Ignore CatalogSnapshot when checking COPY FREEZE prerequisites.

nmisch · nmisch · commit 7932891ab9cc · 2017-11-05T09:26:43.000-08:00
This restores the ability, essentially lost in commit ffaa44c, to use COPY FREEZE under REPEATABLE READ isolation. Back-patch to 9.4, like that commit. Reviewed by Tom Lane. Discussion: https://postgr.es/m/CA+TgmoahWDm-7fperBxzU9uZ99LPMUmEpSXLTw9TmrOgzwnORw@mail.gmail.com
diff --git a/src/backend/commands/copy.c b/src/backend/commands/copy.c
@@ -2317,13 +2317,25 @@ CopyFrom(CopyState cstate)
 	/*
 	 * Optimize if new relfilenode was created in this subxact or one of its
 	 * committed children and we won't see those rows later as part of an
-	 * earlier scan or command. This ensures that if this subtransaction
-	 * aborts then the frozen rows won't be visible after xact cleanup. Note
+	 * earlier scan or command. The subxact test ensures that if this subxact
+	 * aborts then the frozen rows won't be visible after xact cleanup.  Note
 	 * that the stronger test of exactly which subtransaction created it is
-	 * crucial for correctness of this optimisation.
+	 * crucial for correctness of this optimisation. The test for an earlier
+	 * scan or command tolerates false negatives. FREEZE causes other sessions
+	 * to see rows they would not see under MVCC, and a false negative merely
+	 * spreads that anomaly to the current session.
 	 */
 	if (cstate->freeze)
 	{
+		/*
+		 * Tolerate one registration for the benefit of FirstXactSnapshot.
+		 * Scan-bearing queries generally create at least two registrations,
+		 * though relying on that is fragile, as is ignoring ActiveSnapshot.
+		 * Clear CatalogSnapshot to avoid counting its registration.  We'll
+		 * still detect ongoing catalog scans, each of which separately
+		 * registers the snapshot it uses.
+		 */
+		InvalidateCatalogSnapshot();
 		if (!ThereAreNoPriorRegisteredSnapshots() || !ThereAreNoReadyPortals())
 			ereport(ERROR,
 					(errcode(ERRCODE_INVALID_TRANSACTION_STATE),
diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c
@@ -1447,6 +1447,14 @@ DeleteAllExportedSnapshotFiles(void)
 	FreeDir(s_dir);
 }
 
+/*
+ * ThereAreNoPriorRegisteredSnapshots
+ *		Is the registered snapshot count less than or equal to one?
+ *
+ * Don't use this to settle important decisions.  While zero registrations and
+ * no ActiveSnapshot would confirm a certain idleness, the system makes no
+ * guarantees about the significance of one registered snapshot.
+ */
 bool
 ThereAreNoPriorRegisteredSnapshots(void)
 {

Original file line number	Diff line number	Diff line change
`@@ -1447,6 +1447,14 @@ DeleteAllExportedSnapshotFiles(void)`
`1447`	`1447`	`FreeDir(s_dir);`
`1448`	`1448`	`}`
`1449`	`1449`
	`1450`	`+/*`
	`1451`	`+ * ThereAreNoPriorRegisteredSnapshots`
	`1452`	`+ * Is the registered snapshot count less than or equal to one?`
	`1453`	`+ *`
	`1454`	`+ * Don't use this to settle important decisions. While zero registrations and`
	`1455`	`+ * no ActiveSnapshot would confirm a certain idleness, the system makes no`
	`1456`	`+ * guarantees about the significance of one registered snapshot.`
	`1457`	`+ */`
`1450`	`1458`	`bool`
`1451`	`1459`	`ThereAreNoPriorRegisteredSnapshots(void)`
`1452`	`1460`	`{`