Fix snapshot leak if lo_open called on non-existent object.

lo_open registers the currently active snapshot, and checks if the
large object exists after that. Normally, snapshots registered by lo_open
are unregistered at end of transaction when the lo descriptor is closed, but
if we error out before the lo descriptor is added to the list of open
descriptors, it is leaked. Fix by moving the snapshot registration to after
checking if the large object exists.

Reported by Pavel Stehule. Backpatch to 8.4. The snapshot registration
system was introduced in 8.4, so prior versions are not affected (and not
supported, anyway).

1 files changed, 26 insertions, 18 deletions

diff --git a/src/backend/storage/large_object/inv_api.c b/src/backend/storage/large_object/inv_api.c
index e0441f5bf1c..1e7c501bc88 100644
--- a/src/backend/storage/large_object/inv_api.c
+++ b/src/backend/storage/large_object/inv_api.c
@@ -243,39 +243,47 @@ LargeObjectDesc *
 inv_open(Oid lobjId, int flags, MemoryContext mcxt)
 {
 	LargeObjectDesc *retval;
-
-	retval = (LargeObjectDesc *) MemoryContextAlloc(mcxt,
-													sizeof(LargeObjectDesc));
-
-	retval->id = lobjId;
-	retval->subid = GetCurrentSubTransactionId();
-	retval->offset = 0;
+	Snapshot	snapshot = NULL;
+	int			descflags = 0;
 
 	if (flags & INV_WRITE)
 	{
-		retval->snapshot = SnapshotNow;
-		retval->flags = IFS_WRLOCK | IFS_RDLOCK;
+		snapshot = SnapshotNow;
+		descflags = IFS_WRLOCK | IFS_RDLOCK;
 	}
 	else if (flags & INV_READ)
 	{
-		/*
-		 * We must register the snapshot in TopTransaction's resowner, because
-		 * it must stay alive until the LO is closed rather than until the
-		 * current portal shuts down.
-		 */
-		retval->snapshot = RegisterSnapshotOnOwner(GetActiveSnapshot(),
-												TopTransactionResourceOwner);
-		retval->flags = IFS_RDLOCK;
+		snapshot = GetActiveSnapshot();
+		descflags = IFS_RDLOCK;
 	}
 	else
 		elog(ERROR, "invalid flags: %d", flags);
 
 	/* Can't use LargeObjectExists here because it always uses SnapshotNow */
-	if (!myLargeObjectExists(lobjId, retval->snapshot))
+	if (!myLargeObjectExists(lobjId, snapshot))
 		ereport(ERROR,
 				(errcode(ERRCODE_UNDEFINED_OBJECT),
 				 errmsg("large object %u does not exist", lobjId)));
 
+	/*
+	 * We must register the snapshot in TopTransaction's resowner, because
+	 * it must stay alive until the LO is closed rather than until the
+	 * current portal shuts down. Do this after checking that the LO exists,
+	 * to avoid leaking the snapshot if an error is thrown.
+	 */
+	if (snapshot != SnapshotNow)
+		snapshot = RegisterSnapshotOnOwner(snapshot,
+										   TopTransactionResourceOwner);
+
+	/* All set, create a descriptor */
+	retval = (LargeObjectDesc *) MemoryContextAlloc(mcxt,
+													sizeof(LargeObjectDesc));
+	retval->id = lobjId;
+	retval->subid = GetCurrentSubTransactionId();
+	retval->offset = 0;
+	retval->snapshot = snapshot;
+	retval->flags = descflags;
+
 	return retval;
 }