Fix race in SERIALIZABLE READ ONLY.

macdice · macdice · commit af397c6c27b7 · 2023-03-09T16:56:51.000+13:00
Commit bdaabb9 started skipping doomed transactions when building the list of possible conflicts for SERIALIZABLE READ ONLY. That makes sense, because doomed transactions won't commit, but a couple of subtle things broke: 1. If all uncommitted r/w transactions are doomed, a READ ONLY transaction would arbitrarily not benefit from the safe snapshot optimization. It would not be taken immediately, and yet no other transaction would set SXACT_FLAG_RO_SAFE later. 2. In the same circumstances but with DEFERRABLE, GetSafeSnapshot() would correctly exit its wait loop without sleeping and then take the optimization in non-assert builds, but assert builds would fail a sanity check that SXACT_FLAG_RO_SAFE had been set by another transaction. This is similar to the case for PredXact->WritableSxactCount == 0. We should opt out immediately if our possibleUnsafeConflicts list is empty after filtering. The code to maintain the serializable global xmin is moved down below the new opt out site, because otherwise we'd have to reverse its effects before returning. Back-patch to all supported releases. Bug #17368. Reported-by: Alexander Lakhin <exclusion@gmail.com> Discussion: https://postgr.es/m/17116-d6ca217acc180e30%40postgresql.org Discussion: https://postgr.es/m/20110707212159.GF76634%40csail.mit.edu
diff --git a/src/backend/storage/lmgr/predicate.c b/src/backend/storage/lmgr/predicate.c
@@ -1847,24 +1847,6 @@ GetSerializableTransactionSnapshotInt(Snapshot snapshot,
 		return snapshot;
 	}
 
-	/* Maintain serializable global xmin info. */
-	if (!TransactionIdIsValid(PredXact->SxactGlobalXmin))
-	{
-		Assert(PredXact->SxactGlobalXminCount == 0);
-		PredXact->SxactGlobalXmin = snapshot->xmin;
-		PredXact->SxactGlobalXminCount = 1;
-		SerialSetActiveSerXmin(snapshot->xmin);
-	}
-	else if (TransactionIdEquals(snapshot->xmin, PredXact->SxactGlobalXmin))
-	{
-		Assert(PredXact->SxactGlobalXminCount > 0);
-		PredXact->SxactGlobalXminCount++;
-	}
-	else
-	{
-		Assert(TransactionIdFollows(snapshot->xmin, PredXact->SxactGlobalXmin));
-	}
-
 	/* Initialize the structure. */
 	sxact->vxid = vxid;
 	sxact->SeqNo.lastCommitBeforeSnapshot = PredXact->LastSxactCommitSeqNo;
@@ -1902,6 +1884,19 @@ GetSerializableTransactionSnapshotInt(Snapshot snapshot,
 				SetPossibleUnsafeConflict(sxact, othersxact);
 			}
 		}
+
+		/*
+		 * If we didn't find any possibly unsafe conflicts because every
+		 * uncommitted writable transaction turned out to be doomed, then we
+		 * can "opt out" immediately.  See comments above the earlier check for
+		 * PredXact->WritableSxactCount == 0.
+		 */
+		if (SHMQueueEmpty(&sxact->possibleUnsafeConflicts))
+		{
+			ReleasePredXact(sxact);
+			LWLockRelease(SerializableXactHashLock);
+			return snapshot;
+		}
 	}
 	else
 	{
@@ -1910,6 +1905,24 @@ GetSerializableTransactionSnapshotInt(Snapshot snapshot,
 			   (MaxBackends + max_prepared_xacts));
 	}
 
+	/* Maintain serializable global xmin info. */
+	if (!TransactionIdIsValid(PredXact->SxactGlobalXmin))
+	{
+		Assert(PredXact->SxactGlobalXminCount == 0);
+		PredXact->SxactGlobalXmin = snapshot->xmin;
+		PredXact->SxactGlobalXminCount = 1;
+		SerialSetActiveSerXmin(snapshot->xmin);
+	}
+	else if (TransactionIdEquals(snapshot->xmin, PredXact->SxactGlobalXmin))
+	{
+		Assert(PredXact->SxactGlobalXminCount > 0);
+		PredXact->SxactGlobalXminCount++;
+	}
+	else
+	{
+		Assert(TransactionIdFollows(snapshot->xmin, PredXact->SxactGlobalXmin));
+	}
+
 	MySerializableXact = sxact;
 	MyXactDidWrite = false;		/* haven't written anything yet */
 
