Re: A recent message added to pg_upgade

At Fri, 27 Oct 2023 14:57:10 +0530, Amit Kapila <amit.kapila16@gmail.com> wrote in 
> On Fri, Oct 27, 2023 at 2:02 PM Alvaro Herrera <alvherre@alvh.no-ip.org> wrote:
> >
> > On 2023-Oct-27, Kyotaro Horiguchi wrote:
> >
> > > @@ -1433,8 +1433,8 @@ InvalidatePossiblyObsoleteSlot(ReplicationSlotInvalidationCause cause,
> > >               {
> > >                       ereport(ERROR,
> > >                                       errcode(ERRCODE_INVALID_PARAMETER_VALUE),
> > > -                                     errmsg("replication slots must not be invalidated during the upgrade"),
> > > -                                     errhint("\"max_slot_wal_keep_size\" must be set to -1 during the upgrade"));
> >
> > Hmm, if I read this code right, this error is going to be thrown by the
> > checkpointer while finishing a checkpoint.  Fortunately, the checkpoint
> > record has already been written, but I don't know what would happen if
> > this is thrown while trying to write the shutdown checkpoint.  Probably
> > nothing terribly good.
> >
> > I don't think this is useful.  If the setting is invalid during binary
> > upgrade, let's prevent it from being set at all right from the start of
> > the upgrade process.
> 
> We are already forcing the required setting
> "max_slot_wal_keep_size=-1" during the upgrade similar to some of the
> other settings like "full_page_writes". However, the user can provide
> an option for "max_slot_wal_keep_size" in which case it will be
> overridden. Now, I think (a) we can ensure that our setting always
> takes precedence in this case. The other idea is (b) to parse the
> user-provided options and check if "max_slot_wal_keep_size" has a
> value different than expected and raise an error accordingly. Or we
> can simply (c) document the usage of max_slot_wal_keep_size in the
> upgrade. I am not sure whether it is worth complicating the code for
> this as the user shouldn't be using such an option during the upgrade.
> So, I think doing (a) and (c) could be simpler.
> >
> >  In InvalidatePossiblyObsoleteSlot() we could have
> > just an Assert() or elog(PANIC).
> >
> 
> Yeah, we can change to either of those.

This discussion seems like a bit off from my point. I suggested adding
a check for that setting when IsBinaryUpgraded is true at the GUC
level as shown in the attached patch. I believe Álvaro made a similar
suggestion.  While the error message is somewhat succinct, I think it
is sufficient given the low possilibility of the scenario and the fact
that it cannot occur inadvertently.

-- 
Kyotaro Horiguchi
NTT Open Source Software Center