Instance remains in migrating state forever

An unexpected failure during live migration leaves the instance
to migrating state forever.
The issue is that in case of unexpected failure the task state is
set to be same as the instance task state. It should be set to
'None' instead.

Change-Id: I254977fbebf87ac712322f321792a5dfe3f0df0e
Closes-Bug: 1694406

nova/conductor/manager.py

@@ -443,8 +443,10 @@ class ComputeTaskManager(base.Base):
                           ' %(dest)s unexpectedly failed.'),
                       {'instance_id': instance.uuid, 'dest': destination},
                       exc_info=True)
+            # Reset the task state to None to indicate completion of
+            # the operation as it is done in case of known exceptions.
             _set_vm_state(context, instance, ex, vm_states.ERROR,
-                          instance.task_state)
+                          task_state=None)
             migration.status = 'error'
             migration.save()
             raise exception.MigrationError(reason=six.text_type(ex))

nova/tests/unit/conductor/test_conductor.py

@@ -1872,7 +1872,7 @@ class ConductorTaskTestCase(_BaseTaskTestCase, test_compute.BaseTestCase):
                         instance['uuid'],
                         'compute_task', 'migrate_server',
                         dict(vm_state=vm_states.ERROR,
-                             task_state=inst_obj.task_state,
+                             task_state=None,
                              expected_task_state=task_states.MIGRATING,),
                         expected_ex, request_spec)
         self.assertEqual(ex.kwargs['reason'], six.text_type(expected_ex))