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

On Fri, Apr 26, 2024 at 5:56 PM Melanie Plageman
<melanieplageman@gmail.com> wrote:
>
> On Fri, Apr 26, 2024 at 5:28 PM Peter Geoghegan <pg@bowt.ie> wrote:
> >
> > On Fri, Apr 26, 2024 at 4:46 PM Melanie Plageman
> > <melanieplageman@gmail.com> wrote:
> > > I have a more basic question. How could GlobalVisState->maybe_needed
> > > going backwards cause a problem with relfrozenxid? Yes, if
> > > maybe_needed goes backwards, we may not remove a tuple whose xmin/xmax
> > > are older than VacuumCutoffs->OldestXmin. But, if that tuple's
> > > xmin/xmax are older than OldestXmin, then wouldn't we freeze it?
> >
> > You can't freeze every XID older than OldestXmin.
> > heap_prepare_freeze_tuple() isn't prepared for HEAPTUPLE_DEAD tuples,
> > and expects that those will be taken care of by the time it is called.
>
> But, the tuple isn't HEAPTUPLE_DEAD -- it's HEAPTUPLE_RECENTLY_DEAD.

Why? What tuple is this?

> It will always be HEAPTUPLE_RECENTLY_DEAD in 17 and in <= 16, if
> HeapTupleSatisfiesVacuum() returns HEAPTUPLE_DEAD, we wouldn't call
> heap_prepare_freeze_tuple() because of the retry loop.

The retry loop exists precisely because heap_prepare_freeze_tuple()
isn't prepared to deal with HEAPTUPLE_DEAD tuples. So I agree that
that won't be allowed to happen on versions that have the retry loop
(14 - 16).

As Andres pointed out, even if we were to call
heap_prepare_freeze_tuple() with a HEAPTUPLE_DEAD tuple, we'd get a
"can't happen" error (though it's hard to see this because it doesn't
actually rely on the hint bits set in the tuple).

--
Peter Geoghegan