// ExtraInPipe returns a pipe and fd number that will be connected to a
// readable fd in the child process.
//
// See StdinPipe for how to handle close and exit.
func (c *Cmd) ExtraInPipe() (io.WriteCloser, uintptr, error) {
	pr, pw, err := os.Pipe()
	if err != nil {
		return nil, 0, err
	}
	c.ExtraFiles = append(c.ExtraFiles, pr)
	c.closeAfterStart = append(c.closeAfterStart, pr)
	wc := &closeOnce{File: pw}
	c.closeAfterWait = append(c.closeAfterWait, wc)
	return wc, uintptr(len(c.ExtraFiles)) + 2, nil
}

// ExtraOutPipe returns a pipe and a fd number that will be connected to a
// writable fd in the child process.
//
// See StdoutPipe for how to handle close and exit.
func (c *Cmd) ExtraOutPipe() (io.ReadCloser, uintptr, error) {
	pr, pw, err := os.Pipe()
	if err != nil {
		return nil, 0, err
	}
	c.ExtraFiles = append(c.ExtraFiles, pw)
	c.closeAfterStart = append(c.closeAfterStart, pw)
	c.closeAfterWait = append(c.closeAfterWait, pr)
	return pr, uintptr(len(c.ExtraFiles)) + 2, nil
}