Re: Subscription statistics are not dropped at DROP SUBSCRIPTION in some cases

On Thu, Sep 7, 2023 at 10:22 PM Masahiko Sawada <sawada.mshk@gmail.com> wrote:
>
> On Tue, Sep 5, 2023 at 11:32 AM Zhijie Hou (Fujitsu)
> <houzj.fnst@fujitsu.com> wrote:
> >
> > On Monday, September 4, 2023 10:42 PM Masahiko Sawada <sawada.mshk@gmail.com> wrote:
> >
> > Hi,
> >
> > > On Mon, Sep 4, 2023 at 9:38 PM Zhijie Hou (Fujitsu) <houzj.fnst@fujitsu.com>
> > > wrote:
> > > >
> > > > On Wednesday, July 5, 2023 2:53 PM Masahiko Sawada
> > > <sawada.mshk@gmail.com> wrote:
> > > >
> > > > > Thanks for reviewing the patch. Pushed.
> > > >
> > > > My colleague Vignesh noticed that the newly added test cases were failing in
> > > BF animal sungazer[1].
> > >
> > > Thank you for reporting!
> > >
> > > >
> > > > The test failed to drop the slot which is active on publisher.
> > > >
> > > > --error-log--
> > > > This failure is because pg_drop_replication_slot fails with "replication slot
> > > "test_tab2_sub" is active for PID 55771638":
> > > > 2023-09-02 09:00:04.806 UTC [12910732:4] 026_stats.pl LOG:  statement:
> > > > SELECT pg_drop_replication_slot('test_tab2_sub')
> > > > 2023-09-02 09:00:04.807 UTC [12910732:5] 026_stats.pl ERROR:
> > > > replication slot "test_tab2_sub" is active for PID 55771638
> > > > 2023-09-02 09:00:04.807 UTC [12910732:6] 026_stats.pl STATEMENT:
> > > > SELECT pg_drop_replication_slot('test_tab2_sub')
> > > > -------------
> > > >
> > > > I the reason is that the test DISABLEd the subscription before
> > > > dropping the slot, while "ALTER SUBSCRIPTION DISABLE" doesn't wait for
> > > > the walsender to release the slot, so it's possible that the walsender
> > > > is still alive when calling
> > > > pg_drop_replication_slot() to drop the slot on
> > > > publisher(pg_drop_xxxslot() doesn't wait for slot to be released).
> > >
> > > I agree with your analysis.
> > >
> > > >
> > > > I think we can wait for the slot to become inactive before dropping like:
> > > >         $node_primary->poll_query_until('otherdb',
> > > >                 "SELECT NOT EXISTS (SELECT 1 FROM pg_replication_slots
> > > WHERE active_pid IS NOT NULL)"
> > > >         )
> > > >
> > >
> > > I prefer this approach but it would be better to specify the slot name in the
> > > query.
> > >
> > > > Or we can just don't drop the slot as it’s the last testcase.
> > >
> > > Since we might add other tests after that in the future, I think it's better to drop
> > > the replication slot (and subscription).
> > >
> > > >
> > > > Another thing might be worth considering is we can add one parameter
> > > > in
> > > > pg_drop_replication_slot() to let user control whether to wait or not,
> > > > and the test can be fixed as well by passing nowait=false to the func.
> > >
> > > While it could be useful in general we cannot use this approach for this issue
> > > since it cannot be backpatched to older branches. We might want to discuss it
> > > on a new thread.
> > >
> > > I've attached a draft patch to fix this issue. Feedback is very welcome.
> >
> > Thanks, it looks good to me.
>
> Thank you for reviewing the patch.
>
> I'll push the attached patch to master, v16, and v15 tomorrow, barring
> any objections.

Pushed.

Regards,

--
Masahiko Sawada
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