Catch only MessageDeliveryFailure exceptions

There was a todo left to only catch MessageDeliveryFailure exceptions
once they were being propagated properly; the bug related to that [1]
has been closed and the fix is merged [2].  So we can catch only the
correct error and eliminate the TODO.

TrivialFix

[1] https://bugs.launchpad.net/neutron/+bug/1705351
[2] https://review.openstack.org/486706

Change-Id: I1201ee150709b84e6f4b2fa4b108da9ea28983a3

neutron_lib/rpc.py

@@ -25,6 +25,7 @@ from neutron_lib.utils import runtime
 from oslo_config import cfg
 from oslo_log import log as logging
 import oslo_messaging
+from oslo_messaging import exceptions as oslomsg_exc
 from oslo_messaging.rpc import dispatcher
 from oslo_messaging import serializer as om_serializer
 from oslo_service import service
@@ -104,11 +105,7 @@ class _ContextWrapper(object):
     def cast(self, ctxt, method, **kwargs):
         try:
             self._original_context.cast(ctxt, method, **kwargs)
-        except Exception as e:
-            # TODO(kevinbenton): make catch specific to missing exchange once
-            # bug/1705351 is resolved on the oslo.messaging side; if
-            # oslo.messaging auto-creates the exchange, then just remove the
-            # code completely
+        except oslomsg_exc.MessageDeliveryFailure as e:
             LOG.debug("Ignored exception during cast: %s", str(e))
 
 

neutron_lib/tests/unit/test_rpc.py

@@ -15,6 +15,7 @@ import mock
 from oslo_config import cfg
 import oslo_messaging as messaging
 from oslo_messaging import conffixture as messaging_conffixture
+from oslo_messaging import exceptions as oslomsg_exc
 from oslo_messaging.rpc import dispatcher
 import testtools
 
@@ -445,7 +446,7 @@ class CastExceptionTestCase(base.BaseTestCase):
         self.addCleanup(rpc.cleanup)
         rpc.init(CONF)
         rpc.TRANSPORT = mock.MagicMock()
-        rpc.TRANSPORT._send.side_effect = Exception
+        rpc.TRANSPORT._send.side_effect = oslomsg_exc.MessageDeliveryFailure
         target = messaging.Target(version='1.0', topic='testing')
         self.client = rpc.get_client(target)
         self.cast_context = mock.Mock()