code: func fileDescriptor(w io.Writer) (fd int, ok bool) {
  	file, ok := w.(interface{ Fd() uintptr })
  	if ok {
  		fd = int(file.Fd())
  	}
  	return fd, ok
  }
docs: // fileDescriptor returns the file descriptor associated with the
  // writer, or (0, false) if no file descriptor is available.