MultiXact bugs

Hi,

The attached pgbench testcase can reproduce two issues:
1) (takes a bit)
TRAP: FailedAssertion("!(((xid) >= ((TransactionId) 3)))", File:/pruneheap.c", Line: 601)

That's because HeapTupleHeaderGetUpdateXid() ignores aborted updaters
and returns InvalidTransactionId in that case, but
HeapTupleSatisfiesVacuum() returns HEAPTUPLE_DELETE_IN_PROGRESS...

Looks like a 9.3+ issue, and shouldn't have a ll that high impact in
non-assert builds, page pruning will be delayed a bit.

2) we frequently error out in heap_lock_updated_tuple_rec() with
ERROR: unable to fetch updated version of tuple

That's because when we're following a ctid chain, it's perfectly
possible for the updated version of a tuple to already have been
vacuumed/pruned away if the the updating transaction has aborted.

Also looks like a 9.3+ issues to me, slightly higher impact, but in the
end you're just getting some errors under concurrency.

Greetings,

Andres Freund

--
Andres Freund http://www.2ndQuadrant.com/
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