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

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).

Branch
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REL9_0_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/c5c87f065d46eed92f7957ee71f2d81867077867

Modified Files
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src/backend/storage/large_object/inv_api.c |   44 ++++++++++++++++------------
1 file changed, 26 insertions(+), 18 deletions(-)