Catch loading failures if transport_url is not set

The current implementation always tries to get a transport from
oslo.messaging assuming the transport_url option has been set. This is
done to keep backwards compatibility. However, since the default
`rpc_backend` is rabbit, it'll always try to load such driver. The
problem raises when `kombu` is not installed and the `notifier_strategy`
is set to qpid. This will make glance-api fail because it'll try to load
the rabbit driver *before* loading the qpid one.

This patch puts the first load attempt in a try / except block. The
error caught (if any) will be re-raised *just* if transport_url has been
configured, otherwise it'll move forward and try to load the notifier
driver using the old configuration options.

The patch also removes an obsolete test.

Change-Id: I00e653704463ea8245e2960050e0d5e6839f278a
Closes-bug: #1302661
(cherry picked from commit 164d1ab46f)

glance/notifier.py

@@ -77,10 +77,20 @@ class Notifier(object):
 
         publisher_id = CONF.default_publisher_id
 
-        # NOTE(flaper87): Assume the user has configured
-        # the transport url.
-        self._transport = messaging.get_transport(CONF,
-                                                  aliases=_ALIASES)
+        try:
+            # NOTE(flaper87): Assume the user has configured
+            # the transport url.
+            self._transport = messaging.get_transport(CONF,
+                                                      aliases=_ALIASES)
+        except messaging.DriverLoadFailure:
+            # NOTE(flaper87): Catch driver load failures and re-raise
+            # them *just* if the `transport_url` option was set. This
+            # step is intended to keep backwards compatibility and avoid
+            # weird behaviors (like exceptions on missing dependencies)
+            # when the old notifier options are used.
+            if CONF.transport_url is not None:
+                with excutils.save_and_reraise_exception():
+                    LOG.exception(_('Error loading the notifier'))
 
         # NOTE(flaper87): This needs to be checked
         # here because the `get_transport` call

glance/tests/unit/test_notifier.py

@@ -103,11 +103,12 @@ class TestNotifier(utils.BaseTestCase):
         self.assertEqual(str(nfier._transport._driver._url),
                          'qpid:///')
 
-    def test_custom_strategy(self):
-        st = "glance.notifier.notify_noop.NoopStrategy"
-        self.config(notifier_strategy=st)
-        #NOTE(bcwaldon): the fact that Notifier is instantiated means we're ok
-        notifier.Notifier()
+    def test_notifier_strategy(self):
+        self.config(notifier_strategy='qpid')
+        nfier = notifier.Notifier()
+        self.assertIsNotNone(nfier._transport)
+        self.assertEqual(str(nfier._transport._driver._url),
+                         'qpid:///')
 
     def test_transport_url(self):
         transport_url = "qpid://superhost:5672/"