do not log error for missing _save_tags

The instance.save always logs the below error message if instance.tags
are lazy loaded before the save.

AttributeError: 'Instance' object has no attribute '_save_tags'

The tags should never be saved through the instance object so adding an
empty _save_tags function is better than logging an error every time.

Change-Id: Id41b06e7a294d803bd580d38ef04cc0ffb32a3bf

nova/objects/instance.py

@@ -616,6 +616,10 @@ class Instance(base.NovaPersistentObject, base.NovaObject,
         # be dropped.
         pass
 
+    def _save_tags(self, context):
+        # NOTE(gibi): tags are not saved through the instance
+        pass
+
     def _save_flavor(self, context):
         if not any([x in self.obj_what_changed() for x in
                     ('flavor', 'old_flavor', 'new_flavor')]):

nova/tests/unit/objects/test_instance.py

@@ -234,6 +234,25 @@ class _TestInstanceObject(object):
         self.assertEqual(1, len(instance.tags))
         self.assertEqual('foo', instance.tags[0].tag)
 
+    @mock.patch('nova.objects.instance.LOG.exception')
+    def test_save_does_not_log_exception_after_tags_loaded(self, mock_log):
+        instance = objects.Instance(self.context, uuid=uuids.instance,
+                                    user_id=self.context.user_id,
+                                    project_id=self.context.project_id)
+        instance.create()
+        tag = objects.Tag(self.context, resource_id=instance.uuid, tag='foo')
+        tag.create()
+
+        # this will lazy load tags so instance.tags will be set
+        self.assertEqual(1, len(instance.tags))
+
+        # instance.save will try to find a way to save tags but is should not
+        # spam the log with errors
+        instance.display_name = 'foobar'
+        instance.save()
+
+        self.assertFalse(mock_log.called)
+
     @mock.patch.object(db, 'instance_get')
     def test_get_by_id(self, mock_get):
         mock_get.return_value = self.fake_instance