code: func readerSize(br *bufio.Reader) int {
  	br.Reset(sizeReader)
  	br.ReadByte()
  	n := br.Buffered() + 1
  	br.Reset(nil)
  	return n
  }
docs: // readerSize returns buffer size of the given buffered reader.
  // NOTE: current workaround implementation resets underlying io.Reader.