Re: Conflict detection for update_deleted in logical replication

On Fri, May 23, 2025 at 9:21 PM Xuneng Zhou <xunengzhou(at)gmail(dot)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 */ );

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