// It does not actually call p.Wait.
func (p *Process) blockUntilWaitable() (bool, error) {
	var errno syscall.Errno
	for {
		_, errno = wait6(_P_PID, p.Pid, syscall.WEXITED|syscall.WNOWAIT)
		if errno != syscall.EINTR {
			break
		}
	}
	runtime.KeepAlive(p)
	if errno == syscall.ENOSYS {
		return false, nil
	} else if errno != 0 {
		return false, NewSyscallError("wait6", errno)
	}

	var siginfo [16]uint64
	psig := &siginfo[0]
	var e syscall.Errno
	for {
		_, _, e = syscall.Syscall6(syscall.SYS_WAITID, _P_PID, uintptr(p.Pid), uintptr(unsafe.Pointer(psig)), syscall.WEXITED|syscall.WNOWAIT, 0, 0)
		if e != syscall.EINTR {
			break
		}
	}
	runtime.KeepAlive(p)
	if e != 0 {
		// waitid has been available since Linux 2.6.9, but
		// reportedly is not available in Ubuntu on Windows.