From: Antonin Houska <ah@cybertec.at>
To: Amit Kapila <amit.kapila16@gmail.com>
cc: Mihail Nikalayeu <mihailnikalayeu@gmail.com>,
    Andres Freund <andres@anarazel.de>,
    Alvaro Herrera <alvherre@alvh.no-ip.org>,
    Srinath Reddy Sadipiralla <srinath2133@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]
In-reply-to: 
 <CAA4eK1KC6CGN-N2bUffSign8Sw4q6=8d3L-Xh4t+50GCdQb6zw@mail.gmail.com>
References: 
 <CAA4eK1Jg21ODQ7fS2fvN5W_S5kDRhAP5inj3XMRQaa=s-GbYhw@mail.gmail.com>
 <202604071230.b5axxf3qna3m@alvherre.pgsql>
 <cdgw4sbbfcgk6du3iv54r2dgiy4tfywoklbotlmj4irxavdcr3@glxfw5jj277q>
 <227677.1775576304@localhost>
 <pveffyxhnuurhb44uzqlwo3rkyzorkfh2rot7uwzlf2axhfvbp@7nrs2omysxkc>
 <CAA4eK1JhuT5fyTosWDZ+Pgs+j7xEjObTyRMn80uNKgi_ivqHbw@mail.gmail.com>
 <CADzfLwWnbKcb3v8sStdgNE=WNc3uUqx5SiS4zftX2UaEfNzG5w@mail.gmail.com>
 <85813.1777901089@localhost> <27869.1777985266@localhost>
 <CAA4eK1KC6CGN-N2bUffSign8Sw4q6=8d3L-Xh4t+50GCdQb6zw@mail.gmail.com>
Comments: In-reply-to Amit Kapila <amit.kapila16@gmail.com>
   message dated "Sun, 10 May 2026 17:01:04 +0530."
MIME-Version: 1.0
Content-Type: text/plain; charset=utf-8
Content-Transfer-Encoding: quoted-printable
Date: Mon, 11 May 2026 17:17:52 +0200
Message-ID: <70574.1778512672@localhost>
Archived-At: 
 <https://www.postgresql.org/message-id/70574.1778512672%40localhost>
Precedence: bulk

Amit Kapila <amit.kapila16@gmail.com> wrote:

> On Tue, May 5, 2026 at 6:17=E2=80=AFPM Antonin Houska <ah@cybertec.at> wr=
ote:
> >
> > Antonin Houska <ah@cybertec.at> wrote:
> >
> > I think the problem is that with database-specific snapshot,
> > SnapBuildProcessRunningXacts() returns early, w/o adjusting builder->xm=
in
> >
> >         /*
> >          * Database specific transaction info may exist to reach CONSIS=
TENT state
> >          * faster, however the code below makes no use of it. Moreover,=
 such
> >          * record might cause problems because the following normal (cl=
uster-wide)
> >          * record can have lower value of oldestRunningXid. In that cas=
e, let's
> >          * wait with the cleanup for the next regular cluster-wide reco=
rd.
> >          */
> >         if (OidIsValid(running->dbid))
> >                 return;
> >
> > and thus some transactions whose XID is below running->oldestRunningXid=
 may
> > continue to be incorrectly considered running.
> >
> > I originally thought that this should not happen because such transacti=
ons
> > will be added to the builder's array of committed transactions by
> > SnapBuildCommitTxn() anyway. However, I failed to notice that COMMIT re=
cord of
> > a transaction listed in the xl_running_xacts WAL record is not guarante=
ed to
> > follow the xl_running_xacts record in WAL. In other words, even if
> > xl_running_xacts is created before a COMMIT record of the contained
> > transaction, it may end up at higher LSN in WAL. So the cleanup I relie=
d on
> > might not take place.
> >
>=20
> BTW, is it possible to write a test by using injection_points or via
> manual steps (by using debugger, etc) so that we can more clearly
> understand this problem and proposed fix?

So far I could observe the situation in WAL, but have no idea how it can
happen. For example, transaction 49242 gets committed here

rmgr: Transaction len (rec/tot): 46/ 46, tx: 49242, lsn: 0/18BC28C8, prev
0/18BC2890, desc: COMMIT 2026-05-11 16:38:16.603265 CEST

and then it appears in the 'xids' list of RUNNING_XACTS:

rmgr: Standby     len (rec/tot):    106/   106, tx:          0, lsn:
0/18BC3140, prev 0/18BC3100, desc: RUNNING_XACTS nextXid 49255
latestCompletedXid 49241 oldestRunningXid 49242; 13 xacts: 49248 49249 49246
49243 49252 49251 49244 49245 49242 49250 49253 49254 49247; dbid:5


I thought the situation is quite common (and therefore nothing of
SnapBuildProcessRunningXacts() should be skipped), but when trying to
reproduce the problem, I noticed that LogStandbySnapshot() shouldn't allow
that ordering issue when logical decoding is enabled:

	/*
	 * GetRunningTransactionData() acquired ProcArrayLock, we must release it.
	 * For Hot Standby this can be done before inserting the WAL record
	 * because ProcArrayApplyRecoveryInfo() rechecks the commit status using
	 * the clog. For logical decoding, though, the lock can't be released
	 * early because the clog might be "in the future" from the POV of the
	 * historic snapshot. This would allow for situations where we're waiting
	 * for the end of a transaction listed in the xl_running_xacts record
	 * which, according to the WAL, has committed before the xl_running_xacts
	 * record. Fortunately this routine isn't executed frequently, and it's
	 * only a shared lock.
	 */
	if (!logical_decoding_enabled)
		LWLockRelease(ProcArrayLock);

So I don't have the answer right now.

--=20
Antonin Houska
Web: https://www.cybertec-postgresql.com