Avoid resetting Xmax when it's a multi with an aborted update

alvherre · alvherre · commit c6cd27e36b9c · 2013-12-05T12:21:55.000-03:00
HeapTupleSatisfiesUpdate can very easily "forget" tuple locks while checking the contents of a multixact and finding it contains an aborted update, by setting the HEAP_XMAX_INVALID bit. This would lead to concurrent transactions not noticing any previous locks held by transactions that might still be running, and thus being able to acquire subsequent locks they wouldn't be normally able to acquire. This bug was introduced in commit 1ce150b; backpatch this fix to 9.3, like that commit. This change reverts the change to the delete-abort-savept isolation test in 1ce150b, because that behavior change was caused by this bug. Noticed by Andres Freund while investigating a different issue reported by Noah Misch.
diff --git a/src/backend/utils/time/tqual.c b/src/backend/utils/time/tqual.c
@@ -789,13 +789,26 @@ HeapTupleSatisfiesUpdate(HeapTupleHeader tuple, CommandId curcid,
 		if (TransactionIdDidCommit(xmax))
 			return HeapTupleUpdated;
 
-		/* no member, even just a locker, alive anymore */
+		/*
+		 * By here, the update in the Xmax is either aborted or crashed, but
+		 * what about the other members?
+		 */
+
 		if (!MultiXactIdIsRunning(HeapTupleHeaderGetRawXmax(tuple)))
+		{
+			/*
+			 * There's no member, even just a locker, alive anymore, so we can
+			 * mark the Xmax as invalid.
+			 */
 			SetHintBits(tuple, buffer, HEAP_XMAX_INVALID,
 						InvalidTransactionId);
-
-		/* it must have aborted or crashed */
-		return HeapTupleMayBeUpdated;
+			return HeapTupleMayBeUpdated;
+		}
+		else
+		{
+			/* There are lockers running */
+			return HeapTupleBeingUpdated;
+		}
 	}
 
 	if (TransactionIdIsCurrentTransactionId(HeapTupleHeaderGetRawXmax(tuple)))
diff --git a/src/test/isolation/expected/delete-abort-savept.out b/src/test/isolation/expected/delete-abort-savept.out
@@ -23,11 +23,12 @@ key            value
 step s1svp: SAVEPOINT f;
 step s1d: DELETE FROM foo;
 step s1r: ROLLBACK TO f;
-step s2l: SELECT * FROM foo FOR UPDATE;
+step s2l: SELECT * FROM foo FOR UPDATE; <waiting ...>
+step s1c: COMMIT;
+step s2l: <... completed>
 key            value          
 
 1              1              
-step s1c: COMMIT;
 step s2c: COMMIT;
 
 starting permutation: s1l s1svp s1d s1r s2l s2c s1c
@@ -38,12 +39,8 @@ key            value
 step s1svp: SAVEPOINT f;
 step s1d: DELETE FROM foo;
 step s1r: ROLLBACK TO f;
-step s2l: SELECT * FROM foo FOR UPDATE;
-key            value          
-
-1              1              
-step s2c: COMMIT;
-step s1c: COMMIT;
+step s2l: SELECT * FROM foo FOR UPDATE; <waiting ...>
+invalid permutation detected
 
 starting permutation: s1l s1svp s1d s2l s1r s1c s2c
 step s1l: SELECT * FROM foo FOR KEY SHARE;
