Re: [PoC] pg_upgrade: allow to upgrade publisher node

On Thu, Nov 30, 2023 at 6:49 PM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
>
> On Wed, Nov 29, 2023 at 7:33 AM Hayato Kuroda (Fujitsu)
> <kuroda(dot)hayato(at)fujitsu(dot)com> wrote:
> >
> > > > Actually, we do not expect that it won't input NULL. IIUC all of slots have
> > > > slot_name, and subquery uses its name. But will it be kept forever? I think we
> > > > can avoid any risk.
> > > >
> > > > > I've not tested it yet but even if it returns NULL, perhaps
> > > > > get_old_cluster_logical_slot_infos() would still set curr->caught_up
> > > > > to false, no?
> > > >
> > > > Hmm. I checked the C99 specification [1] of strcmp, but it does not define the
> > > > case when the NULL is input. So it depends implementation.
> > >
> > > I think PQgetvalue() returns an empty string if the result value is null.
> > >
> >
> > Oh, you are right... I found below paragraph from [1].
> >
> > > An empty string is returned if the field value is null. See PQgetisnull to distinguish
> > > null values from empty-string values.
> >
> > So I agree what you said - current code can accept NULL.
> > But still not sure the error message is really good or not.
> > If we regard an empty string as false, the slot which has empty name will be reported like:
> > "The slot \"\" has not consumed the WAL yet" in check_old_cluster_for_valid_slots().
> > Isn't it inappropriate?
> >
>
> I see your point that giving a better message (which would tell the
> actual problem) to the user in this case also has a value. OTOH, as
> you said, this case won't happen in practical scenarios, so I am fine
> either way with a slight tilt toward retaining a better error message
> (aka the current way). Sawada-San/Bharath, do you have any suggestions
> on this?

TBH I'm not sure the error message is much helpful for users more than
the message "The slot \"\" has not consumed the WAL yet" in practice.
In either case, the messages just tell the user the slot name passed
to the function was not appropriate. Rather, I'm a bit concerned that
we create a precedent that we make a function non-strict to produce an
error message only for unrealistic cases. Please point out if we
already have such precedents. Other functions in pg_upgrade_support.c
such as binary_upgrade_set_next_pg_tablespace_oid() are not called if
the argument is NULL since it's a strict function. But if null was
passed in (where should not happen in practice), pg_upgrade would fail
with an error message or would finish while leaving the cluster in an
inconsistent state, I've not tested. Why do we want to care about the
argument being NULL only in
binary_upgrade_logical_slot_has_caught_up()?

Regards,

--
Masahiko Sawada
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