Gracefully handle cancelling all events more than once

Check to see if we can iterate over events if cancel_all_events
is called more than once, so we can avoid a NoneType error.

Change-Id: Ie0baeb17ffb1a6421bcc7d0fbcdf24afc6ede14c
Closes-Bug: #1549401

nova/compute/manager.py

@@ -581,6 +581,9 @@ class InstanceEvents(object):
         return _clear_events()
 
     def cancel_all_events(self):
+        if self._events is None:
+            LOG.debug('Unexpected attempt to cancel events during shutdown.')
+            return
         our_events = self._events
         # NOTE(danms): Block new events
         self._events = None

nova/tests/unit/compute/test_compute_mgr.py

@@ -2057,6 +2057,8 @@ class ComputeManagerUnitTestCase(test.NoDBTestCase):
             }
         }
         self.compute.instance_events.cancel_all_events()
+        # call it again to make sure we handle that gracefully
+        self.compute.instance_events.cancel_all_events()
         self.assertTrue(fake_eventlet_event.send.called)
         event = fake_eventlet_event.send.call_args_list[0][0][0]
         self.assertEqual('network-vif-plugged', event.name)