code: func (ms *FileMsgStore) writeIndex(w io.Writer, seq uint64, offset, timestamp int64, msgSize int) error {
  	_buf := [msgIndexRecSize]byte{}
  	buf := _buf[:]
  	ms.addIndex(buf, seq, offset, timestamp, msgSize)
  	_, err := w.Write(buf[:msgIndexRecSize])
  	return err
  }
docs: // writeIndex writes a message index record to the writer `w`