Re: pgsql: Implement pg_wal_replay_wait() stored procedure

Hi, Alexander!

On Tue, 22 Oct 2024 at 13:26, Alexander Korotkov <aekorotkov(at)gmail(dot)com>
wrote:

> On Wed, Oct 16, 2024 at 11:20 PM Alexander Korotkov
> <aekorotkov(at)gmail(dot)com> wrote:
> >
> > On Wed, Oct 16, 2024 at 10:35 PM Peter Eisentraut <peter(at)eisentraut(dot)org>
> wrote:
> > > On 02.09.24 01:55, Alexander Korotkov wrote:
> > > > On Mon, Sep 2, 2024 at 2:28 AM Michael Paquier <michael(at)paquier(dot)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.

0001:
Looks completely ok.

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?

0003:
Besides refactor it looks that deleteLSNWaiter() is added
in WaitForLSNReplay. Maybe it's worth mentioning in the commit message.
Maybe refactoring for loop for assigning result variable and breaking a
loop instead of immediate return would look better and lead to natural call
of after the loop before returning.

0004:

Comment:
+ * Waits until recovery replays the target LSN with optional timeout.
Throw
+ * an error on failure.
may need mentioning "Unless no_error provided throws an error on failure"

Otherwise the patch looks good.

Regards,
Pavel Borisov
Supabase