Re: pg_createsubscriber - more logging to say if there are no pubs to drop

On Tue, Oct 14, 2025 at 4:29 PM Peter Smith <smithpb2250@gmail.com> wrote:
>
> On Wed, Oct 15, 2025 at 4:42 AM Masahiko Sawada <sawada.mshk@gmail.com> wrote:
> >
> > On Wed, Oct 8, 2025 at 6:34 PM Peter Smith <smithpb2250@gmail.com> wrote:
> > >
> > > Hi hackers,
> > >
> > > While reviewing pg_createsubscriber in another thread, I found some of
> > > the current logging to be confusing. Specifically, there is one part
> > > that drops all existing publications. Sometimes it might look like
> > > this:
> > >
> > > ----------
> > > pg_createsubscriber: dropping all existing publications in database "db2"
> > > pg_createsubscriber: dropping publication "pub_exists1" in database "db2"
> > > pg_createsubscriber: dropping publication "pub_exists2" in database "db2"
> > > pg_createsubscriber: dropping publication "pub_exists3" in database "db2"
> > > ----------
> > >
> > > ~~~
> > >
> > > OTOH, if there is nothing found to be dropped, then the logging just says:
> > >
> > > ----------
> > > pg_createsubscriber: dropping all existing publications in database "db2"
> > > ----------
> > >
> > > That's the scenario that I found ambiguous. You can't be sure from the
> > > logs what happened:
> > > - Were there publications found, and were they dropped silently?
> > > - Did it not find anything to drop?
> > >
> > > ~~~
> > >
> > > Here is a small patch to remove that doubt. Now, if there is nothing
> > > found, the logging would look like:
> > >
> > > ----------
> > > pg_createsubscriber: dropping all existing publications in database "db2"
> > > pg_createsubscriber: no publications found
> > > ----------
> > >
> > > Thoughts?
> >
> > Thank you for the patch!
> >
> > It sounds like a reasonable improvement. I'll push the patch, barring
> > any objections.
> >
>
> Hm.. I'm wondering now if this patch was correct :-(
>
> I had encountered this problem and made this patch while reviewing
> another thread [1], but that other patch is still in development flux
> and so the logging was changing, and may have been broken at the time
> I wrote this one.
>
> In hindsight, I am not sure if this "no publication found" log is
> working as intended for the master code.
>
> ~~~
>
> For example, during a '--dry-run' the create/drop publication in
> pg_createsubscriber are NOP but they still do the appropriate logging.
>
> So, it is true that PQntuples(res) can be 0, because for dry run
> create_publication is NOP.
> But the dry run will still show logging for the create and for the drop.
>
> I hacked one of the test cases:
>
> # PS HACK
> command_ok(
>     [
>         'pg_createsubscriber',
>         '--verbose',
>         '--dry-run',
>         '--recovery-timeout' => $PostgreSQL::Test::Utils::timeout_default,
>         '--pgdata' => $node_s->data_dir,
>         '--publisher-server' => $node_p->connstr($db1),
>         '--socketdir' => $node_s->host,
>         '--subscriber-port' => $node_s->port,
>         '--publication' => 'pub1',
>         '--publication' => 'pub2',
>         '--subscription' => 'sub1',
>         '--subscription' => 'sub2',
>         '--database' => $db1,
>         '--database' => $db2,
>         '--clean' => 'publications',
>     ],
>     'PS HACK');
>
> This gives a logging result like:
>
> pg_createsubscriber: dropping all existing publications in database "xxx"
> 2025-10-15 10:18:53.583 AEDT client backend[31402]
> 040_pg_createsubscriber.pl LOG:  statement: SELECT pubname FROM
> pg_catalog.pg_publication;
> pg_createsubscriber: no publications found
> pg_createsubscriber: dropping publication "pub2" in database "xxx"
>
> ~~~
>
> First saying "no publications found", and then logging a drop anyway
> may seem contrary to the user.
>
> That was not the intended outcome for this patch.

Hmm. So the subscriber should have at least one publication in
non-dry-run cases, and in dry-run cases it has no subscription but
shows the "dropping publication" log, is that right? If the newly
added log could rather confuse users, we should revert the change.


Regards,

--
Masahiko Sawada
Amazon Web Services: https://aws.amazon.com