Horton

Re: BUG #17255: Server crashes in index_delete_sort_cmp() due to race condition with vacuum

Peter Geoghegan <pg@bowt.ie>

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From: Peter Geoghegan <pg@bowt.ie>
To: Andres Freund <andres@anarazel.de>
Cc: Dmitry Dolgov <9erthalion6@gmail.com>, Alexander Lakhin <exclusion@gmail.com>, Matthias van de Meent <boekewurm+postgres@gmail.com>, PostgreSQL mailing lists <pgsql-bugs@lists.postgresql.org>
Date: 2021-11-11T01:37:38Z
Lists: pgsql-bugs 
On Wed, Nov 10, 2021 at 4:47 PM Andres Freund <andres@anarazel.de> wrote:
> On 2021-11-10 13:04:43 -0800, Peter Geoghegan wrote:
> > On Wed, Nov 10, 2021 at 11:20 AM Andres Freund <andres@anarazel.de> wrote:
> > > The way this definitely breaks - I have been able to reproduce this in
> > > isolation - is when one tuple is processed twice by heap_prune_chain(), and
> > > the result of HeapTupleSatisfiesVacuum() changes from
> > > HEAPTUPLE_DELETE_IN_PROGRESS to DEAD.
> >
> > I had no idea that that was now possible. I really think that this
> > ought to be documented centrally.
>
> Where would you suggest?

Offhand I'd say that it would be a good idea to add comments over the
call to vacuum_set_xid_limits() made from vacuumlazy.c.

You might also move the call to GlobalVisTestFor() out of
lazy_scan_heap(), so that it gets called right after
vacuum_set_xid_limits(). That would make the new explanation easier to
follow, since you are after all explaining the relationship between
OldestXmin (or the vacuum_set_xid_limits() call itself) and vistest
(or the GlobalVisTestFor() call itself).

Why do they have to be called in that order? Or do they? I noticed
that "make check-world" won't break if you switch the order.

I assume that you're going to want to say something about what needs
to happen in lazy_scan_prune() in these new comments -- since that is
where the relationship between these two things is most crucial.

Thanks
-- 
Peter Geoghegan

Commits

  1. vacuumlazy.c: Standardize rel_pages terminology.

  2. vacuumlazy.c: document vistest and OldestXmin.

  3. heap pruning: Only call BufferGetBlockNumber() once.

  4. Fix possible HOT corruption when RECENTLY_DEAD changes to DEAD while pruning.

  5. Assert redirect pointers are sensible after heap_page_prune().

  6. Remove tupgone special case from vacuumlazy.c.

  7. Recycle nbtree pages deleted during same VACUUM.

  8. snapshot scalability: Don't compute global horizons while building snapshots.

  9. Refactor heap_page_prune so that instead of changing item states on-the-fly,