Protect against SnapshotNow race conditions in pg_tablespace scans.

Use of SnapshotNow is known to expose us to race conditions if the tuple(s)
being sought could be updated by concurrently-committing transactions.
CREATE DATABASE and DROP DATABASE are particularly exposed because they do
heavyweight filesystem operations during their scans of pg_tablespace,
so that the scans run for a very long time compared to most.  Furthermore,
the potential consequences of a missed or twice-visited row are nastier
than average:

* createdb() could fail with a bogus "file already exists" error, or
  silently fail to copy one or more tablespace's worth of files into the
  new database.

* remove_dbtablespaces() could miss one or more tablespaces, thus failing
  to free filesystem space for the dropped database.

* check_db_file_conflict() could likewise miss a tablespace, leading to an
  OID conflict that could result in data loss either immediately or in
  future operations.  (This seems of very low probability, though, since a
  duplicate database OID would be unlikely to start with.)

Hence, it seems worth fixing these three places to use MVCC snapshots, even
though this will someday be superseded by a generic solution to SnapshotNow
race conditions.

Back-patch to all active branches.

Stephen Frost and Tom Lane

src/backend/commands/dbcommands.c

@@ -131,6 +131,7 @@ createdb(const CreatedbStmt *stmt)
 	int			notherbackends;
 	int			npreparedxacts;
 	createdb_failure_params fparms;
+	Snapshot	snapshot;
 
 	/* Extract options from the statement node tree */
 	foreach(option, stmt->options)
@@ -534,6 +535,29 @@ createdb(const CreatedbStmt *stmt)
 	 */
 	RequestCheckpoint(CHECKPOINT_IMMEDIATE | CHECKPOINT_FORCE | CHECKPOINT_WAIT);
 
+	/*
+	 * Take an MVCC snapshot to use while scanning through pg_tablespace.  For
+	 * safety, register the snapshot (this prevents it from changing if
+	 * something else were to request a snapshot during the loop).
+	 *
+	 * Traversing pg_tablespace with an MVCC snapshot is necessary to provide
+	 * us with a consistent view of the tablespaces that exist.  Using
+	 * SnapshotNow here would risk seeing the same tablespace multiple times,
+	 * or worse not seeing a tablespace at all, if its tuple is moved around
+	 * by a concurrent update (eg an ACL change).
+	 *
+	 * Inconsistency of this sort is inherent to all SnapshotNow scans, unless
+	 * some lock is held to prevent concurrent updates of the rows being
+	 * sought.	There should be a generic fix for that, but in the meantime
+	 * it's worth fixing this case in particular because we are doing very
+	 * heavyweight operations within the scan, so that the elapsed time for
+	 * the scan is vastly longer than for most other catalog scans.  That
+	 * means there's a much wider window for concurrent updates to cause
+	 * trouble here than anywhere else.  XXX this code should be changed
+	 * whenever a generic fix is implemented.
+	 */
+	snapshot = RegisterSnapshot(GetLatestSnapshot());
+
 	/*
 	 * Once we start copying subdirectories, we need to be able to clean 'em
 	 * up if we fail.  Use an ENSURE block to make sure this happens.  (This
@@ -551,7 +575,7 @@ createdb(const CreatedbStmt *stmt)
 		 * each one to the new database.
 		 */
 		rel = heap_open(TableSpaceRelationId, AccessShareLock);
-		scan = heap_beginscan(rel, SnapshotNow, 0, NULL);
+		scan = heap_beginscan(rel, snapshot, 0, NULL);
 		while ((tuple = heap_getnext(scan, ForwardScanDirection)) != NULL)
 		{
 			Oid			srctablespace = HeapTupleGetOid(tuple);
@@ -656,6 +680,9 @@ createdb(const CreatedbStmt *stmt)
 	PG_END_ENSURE_ERROR_CLEANUP(createdb_failure_callback,
 								PointerGetDatum(&fparms));
 
+	/* Free our snapshot */
+	UnregisterSnapshot(snapshot);
+
 	return dboid;
 }
 
@@ -1715,9 +1742,20 @@ remove_dbtablespaces(Oid db_id)
 	Relation	rel;
 	HeapScanDesc scan;
 	HeapTuple	tuple;
+	Snapshot	snapshot;
+
+	/*
+	 * As in createdb(), we'd better use an MVCC snapshot here, since this
+	 * scan can run for a long time.  Duplicate visits to tablespaces would be
+	 * harmless, but missing a tablespace could result in permanently leaked
+	 * files.
+	 *
+	 * XXX change this when a generic fix for SnapshotNow races is implemented
+	 */
+	snapshot = RegisterSnapshot(GetLatestSnapshot());
 
 	rel = heap_open(TableSpaceRelationId, AccessShareLock);
-	scan = heap_beginscan(rel, SnapshotNow, 0, NULL);
+	scan = heap_beginscan(rel, snapshot, 0, NULL);
 	while ((tuple = heap_getnext(scan, ForwardScanDirection)) != NULL)
 	{
 		Oid			dsttablespace = HeapTupleGetOid(tuple);
@@ -1763,6 +1801,7 @@ remove_dbtablespaces(Oid db_id)
 
 	heap_endscan(scan);
 	heap_close(rel, AccessShareLock);
+	UnregisterSnapshot(snapshot);
 }
 
 /*
@@ -1784,9 +1823,19 @@ check_db_file_conflict(Oid db_id)
 	Relation	rel;
 	HeapScanDesc scan;
 	HeapTuple	tuple;
+	Snapshot	snapshot;
+
+	/*
+	 * As in createdb(), we'd better use an MVCC snapshot here; missing a
+	 * tablespace could result in falsely reporting the OID is unique, with
+	 * disastrous future consequences per the comment above.
+	 *
+	 * XXX change this when a generic fix for SnapshotNow races is implemented
+	 */
+	snapshot = RegisterSnapshot(GetLatestSnapshot());
 
 	rel = heap_open(TableSpaceRelationId, AccessShareLock);
-	scan = heap_beginscan(rel, SnapshotNow, 0, NULL);
+	scan = heap_beginscan(rel, snapshot, 0, NULL);
 	while ((tuple = heap_getnext(scan, ForwardScanDirection)) != NULL)
 	{
 		Oid			dsttablespace = HeapTupleGetOid(tuple);
@@ -1812,6 +1861,8 @@ check_db_file_conflict(Oid db_id)
 
 	heap_endscan(scan);
 	heap_close(rel, AccessShareLock);
+	UnregisterSnapshot(snapshot);
+
 	return result;
 }