code: func (d *FSDirectory) FileLength(name string) (n int64, err error) {
  	d.EnsureOpen()
  	fi, err := os.Stat(filepath.Join(d.path, name))
  	if err != nil {
  		return 0, err
  	}
  	return fi.Size(), nil
  }
docs: // Returns the length in bytes of a file in the directory.