Correct potential deadlock in ClientStorage:tpc_begin by being sure to release...

Correct potential deadlock in ClientStorage:tpc_begin by being sure to release locks in a finally block.

src/ZEO/ClientStorage.py

@@ -1111,18 +1111,19 @@ class ClientStorage(object):
         if self._is_read_only:
             raise POSException.ReadOnlyError()
         self._tpc_cond.acquire()
+        try:
             self._midtxn_disconnect = 0
             while self._transaction is not None:
                 # It is allowable for a client to call two tpc_begins in a
                 # row with the same transaction, and the second of these
                 # must be ignored.
                 if self._transaction == txn:
-                self._tpc_cond.release()
                     raise POSException.StorageTransactionError(
                         "Duplicate tpc_begin calls for same transaction")
 
                 self._tpc_cond.wait(30)
             self._transaction = txn
+        finally:
             self._tpc_cond.release()
 
         try:
@@ -1143,8 +1144,10 @@ class ClientStorage(object):
         # the right way to set self._transaction to None
         # calls notify() on _tpc_cond in case there are waiting threads
         self._tpc_cond.acquire()
+        try:
             self._transaction = None
             self._tpc_cond.notify()
+        finally:
             self._tpc_cond.release()
 
     def lastTransaction(self):