Re: pgsql: Implement pg_wal_replay_wait() stored procedure

Hi, Pavel!

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.
>
> 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?

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.

> 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.

I don't think we would get much simplicity/readability by breaking
loop instead of immediate return.  However, I reflected the changes in
the commit message.  Also I reflected that we don't distinguish any
more seeing !RecoveryInProgress() in different places.

> 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"

Changed as you proposed.

> Otherwise the patch looks good.

Thanks!

------
Regards,
Alexander Korotkov
Supabase