Track the attempted method when raising UnsupportedVersion

This helps in circumstances where the only stacktrace visible
is on the target side (e.g. due to cast or fanout operations)
and without the method information is difficult to root cause
what is going on.

Change-Id: I070ecda4d56186fcda8e8bfc49bf723928998393
Closes-bug: #1340277

oslo/messaging/rpc/dispatcher.py

@@ -67,10 +67,13 @@ class NoSuchMethod(RPCDispatcherError, AttributeError):
 class UnsupportedVersion(RPCDispatcherError):
     "Raised if there is no endpoint which supports the requested version."
 
-    def __init__(self, version):
+    def __init__(self, version, method=None):
         msg = "Endpoint does not support RPC version %s" % version
+        if method:
+            msg = "%s. Attempted method: %s" % (msg, method)
         super(UnsupportedVersion, self).__init__(msg)
         self.version = version
+        self.method = method
 
 
 class RPCDispatcher(object):
@@ -183,4 +186,4 @@ class RPCDispatcher(object):
         if found_compatible:
             raise NoSuchMethod(method)
         else:
-            raise UnsupportedVersion(version)
+            raise UnsupportedVersion(version, method=method)

tests/rpc/test_dispatcher.py

@@ -111,6 +111,8 @@ class TestDispatcher(test_utils.BaseTestCase):
                 elif isinstance(ex, messaging.UnsupportedVersion):
                     self.assertEqual(self.msg.get('version', '1.0'),
                                      ex.version)
+                    if ex.method:
+                        self.assertEqual(self.msg.get('method'), ex.method)
             else:
                 self.assertTrue(self.success, failure)
                 self.assertIsNone(failure)