Cloned from launchpad bug 1397153.

Description:

As the code in conductor:

        try:
            live_migrate.execute(context, instance, destination,
                             block_migration, disk_over_commit)
        except (exception.NoValidHost,
                exception.ComputeServiceUnavailable,
                exception.InvalidHypervisorType,
                ....) as ex:
            with excutils.save_and_reraise_exception():
                # TODO(johngarbutt) - eventually need instance actions here
                request_spec = {'instance_properties': {
                    'uuid': instance['uuid'], },
                }
                scheduler_utils.set_vm_state_and_notify(context,
                        'compute_task', 'migrate_server',
                        dict(vm_state=instance['vm_state'],
                             task_state=None,
                             expected_task_state=task_states.MIGRATING,),
                        ex, request_spec, self.db)
        except Exception as ex:
            LOG.error(_LE('Migration of instance %(instance_id)s to host'
                          ' %(dest)s unexpectedly failed.'),
                      {'instance_id': instance['uuid'], 'dest': destination},
                      exc_info=True)
            raise exception.MigrationError(reason=six.text_type(ex))


When there is any unexpected exception raised from 'live_migrate.execute', the code will raise MigrationError, then the instance stuck at vm_state:active task_state: migrating and return 400 to user. This is confuse for user.

Specification URL (additional information):

https://bugs.launchpad.net/nova/+bug/1397153