Support both olso.m v4 and v5

Oslo.Messaging v5 made some API changes which broke us in
I5f23e23644e90919cb67f81fc306ee85c5e09974. Update our
code to "detect" this API change, and behave accordingly.

Closes-Bug: 1580139
Partial-Bug: 1412977
Change-Id: I5b44cd2ded32d90be6efda7b5238df72ce6c6cbd

designate/rpc.py

@@ -26,9 +26,12 @@ __all__ = [
     'TRANSPORT_ALIASES',
 ]
 
+import inspect
+
 from oslo_config import cfg
 import oslo_messaging as messaging
 from oslo_messaging import server as msg_server
+from oslo_messaging.rpc import server as rpc_server
 from oslo_messaging.rpc import dispatcher as rpc_dispatcher
 from oslo_serialization import jsonutils
 
@@ -174,6 +177,7 @@ class RequestContextSerializer(messaging.Serializer):
 
 class RPCDispatcher(rpc_dispatcher.RPCDispatcher):
     def _dispatch(self, *args, **kwds):
+        # TODO(kiall): Remove when oslo.messaging 5 is the min in requirements
         try:
             return super(RPCDispatcher, self)._dispatch(*args, **kwds)
         except Exception as e:
@@ -182,6 +186,15 @@ class RPCDispatcher(rpc_dispatcher.RPCDispatcher):
             else:
                 raise
 
+    def dispatch(self, *args, **kwds):
+        try:
+            return super(RPCDispatcher, self).dispatch(*args, **kwds)
+        except Exception as e:
+            if getattr(e, 'expected', False):
+                raise rpc_dispatcher.ExpectedException()
+            else:
+                raise
+
 
 def get_transport_url(url_str=None):
     return messaging.TransportURL.parse(CONF, url_str, TRANSPORT_ALIASES)
@@ -204,8 +217,21 @@ def get_server(target, endpoints, serializer=None):
         serializer = DesignateObjectSerializer()
     serializer = RequestContextSerializer(serializer)
 
-    dispatcher = RPCDispatcher(target, endpoints, serializer)
-    return msg_server.MessageHandlingServer(TRANSPORT, dispatcher, 'eventlet')
+    # TODO(kiall): Remove when oslo.messaging 5 is the min in requirements
+    argspec = inspect.getargspec(rpc_dispatcher.RPCDispatcher.__init__)
+    if 'target' in argspec.args:
+        # We're on oslo.messaging < 5
+        dispatcher = RPCDispatcher(target, endpoints, serializer)
+
+        return msg_server.MessageHandlingServer(
+            TRANSPORT, dispatcher, 'eventlet')
+
+    else:
+        # We're on oslo.messaging >= 5
+        dispatcher = RPCDispatcher(endpoints, serializer)
+
+        return rpc_server.RPCServer(
+            TRANSPORT, target, dispatcher, 'eventlet')
 
 
 def get_listener(targets, endpoints, serializer=None):