Make guard to have only one pending reconnect

ControlConnection.reconnect could be calld from many places in parallel.
When it happens you get to see streak for reconnects happning one by
one.
This commit adds guard that makes sure there is only one pending
reconnect.

Author: dkropachev

cassandra/cluster.py

@@ -3546,30 +3546,6 @@ class UserTypeDoesNotExist(Exception):
     pass
 
 
-class _ControlReconnectionHandler(_ReconnectionHandler):
-    """
-    Internal
-    """
-
-    def __init__(self, control_connection, *args, **kwargs):
-        _ReconnectionHandler.__init__(self, *args, **kwargs)
-        self.control_connection = weakref.proxy(control_connection)
-
-    def try_reconnect(self):
-        return self.control_connection._reconnect_internal()
-
-    def on_reconnection(self, connection):
-        self.control_connection._set_new_connection(connection)
-
-    def on_exception(self, exc, next_delay):
-        # TODO only overridden to add logging, so add logging
-        if isinstance(exc, AuthenticationFailed):
-            return False
-        else:
-            log.debug("Error trying to reconnect control connection: %r", exc)
-            return True
-
-
 def _watch_callback(obj_weakref, method_name, *args, **kwargs):
     """
     A callback handler for the ControlConnection that tolerates
@@ -3662,6 +3638,7 @@ def __init__(self, cluster, timeout,
 
         self._reconnection_handler = None
         self._reconnection_lock = RLock()
+        self._reconnection_pending = False
 
         self._event_schedule_times = {}
 
@@ -3818,33 +3795,43 @@ def reconnect(self):
         if self._is_shutdown:
             return
 
+        if self._reconnection_pending:
+            return
+        self._reconnection_pending = True
+
         self._submit(self._reconnect)
 
-    def _reconnect(self):
+    def _reconnect(self, schedule = None):
         log.debug("[control connection] Attempting to reconnect")
+        if self._is_shutdown:
+            return
+
         try:
             self._set_new_connection(self._reconnect_internal())
+            self._reconnection_pending = False
+            return
         except NoHostAvailable:
-            # make a retry schedule (which includes backoff)
-            schedule = self._cluster.reconnection_policy.new_schedule()
+            log.debug("[control connection] Reconnection plan is exhausted, scheduling new reconnection attempt")
+        except Exception as ex:
+            log.debug("[control connection] Unexpected exception during reconnect, scheduling new reconnection attempt: %s", ex)
 
-            with self._reconnection_lock:
+        if schedule is None:
+            schedule = self._cluster.reconnection_policy.new_schedule()
 
-                # cancel existing reconnection attempts
-                if self._reconnection_handler:
-                    self._reconnection_handler.cancel()
+        try:
+            next_delay = next(schedule)
+        except StopIteration:
+            # the schedule has been exhausted
+            schedule = self._cluster.reconnection_policy.new_schedule()
+            try:
+                next_delay = next(schedule)
+            except StopIteration:
+                next_delay = 0
 
-                # when a connection is successfully made, _set_new_connection
-                # will be called with the new connection and then our
-                # _reconnection_handler will be cleared out
-                self._reconnection_handler = _ControlReconnectionHandler(
-                    self, self._cluster.scheduler, schedule,
-                    self._get_and_set_reconnection_handler,
-                    new_handler=None)
-                self._reconnection_handler.start()
-        except Exception:
-            log.debug("[control connection] error reconnecting", exc_info=True)
-            raise
+        if next_delay == 0:
+            self._submit(self._reconnect)
+        else:
+            self._cluster.scheduler.schedule(next_delay, partial(self._reconnect, schedule))
 
     def _get_and_set_reconnection_handler(self, new_handler):
         """

tests/integration/standard/test_cluster.py

@@ -326,6 +326,30 @@ def test_invalid_protocol_negotation(self):
                 cluster.connect()
             cluster.shutdown()
 
+    def test_control_connection_reconnect(self):
+        """
+        Ensure clusters that connect on a keyspace, do
+        """
+        cassandra.cluster.log.setLevel(logging.DEBUG)
+
+        cluster = TestCluster()
+        _ = cluster.connect()
+
+        cluster.control_connection._reconnect_internal = Mock(wraps=cluster.control_connection._reconnect_internal)
+
+        cluster.control_connection.reconnect()
+        cluster.control_connection.reconnect()
+        cluster.control_connection.reconnect()
+        cluster.control_connection.reconnect()
+
+        while cluster.control_connection._reconnection_pending:
+            time.sleep(0.1)
+
+        self.assertFalse(cluster.control_connection._connection.is_closed)
+        self.assertFalse(cluster.control_connection._connection.is_defunct)
+        self.assertTrue(cluster.control_connection.refresh_schema())
+        self.assertEqual(1, len(cluster.control_connection._reconnect_internal.mock_calls))
+
     def test_connect_on_keyspace(self):
         """
         Ensure clusters that connect on a keyspace, do