Re: pgsql: Implement pg_wal_replay_wait() stored procedure

On Wed, Oct 23, 2024 at 9:02 AM Pavel Borisov <pashkin.elfe@gmail.com> wrote:
> On Wed, 23 Oct 2024 at 00:12, Alexander Korotkov <aekorotkov@gmail.com> wrote:
>> Thank you for your review.
>>
>> On Tue, Oct 22, 2024 at 4:30 PM Pavel Borisov <pashkin.elfe@gmail.com> wrote:
>> > On Tue, 22 Oct 2024 at 13:26, Alexander Korotkov <aekorotkov@gmail.com> wrote:
>> >>
>> >> On Wed, Oct 16, 2024 at 11:20 PM Alexander Korotkov
>> >> <aekorotkov@gmail.com> wrote:
>> >> >
>> >> > On Wed, Oct 16, 2024 at 10:35 PM Peter Eisentraut <peter@eisentraut.org> wrote:
>> >> > > On 02.09.24 01:55, Alexander Korotkov wrote:
>> >> > > > On Mon, Sep 2, 2024 at 2:28 AM Michael Paquier <michael@paquier.xyz> wrote:
>> >> > > >> On Sun, Sep 01, 2024 at 10:35:27PM +0300, Alexander Korotkov wrote:
>> >> > > >>> This path hasn't changes since the patch revision when it was a
>> >> > > >>> utility command.  I agree that this doesn't look like proper path for
>> >> > > >>> stored procedure.  But I don't think src/backend/utils/adt is
>> >> > > >>> appropriate path either, because it's not really about data type.
>> >> > > >>> pg_wal_replay_wait() looks a good neighbor for
>> >> > > >>> pg_wal_replay_pause()/pg_wal_replay_resume() and other WAL-related
>> >> > > >>> functions.  So, what about moving it to src/backend/access/transam?
>> >> > > >>
>> >> > > >> Moving the new function to xlogfuncs.c while publishing
>> >> > > >> WaitForLSNReplay() makes sense to me.
>> >> > > >
>> >> > > > Thank you for proposal.  I like this.
>> >> > > >
>> >> > > > Could you, please, check the attached patch?
>> >> > >
>> >> > > We still have stuff in src/backend/commands/waitlsn.c that is nothing
>> >> > > like a "command".  You have moved some stuff elsewhere, but what are you
>> >> > > planning to do with the rest?
>> >> >
>> >> > Thank you for spotting this another time.  What about moving that
>> >> > somewhere like src/backend/access/transam/xlogwait.c ?
>> >>
>> >> Implemented this as a separate patch (0001).  Also rebased other
>> >> pending patches on that.  0004 now revises header comment of
>> >> xlogwait.c with new procedure signature.
>> >
>> >
>> > I've looked at v5 of a patchset.
>>
>> > 0002:
>> >
>> > As stated in latch.c
>> >
>> > - WL_POSTMASTER_DEATH: Wait for postmaster to die
>> > - WL_EXIT_ON_PM_DEATH: Exit immediately if the postmaster dies
>> >
>> >  * wakeEvents must include either WL_EXIT_ON_PM_DEATH for automatic exit
>> >  * if the postmaster dies or WL_POSTMASTER_DEATH for a flag set in the
>> >  * return value if the postmaster dies
>> >
>> > It's not completely clear to me if these comments need some clarification (not related to the patchset), or if we should look for WL_EXIT_ON_PM_DEATH for immediately FATAL. Or waiting for postmaster to die on WL_POSTMASTER_DEATH instead of just fatal immediately?
>>
>> As I get from the code, WL_EXIT_ON_PM_DEATH cause process to just
>> proc_exit(1) without throwing FATAL.  So, in the most of situations we
>> do throw FATAL after seeing WL_POSTMASTER_DEATH event.  So, it's
>> reasonable to do the same here.  But indeed, this is a question (not
>> related to this patch) whether WL_EXIT_ON_PM_DEATH should cause
>> process to throw FATAL.
>
>
> Libpq ends up with FATAL on WL_POSTMASTER_DEATH.
> In a backend code on WL_POSTMASTER_DEATH: SyncRepWaitForLSN() sets ProcDiePending but don't FATAL. Walsender exits proc_exit(1).
> I suppose WL_POSTMASTER_DEATH expected behavior is "Do whatever you want: wait for postmaster to die or end up immediately".
> I think path 0002 is good.
>
> I looked through patches v6 and I think they're all good now.

Thank you.  I will push this patchset if now objections.

------
Regards,
Alexander Korotkov
Supabase