// WaitWrite waits until data can be written to fd.
func (fd *FD) WaitWrite() error {
	return fd.pd.waitWrite(fd.isFile)
}

// WriteOnce is for testing only. It makes a single write call.
func (fd *FD) WriteOnce(p []byte) (int, error) {
	if err := fd.writeLock(); err != nil {
		return 0, err
	}
	defer fd.writeUnlock()
	return ignoringEINTRIO(syscall.Write, fd.Sysfd, p)
}