Fix improper abort during update chain locking

In 247c76a98909, I added some code to do fine-grained checking of
MultiXact status of locking/updating transactions when traversing an
update chain.  There was a thinko in that patch which would have the
traversing abort, that is return HeapTupleUpdated, when the other
transaction is a committed lock-only.  In this case we should ignore it
and return success instead.  Of course, in the case where there is a
committed update, HeapTupleUpdated is the correct return value.

A user-visible symptom of this bug is that in REPEATABLE READ and
SERIALIZABLE transaction isolation modes spurious serializability errors
can occur:
  ERROR:  could not serialize access due to concurrent update

In order for this to happen, there needs to be a tuple that's key-share-
locked and also updated, and the update must abort; a subsequent
transaction trying to acquire a new lock on that tuple would abort with
the above error.  The reason is that the initial FOR KEY SHARE is seen
as committed by the new locking transaction, which triggers this bug.
(If the UPDATE commits, then the serialization error is correctly
reported.)

When running a query in READ COMMITTED mode, what happens is that the
locking is aborted by the HeapTupleUpdated return value, then
EvalPlanQual fetches the newest version of the tuple, which is then the
only version that gets locked.  (The second time the tuple is checked
there is no misbehavior on the committed lock-only, because it's not
checked by the code that traverses update chains; so no bug.) Only the
newest version of the tuple is locked, not older ones, but this is
harmless.

The isolation test added by this commit illustrates the desired
behavior, including the proper serialization errors that get thrown.

Backpatch to 9.3.

1 files changed, 16 insertions, 3 deletions

diff --git a/src/backend/access/heap/heapam.c b/src/backend/access/heap/heapam.c
index 8d596202ba4..2035a2158f1 100644
--- a/src/backend/access/heap/heapam.c
+++ b/src/backend/access/heap/heapam.c
@@ -4859,10 +4859,23 @@ test_lockmode_for_conflict(MultiXactStatus status, TransactionId xid,
 	else if (TransactionIdDidCommit(xid))
 	{
 		/*
-		 * If the updating transaction committed, what we do depends on whether
-		 * the lock modes conflict: if they do, then we must report error to
-		 * caller.  But if they don't, we can fall through to lock it.
+		 * The other transaction committed.  If it was only a locker, then the
+		 * lock is completely gone now and we can return success; but if it
+		 * was an update, then what we do depends on whether the two lock
+		 * modes conflict.	If they conflict, then we must report error to
+		 * caller. But if they don't, we can fall through to allow the current
+		 * transaction to lock the tuple.
+		 *
+		 * Note: the reason we worry about ISUPDATE here is because as soon as
+		 * a transaction ends, all its locks are gone and meaningless, and
+		 * thus we can ignore them; whereas its updates persist.  In the
+		 * TransactionIdIsInProgress case, above, we don't need to check
+		 * because we know the lock is still "alive" and thus a conflict needs
+		 * always be checked.
 		 */
+		if (!ISUPDATE_from_mxstatus(status))
+			return HeapTupleMayBeUpdated;
+
 		if (DoLockModesConflict(LOCKMODE_from_mxstatus(status),
 								LOCKMODE_from_mxstatus(wantedstatus)))
 			/* bummer */