Horton

Re: Conflict detection for update_deleted in logical replication

Dilip Kumar <dilipbalaut@gmail.com>

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From: Dilip Kumar <dilipbalaut@gmail.com>
To: Xuneng Zhou <xunengzhou@gmail.com>
Cc: "Zhijie Hou (Fujitsu)" <houzj.fnst@fujitsu.com>, shveta malik <shveta.malik@gmail.com>, vignesh C <vignesh21@gmail.com>, Masahiko Sawada <sawada.mshk@gmail.com>, Nisha Moond <nisha.moond412@gmail.com>, "Hayato Kuroda (Fujitsu)" <kuroda.hayato@fujitsu.com>, Amit Kapila <amit.kapila16@gmail.com>, pgsql-hackers@lists.postgresql.org
Date: 2025-05-24T04:58:50Z
Lists: pgsql-hackers 
On Sat, May 24, 2025 at 10:04 AM Dilip Kumar <dilipbalaut@gmail.com> wrote:
>
> On Fri, May 23, 2025 at 9:21 PM Xuneng Zhou <xunengzhou@gmail.com> wrote:
> >
> Looking at v31-0001 again, most of it looks fine except this logic of
> getting the commit_ts after marking the transaction in commit.  I see
> in RecordTransactionCommit(), we are setting this flag
> (DELAY_CHKPT_IN_COMMIT) to put the transaction in commit state[1], and
> after that we insert the commit log[2], but I noticed that there we
> call GetCurrentTransactionStopTimestamp() for acquiring the commit-ts
> and IIUC we want to ensure that commit-ts timestamp should be after we
> set the transaction in commit with (DELAY_CHKPT_IN_COMMIT), but
> question is, is it guaranteed that the place we are calling
> GetCurrentTransactionStopTimestamp() will always give us the current
> timestamp? Because if you see this function, it may return
> 'xactStopTimestamp' as well if that is already set.  I am still
> digging a bit more. Is there a possibility that 'xactStopTimestamp' is
> already set during some interrupt handling when
> GetCurrentTransactionStopTimestamp() is already called by
> pgstat_report_stat(), or is it guaranteed that during
> RecordTransactionCommit we will call this first time?
>
> If we have already ensured this then I think adding a comment to
> explain the same will be really useful.
>
> [1]
> @@ -1537,7 +1541,7 @@ RecordTransactionCommit(void)
>   */
>   if (markXidCommitted)
>   {
> - MyProc->delayChkptFlags &= ~DELAY_CHKPT_START;
> + MyProc->delayChkptFlags &= ~DELAY_CHKPT_IN_COMMIT;
>   END_CRIT_SECTION();
>   }
>
> [2]
> /*
> * Insert the commit XLOG record.
> */
> XactLogCommitRecord(GetCurrentTransactionStopTimestamp(),
>                                        nchildren, children, nrels, rels,
>                                        ndroppedstats, droppedstats,
>                                        nmsgs, invalMessages,
>                                        RelcacheInitFileInval,
>                                        MyXactFlags,
>                                        InvalidTransactionId, NULL /*
> plain commit */ );

IMHO, this should not be an issue as the only case where
'xactStopTimestamp' is set for the current process is from
ProcessInterrupts->pgstat_report_stat->
GetCurrentTransactionStopTimestamp, and this call sequence is only
possible when transaction blockState is TBLOCK_DEFAULT.  And that is
only set after RecordTransactionCommit() is called, so logically,
RecordTransactionCommit() should always be the first one to set the
'xactStopTimestamp'.  But I still think this is a candidate for
comments, or even better,r if somehow it can be ensured by some
assertion, maybe by passing a parameter in
GetCurrentTransactionStopTimestamp() that if this is called from
RecordTransactionCommit() then 'xactStopTimestamp' must not already be
set.

-- 
Regards,
Dilip Kumar
EnterpriseDB: http://www.enterprisedb.com

Commits

  1. Fix intermittent BF failures in 035_conflicts.

  2. Resume conflict-relevant data retention automatically.

  3. Fix intermittent test failure introduced in 6456c6e2c4.

  4. Fix Coverity issue reported in commit a850be2fe.

  5. Add test to prevent premature removal of conflict-relevant data.

  6. Post-commit review fixes for 228c370868.

  7. Add max_retention_duration option to subscriptions.

  8. Detect and report update_deleted conflicts.

  9. Preserve conflict-relevant data during logical replication.

  10. Integrate FullTransactionIds deeper into two-phase code

  11. Improve checks for GUC recovery_target_timeline

  12. Prevent excessive delays before launching new logrep workers.

  13. Keep WAL segments by slot's last saved restart LSN

  14. Rework some code handling pg_subscription data in psql and pg_dump

  15. Use generateClonedIndexStmt to propagate CREATE INDEX to partitions.