Re: Adding REPACK [concurrently]

Antonin Houska <ah@cybertec.at> wrote:

> Amit Kapila <amit.kapila16@gmail.com> wrote:
> 
> > On Tue, May 5, 2026 at 6:17 PM Antonin Houska <ah@cybertec.at> wrote:
> > >
> > > Antonin Houska <ah@cybertec.at> wrote:
> > >
> > > I think the problem is that with database-specific snapshot,
> > > SnapBuildProcessRunningXacts() returns early, w/o adjusting builder->xmin
> > >
> > >         /*
> > >          * Database specific transaction info may exist to reach CONSISTENT state
> > >          * faster, however the code below makes no use of it. Moreover, such
> > >          * record might cause problems because the following normal (cluster-wide)
> > >          * record can have lower value of oldestRunningXid. In that case, let's
> > >          * wait with the cleanup for the next regular cluster-wide record.
> > >          */
> > >         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 transactions
> > > will be added to the builder's array of committed transactions by
> > > SnapBuildCommitTxn() anyway. However, I failed to notice that COMMIT record of
> > > a transaction listed in the xl_running_xacts WAL record is not guaranteed 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 relied on
> > > might not take place.
> > >
> > 
> > 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.

I think now that "waiting for the end of a transaction listed in the
xl_running_xacts record" in the comment is about transaction removal from
procarray. However, the COMMIT record can still be ahead of xl_running_xacts
because RecordTransactionCommit() is called before
ProcArrayEndTransaction(). I'll think again if the whole problem can be
reproduced with injection points.

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