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: 2021-11-12T04:22:08Z
Lists: pgsql-bugs 
On Thu, Nov 11, 2021 at 4:58 PM Peter Geoghegan <pg@bowt.ie> wrote:
> > What prevents the scenario that some other backend e.g. has a snapshot with
> > xmin=xmax=RECENTLY_DEAD-row. If the RECENTLY_DEAD row has an xid that is later
> > than the DEAD row, this afaict would make it perfectly legal to prune the DEAD
> > row, but *not* the RECENTLY_DEAD one.
>
> I'll need to think about this very carefully. I didn't think it was
> worth blocking v3 on, though naturally it's a big concern.

If we're to traverse HOT chains right to the end in
heap_prune_chain(), reading even LIVE tuples (per the approach
proposed in my bugfix patch), we probably need to be more careful
about concurrently aborted xacts -- relying on the usual
!HeapTupleHeaderIsHotUpdated(htup) test doesn't seem safe.

Imagine if we land on a concurrently-aborted DEAD tuple at the end of
a physical HOT chain -- this might not be caught before we test the
previous tuple in the chain using HeapTupleHeaderIsHotUpdated(htup) --
the abort might happen just as we land on the final/aborted tuple. We
certainly shouldn't conclude that the whole HOT chain is now DEAD,
just because that one tuple is dead.

That definitely cannot happen on HEAD, I think, because we just give
up as soon as we see anything that isn't either DEAD or RECENTLY_DEAD.
But maybe it's possible with the patch.

-- 
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,