Re: BUG #18979: pg_upgrade to PG17 fails if max_slot_wal_keep_size is not set to -1

"David G. Johnston" <david(dot)g(dot)johnston(at)gmail(dot)com> writes:
> On Sun, Jul 6, 2025 at 8:26 AM Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>> However, maybe instead of having check_max_slot_wal_keep_size
>> throw an error about this, we could make it just silently keep
>> the value as -1.

> Can't we just move this to postmaster.c ~ line 850 ?

max_slot_wal_keep_size is marked PGC_SIGHUP, so in principle it
could be changed after postmaster start. So if we want a server-side
defense, I don't believe checking at postmaster start is adequate.

In practice, as long as pg_upgrade provides that -c switch, I don't
believe any other GUC source that is allowed to set a PGC_SIGHUP
GUC would override the -c switch. So the need for any server-side
defense isn't obvious to me.

> This seems no different than wal_level and summarize_wal having a
> co-dependency such that intermediate invalid states must be allowed to
> exist so long as what the server ends up running under is valid.

I think that code doesn't do what its author hoped :-(

Anyway, I found the thread for commit 8bfb231b4 which installed
this code [1], and I'm going to go complain there.

regards, tom lane

[1] https://www.postgresql.org/message-id/flat/20231027.115759.2206827438943188717.horikyota.ntt%40gmail.com