Horton

Re: Conflict detection for update_deleted in logical replication

Nisha Moond <nisha.moond412@gmail.com>

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From: Nisha Moond <nisha.moond412@gmail.com>
To: "Zhijie Hou (Fujitsu)" <houzj.fnst@fujitsu.com>
Cc: Amit Kapila <amit.kapila16@gmail.com>, shveta malik <shveta.malik@gmail.com>, Masahiko Sawada <sawada.mshk@gmail.com>, "Hayato Kuroda (Fujitsu)" <kuroda.hayato@fujitsu.com>, pgsql-hackers <pgsql-hackers@postgresql.org>, vignesh C <vignesh21@gmail.com>, Dilip Kumar <dilipbalaut@gmail.com>
Date: 2025-08-30T07:40:56Z
Lists: pgsql-hackers 
On Sat, Aug 30, 2025 at 10:17 AM Nisha Moond <nisha.moond412@gmail.com> wrote:
>
> On Fri, Aug 29, 2025 at 11:49 AM Zhijie Hou (Fujitsu)
> <houzj.fnst@fujitsu.com> wrote:
> >
> > Here is the new version patch set which also addressed Shveta's comments[1].
> >
>
> Thanks for the patches here, I tested the v68-001 patch alone, please
> find review comments -
>

Further review comments for v68-0001 patch -

3) v68 seems to have introduced a bug:

@@ -1254,9 +1257,12 @@ ApplyLauncherMain(Datum main_arg)
  /*
  * Compute the minimum xmin required to protect dead tuples
  * required for conflict detection among all running apply
- * workers that enables retain_dead_tuples.
+ * workers.
  */
- if (sub->retaindeadtuples && can_advance_xmin)
+ if (TransactionIdIsValid(MyReplicationSlot->data.xmin) &&
+ sub->retaindeadtuples &&
+ sub->retentionactive &&
+ can_update_xmin)
  compute_min_nonremovable_xid(w, &xmin);

The new check "TransactionIdIsValid(MyReplicationSlot->data.xmin)" can
cause a segmentation fault in the launcher when a default subscription
is created (i.e., retain_dead_tuples=off) and the conflict slot does
not exist.
Perhaps it should first check "sub->retaindeadtuples" before accessing the slot.

--
Thanks,
Nisha