code: func lookupUserid(id int) string {
  	u, err := user.LookupId(strconv.Itoa(id))
  	if err == nil {
  		return u.Username
  	}
  	if _, ok := err.(user.UnknownUserIdError); ok {
  		return ""
  	}
  	if parsedPasswd {
  		return ""
  	}
  	parsedPasswd = true
  	populateMap(uidName, nil, "/etc/passwd")
  	return uidName[id]
  }
docs: // lookupMu is held