Fix getting nwinfo for Instance obj

This fixes the get_nw_info_for_instance() method in nova/compute/utils.

If an instance object has no 'info_cache' (means we lack an entry in the
instance_info_caches table for the instance), don't try to access the
'network_info' attribute.  Just return a network model with empty
networking information like the dict version of an instance would do.

Change-Id: Ib1f49f10cefeefc55a87fea69b6fbc772c978dca
Closes-bug: 1243291

nova/compute/utils.py

@@ -346,6 +346,8 @@ def notify_about_aggregate_update(context, event_suffix, aggregate_payload):
 
 def get_nw_info_for_instance(instance):
     if isinstance(instance, instance_obj.Instance):
+        if instance.info_cache is None:
+            return network_model.NetworkInfo.hydrate([])
         return instance.info_cache.network_info
     # FIXME(comstud): Transitional while we convert to objects.
     info_cache = instance['info_cache'] or {}

nova/tests/compute/test_compute_utils.py

@@ -697,3 +697,18 @@ def fake_show(ctx, image_id):
 
         expected = {'properties': 'DONTCARE'}
         self.assertThat(expected, matchers.DictMatches(image_meta))
+
+
+class ComputeUtilsGetNWInfo(test.TestCase):
+    def test_instance_object_none_info_cache(self):
+        inst = fake_instance.fake_instance_obj('fake-context',
+                                               expected_attrs=['info_cache'])
+        self.assertEqual(None, inst.info_cache)
+        result = compute_utils.get_nw_info_for_instance(inst)
+        self.assertEqual(jsonutils.dumps([]), result.json())
+
+    def test_instance_dict_none_info_cache(self):
+        inst = fake_instance.fake_db_instance(info_cache=None)
+        self.assertEqual(None, inst['info_cache'])
+        result = compute_utils.get_nw_info_for_instance(inst)
+        self.assertEqual(jsonutils.dumps([]), result.json())