code: func fileSize(filename string) (size int64, ok bool) {
  	info, err := os.Stat(filename)
  	ok = err == nil
  	if ok {
  		size = info.Size()
  	}
  	return
  }
docs: // fileSize returns the size of the specified file; ok will be false
  // if the file does not exist or is not an ordinary file