Follow up to I3e28c0163dc14dacf847c5a69730ba2e29650370

This addresses a few nits in the referenced change:

* https://review.opendev.org/#/c/635646/48/nova/conductor/tasks/cross_cell_migrate.py@540
* https://review.opendev.org/#/c/635646/48/nova/conductor/tasks/cross_cell_migrate.py@571
* https://review.opendev.org/#/c/635646/48/nova/tests/unit/conductor/tasks/test_cross_cell_migrate.py@997

Part of blueprint cross-cell-resize

Change-Id: I3deb84cde1b25247cc5721d13a6a35cc8faeee46

Author: mriedem

nova/conductor/tasks/cross_cell_migrate.py

@@ -537,8 +537,9 @@ def _copy_latest_fault(self, source_cell_context):
             fault = objects.InstanceFault.get_latest_for_instance(
                 self.context, self.instance.uuid)
             if fault:
-                fault = clone_creatable_object(source_cell_context, fault)
-                fault.create()
+                fault_clone = clone_creatable_object(source_cell_context,
+                                                     fault)
+                fault_clone.create()
         except Exception:
             LOG.exception(
                 'Failed to copy instance fault from target cell DB',
@@ -568,9 +569,10 @@ def _copy_finish_snapshot_based_resize_at_dest_event(
                 # create it in the source cell DB.
                 for event in events:
                     if event.event == event_name:
-                        event = clone_creatable_object(
+                        event_clone = clone_creatable_object(
                             source_cell_context, event)
-                        event.create(action.instance_uuid, action.request_id)
+                        event_clone.create(action.instance_uuid,
+                                           action.request_id)
                         break
                 else:
                     LOG.warning('Failed to find InstanceActionEvent with '

nova/tests/unit/conductor/tasks/test_cross_cell_migrate.py

@@ -969,7 +969,8 @@ def test_execute(self):
         self.assertEqual(self.target_context.cell_uuid,
                          mapping.cell_mapping.uuid)
 
-    def test_finish_snapshot_based_resize_at_dest_fails(self):
+    @mock.patch('nova.objects.InstanceMapping.save')
+    def test_finish_snapshot_based_resize_at_dest_fails(self, mock_im_save):
         """Tests when the finish_snapshot_based_resize_at_dest compute method
         raises an error.
         """
@@ -994,6 +995,8 @@ def test_finish_snapshot_based_resize_at_dest_fails(self):
         # copied from the target cell DB to the source cell DB.
         copy_fault.assert_called_once_with(self.source_context)
         copy_event.assert_called_once_with(self.source_context)
+        # Assert the instance mapping was never updated.
+        mock_im_save.assert_not_called()
 
     def test_copy_latest_fault(self):
         """Tests _copy_latest_fault working as expected"""