or LOGIN

  8. Add missing initialization

  9. Simplify declaration of memcpy target

  10. Reserve replication slots specifically for REPACK

  11. doc: Add an example of REPACK (CONCURRENTLY)

  12. Allow logical replication snapshots to be database-specific

  13. Avoid different-size pointer-to-integer cast

  14. Fix valgrind failure

  15. Add CONCURRENTLY option to REPACK

  16. Rename cluster.c to repack.c (and corresponding .h)

  17. Allow index_create to suppress index_build progress reporting

  18. Make index_concurrently_create_copy more general

  19. Document the 'command' column of pg_stat_progress_repack

  20. Introduce the REPACK command

  21. Toggle logical decoding dynamically based on logical slot presence.

  22. Split vacuumdb to create vacuuming.c/h

  23. Remove ReorderBufferTupleBuf structure.

  24. Revert changes to CONCURRENTLY that "sped up" Xmin advance

  25. VACUUM: ignore indexing operations with CONCURRENTLY