>From 357f7521384df34c697b3544115622520a6a0e9f Mon Sep 17 00:00:00 2001
From: Heikki Linnakangas <heikki.linnakangas@iki.fi>
Date: Mon, 30 Sep 2013 11:29:09 +0300
Subject: [PATCH 1/1] 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).
---
 src/backend/storage/large_object/inv_api.c | 44 ++++++++++++++++++------------
 1 file 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 fb91571..d248743 100644
--- a/src/backend/storage/large_object/inv_api.c
+++ b/src/backend/storage/large_object/inv_api.c
@@ -240,29 +240,18 @@ 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 = NULL;		/* instantaneous MVCC snapshot */
-		retval->flags = IFS_WRLOCK | IFS_RDLOCK;
+		snapshot = NULL;		/* instantaneous MVCC snapshot */
+		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
 		ereport(ERROR,
@@ -271,11 +260,30 @@ inv_open(Oid lobjId, int flags, MemoryContext mcxt)
 						flags)));
 
 	/* Can't use LargeObjectExists here because we need to specify snapshot */
-	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)
+		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;
 }
 
-- 
1.8.4.rc3