Handle setting same keyspace twice

Before this fix, the second "USE system" query would hang until an
OperationTimedOut exception was raised.

Fixes PYTHON-38

cassandra/cluster.py

@@ -1177,6 +1177,10 @@ class Session(object):
         remaining_callbacks = set(self._pools.values())
         errors = {}
 
+        if not remaining_callbacks:
+            callback(errors)
+            return
+
         def pool_finished_setting_keyspace(pool, host_errors):
             remaining_callbacks.remove(pool)
             if host_errors:

cassandra/connection.py

@@ -322,6 +322,7 @@ class Connection(object):
         occurred, otherwise :const:`None`.
         """
         if not keyspace or keyspace == self.keyspace:
+            callback(self, None)
             return
 
         query = QueryMessage(query='USE "%s"' % (keyspace,),

cassandra/pool.py

@@ -532,6 +532,10 @@ class HostConnectionPool(object):
         remaining_callbacks = set(self._connections)
         errors = []
 
+        if not remaining_callbacks:
+            callback(self, errors)
+            return
+
         def connection_finished_setting_keyspace(conn, error):
             remaining_callbacks.remove(conn)
             if error:

tests/integration/test_cluster.py

@@ -69,6 +69,12 @@ class ClusterTests(unittest.TestCase):
         result2 = session2.execute("SELECT * FROM test")
         self.assertEquals(result, result2)
 
+    def test_set_keyspace_twice(self):
+        cluster = Cluster()
+        session = cluster.connect()
+        session.execute("USE system")
+        session.execute("USE system")
+
     def test_default_connections(self):
         """
         Ensure errors are not thrown when using non-default policies