Re: BUG #17257: (auto)vacuum hangs within lazy_scan_prune()

On Mon, Jan 8, 2024 at 1:21 PM Noah Misch <noah@leadboat.com> wrote:
> > I see no reason why it
> > matters if OldestXmin goes backwards across two VACUUM operations, so
> > I haven't tried to avoid that.
>
> That may be fully okay, or we may want to clamp OldestXmin to be no older than
> relfrozenxid.  I don't feel great about the system moving relfrozenxid
> backward unless it observed an older XID, and observing an older XID would be
> a corruption signal.  I don't have a specific way non-monotonic relfrozenxid
> breaks things, though.

We're already prepared for this -- relfrozenxid simply cannot go
backwards, regardless of what vacuumlazy.c thinks. That is,
vac_update_relstats() won't accept a new relfrozenxid that is < its
existing value (unless it's a value "from the future", which is a way
of recovering after historical pg_upgrade-related corruption bugs).

If memory serves it doesn't take much effort to exercise the relevant
code within vac_update_relstats(). I'm pretty sure that the regression
tests will fail if you run them after removing its defensive
no-older-relfrozenxid test (though I haven't checked recently).

--
Peter Geoghegan