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

Dear Sawada-san,

> > 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?

(Note again - currently we do not find such a case, so it may be overkill)

[1]: https://www.postgresql.org/docs/devel/libpq-exec.html#LIBPQ-PQGETVALUE

Best Regards,
Hayato Kuroda
FUJITSU LIMITED