'Get restore point snapshot name for incremental backup.
If the backup was not incremental (determined by the fact that the
base has no snapshots/restore points), None is returned. Otherwise, the
restore point associated with backup_id is returned.'

def _get_restore_point(self, base_name, backup_id):

    with rbd_driver.RADOSClient(self, self._ceph_backup_pool) as client:
        base_rbd = self.rbd.Image(client.ioctx, base_name, read_only=True)
        try:
            restore_point = self._get_backup_snap_name(base_rbd, base_name, backup_id)
        finally:
            base_rbd.close()
    return restore_point
