docs: def clean_old_jobs():
      '''
      Clean out minions's return data for old jobs.
  
      Normally, hset 'ret:<jid>' are saved with a TTL, and will eventually
      get cleaned by redis.But for jobs with some very late minion return, the
      corresponding hset's TTL will be refreshed to a too late timestamp, we'll
      do manually cleaning here.
      '''
      serv = _get_serv(ret=None)
      ret_jids = serv.keys('ret:*')
      living_jids = set(serv.keys('load:*'))
      to_remove = []
      for ret_key in ret_jids:
          load_key = ret_key.replace('ret:', 'load:', 1)
          if load_key not in living_jids:
              to_remove.append(ret_key)
      if to_remove:
          serv.delete(*to_remove)
          log.debug('clean old jobs: %s', to_remove)