code: func getSourceHashFromInfos(infos []copyInfo) string {
  	if len(infos) == 1 {
  		return infos[0].hash
  	}
  	var hashs []string
  	for _, info := range infos {
  		hashs = append(hashs, info.hash)
  	}
  	return hashStringSlice("multi", hashs)
  }
docs: // For backwards compat, if there's just one info then use it as the
  // cache look-up string, otherwise hash 'em all into one