control_bus service has only 1 heartbeat thread

control_service class expects to have only one heartbeat thread.
But the service launchs a heartbeat thread everytime when
control_bus.start() is called. Thus control_bus has 3 methods
and sends 3 heartbeat messages always.

This adds a barrier to stop creating extra heartbeat threads in the class.

Change-Id: I28f06eef3548e07367a3c3757892112a87391ce5
Closes-Bug: #1622865

congress/dse2/control_bus.py

@@ -146,6 +146,10 @@ class DseNodeControlBus(data_service.DataService):
 
     # Note(thread-safety): blocking function
     def start(self):
+        if self._running:
+            LOG.debug('control bus on %s already started.' % self.node.node_id)
+            return
+
         LOG.debug("<%s> Starting DSE control bus", self.node.node_id)
         super(DseNodeControlBus, self).start()
 

congress/dse2/dse_node.py

@@ -174,7 +174,8 @@ class DseNode(object):
             self.transport, service._target, service.rpc_endpoints(),
             executor='eventlet')
 
-        service.start()
+        if self._running:
+            service.start()
 
         LOG.debug('<%s> Service %s RPC Server listening on %s',
                   self.node_id, service.service_id, service._target)