Comment 2 for bug 1871925

Note: I find a fourth place in the code where an instance mapping could be updated with a cell_id [4], but I'm not sure if that code is ever reached because it's in the build_instances method, which is only called during reschedules, as far as I know:

            # This is populated in scheduler_utils.populate_retry
            num_attempts = local_filter_props.get('retry',
                                                  {}).get('num_attempts', 1)
            if num_attempts <= 1:
                # If this is a reschedule the instance is already mapped to
                # this cell and the BuildRequest is already deleted so ignore
                # the logic below.
                inst_mapping = self._populate_instance_mapping(context,
                                                               instance,
                                                               host)

[4] https://github.com/openstack/nova/blob/7a71408a79dc81f344ee6c7760fa881afb935dfc/nova/conductor/manager.py#L817