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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).
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1 parent 514b319 commit 357f7521384df34c697b3544115622520a6a0e9f @hlinnaka hlinnaka committed Sep 30, 2013
Showing with 26 additions and 18 deletions.
  1. +26 −18 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;
}

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