pgsql: Fix deadlock at startup, if max_prepared_transactions is too sma

Fix deadlock at startup, if max_prepared_transactions is too small.

When the startup process recovers transactions by scanning pg_twophase
directory, it should clear MyLockedGxact after it's done processing each
transaction. Like we do during normal operation, at PREPARE TRANSACTION.
Otherwise, if the startup process exits due to an error, it will try to
clear the locking_backend field of the last recovered transaction. That's
usually harmless, but if the error happens in MarkAsPreparing, while
holding TwoPhaseStateLock, the shmem-exit hook will try to acquire
TwoPhaseStateLock again, and deadlock with itself.

This fixes bug #13128 reported by Grant McAlister. The bug was introduced
by commit bb38fb0d, so backpatch to all supported versions like that
commit.

Branch
------
REL9_3_STABLE

Details
-------
http://git.postgresql.org/pg/commitdiff/f73ebd766a4903ab937b28c3cc90a53b6dcb61f4

Modified Files
--------------
src/backend/access/transam/twophase.c | 6 ++++++
1 file changed, 6 insertions(+)