Python pty.fork() Examples

def __init__(self, args, close_stderr=False):
    pid, fd = pty.fork()
    if pid == 0:
      # We're the child. Transfer control to command.
      if close_stderr:
        dev_null = os.open('/dev/null', 0)
        os.dup2(dev_null, 2)
      os.execvp(args[0], args)
    else:
      # Disable echoing.
      attr = termios.tcgetattr(fd)
      attr[3] = attr[3] & ~termios.ECHO
      termios.tcsetattr(fd, termios.TCSANOW, attr)
      # Set up a file()-like interface to the child process
      self.r = os.fdopen(fd, 'r', 1)
      self.w = os.fdopen(os.dup(fd), 'w', 1) 

def test_child_gets_controlling_tty_if_we_had_one():
    """If dumb-init has a controlling TTY, it should give it to the child.

    To test this, we make a new TTY then exec "dumb-init bash" and ensure that
    the shell has working job control.
    """
    pid, sfd = pty.fork()
    if pid == 0:
        os.execvp('dumb-init', ('dumb-init', 'bash', '-m'))
    else:
        ttyflags(sfd)

        # We might get lots of extra output from the shell, so print something
        # we can match on easily.
        assert os.write(sfd, b'echo "flags are: [[$-]]"\n') == 25
        assert os.write(sfd, b'exit 0\n') == 7
        output = readall(sfd)
        assert os.waitpid(pid, 0) == (pid, 0), output

        m = re.search(b'flags are: \\[\\[([a-zA-Z]+)\\]\\]\n', output)
        assert m, output

        # "m" is job control
        flags = m.group(1)
        assert b'm' in flags 

def handle(self):
        masterFd, slaveFd = pty.openpty()
        try:
            # if we're not in the main thread, this will not work.
            signal.signal(signal.SIGTTOU, signal.SIG_IGN)
        except:  # noqa
            pass
        pid = os.fork()
        if pid:
            os.close(masterFd)
            raise SocketConnected(slaveFd, pid)
            # make parent process the pty slave - the opposite of
            # pty.fork().  In this setup, the parent process continues
            # to act normally, while the child process performs the
            # logging.  This makes it simple to kill the logging process
            # when we are done with it and restore the parent process to
            # normal, unlogged operation.
        else:
            os.close(slaveFd)
            try:
                protocol = TelnetServerProtocolHandler(self.request, masterFd)
                protocol.handle()
            finally:
                os.close(masterFd)
                os._exit(1) 

def SpawnPTY(argv):
  """Spawn a process and connect its controlling terminal to a PTY.

  Create a new PTY device and spawn a process with the controlling
  terminal of the child process set to the slave end of the new PTY.

  Args:
    argv: arguments (including executable name) for the child process.

  Returns:
    A pair containing the PID of the child process and the file
    descriptor for the master end of the new PTY device.
  """

  assert argv, 'SpawnPTY: argv is an empty list'

  (pid, fd) = pty.fork()
  if pid == 0:
    try:
      os.execvp(argv[0], argv)
    except OSError as err:
      print "# Error: cannot execute program '%s'" % argv[0]
      print '# %s\n%s' % (str(err), chr(4))
      sys.exit(1)
  return (pid, fd) 

def spawn(self):
        env = self.env
        env['TERM'] = 'linux'

        self.pid, self.master = pty.fork()

        if self.pid == 0:
            if callable(self.command):
                try:
                    try:
                        self.command()
                    except:
                        sys.stderr.write(traceback.format_exc())
                        sys.stderr.flush()
                finally:
                    os._exit(0)
            else:
                os.execvpe(self.command[0], self.command, env)

        if self.main_loop is None:
            fcntl.fcntl(self.master, fcntl.F_SETFL, os.O_NONBLOCK)

        atexit.register(self.terminate) 

def handle(self):
        masterFd, slaveFd = pty.openpty()

        try:
            # if we're not in the main thread, this will not work.
            signal.signal(signal.SIGTTOU, signal.SIG_IGN)
        except:
            pass
        pid = os.fork()
        if pid:
            os.close(masterFd)
            raise SocketConnected(slaveFd, pid)
            # make parent process the pty slave - the opposite of
            # pty.fork().  In this setup, the parent process continues
            # to act normally, while the child process performs the
            # logging.  This makes it simple to kill the logging process
            # when we are done with it and restore the parent process to
            # normal, unlogged operation.
        else:
            os.close(slaveFd)
            try:
                protocol = TelnetServerProtocolHandler(self.request, masterFd)
                protocol.handle()
            finally:
                os.close(masterFd)
                os._exit(1) 

def test_inkey_0s_raw_ctrl_c():
    "0-second inkey with raw allows receiving ^C."
    pid, master_fd = pty.fork()
    if pid is 0:  # child
        try:
            cov = __import__('cov_core_init').init()
        except ImportError:
            cov = None
        term = TestTerminal()
        read_until_semaphore(sys.__stdin__.fileno(), semaphore=SEMAPHORE)
        with term.raw():
            os.write(sys.__stdout__.fileno(), RECV_SEMAPHORE)
            inp = term.inkey(timeout=0)
            os.write(sys.__stdout__.fileno(), inp.encode('latin1'))
        if cov is not None:
            cov.stop()
            cov.save()
        os._exit(0)

    with echo_off(master_fd):
        os.write(master_fd, SEND_SEMAPHORE)
        # ensure child is in raw mode before sending ^C,
        read_until_semaphore(master_fd)
        os.write(master_fd, u'\x03'.encode('latin1'))
        stime = time.time()
        output = read_until_eof(master_fd)
    pid, status = os.waitpid(pid, 0)
    if os.environ.get('TRAVIS', None) is not None:
        # For some reason, setraw has no effect travis-ci,
        # is still accepts ^C, causing system exit on py26,
        # but exit 0 on py27, and either way on py33
        # .. strange, huh?
        assert output in (u'', u'\x03')
        assert os.WEXITSTATUS(status) in (0, 2)
    else:
        assert (output == u'\x03' or
                output == u'' and not os.isatty(0))
        assert os.WEXITSTATUS(status) == 0
    assert math.floor(time.time() - stime) == 0.0 

def test_inkey_0s_cbreak_sequence():
    "0-second inkey with multibyte sequence; should decode immediately."
    pid, master_fd = pty.fork()
    if pid is 0:  # child
        try:
            cov = __import__('cov_core_init').init()
        except ImportError:
            cov = None
        term = TestTerminal()
        os.write(sys.__stdout__.fileno(), SEMAPHORE)
        with term.cbreak():
            inp = term.inkey(timeout=0)
            os.write(sys.__stdout__.fileno(), inp.name.encode('ascii'))
            sys.stdout.flush()
        if cov is not None:
            cov.stop()
            cov.save()
        os._exit(0)

    with echo_off(master_fd):
        os.write(master_fd, u'\x1b[D'.encode('ascii'))
        read_until_semaphore(master_fd)
        stime = time.time()
        output = read_until_eof(master_fd)
    pid, status = os.waitpid(pid, 0)
    assert output == u'KEY_LEFT'
    assert os.WEXITSTATUS(status) == 0
    assert math.floor(time.time() - stime) == 0.0 

def test_inkey_1s_cbreak_input():
    "1-second inkey w/multibyte sequence; should return after ~1 second."
    pid, master_fd = pty.fork()
    if pid is 0:  # child
        try:
            cov = __import__('cov_core_init').init()
        except ImportError:
            cov = None
        term = TestTerminal()
        os.write(sys.__stdout__.fileno(), SEMAPHORE)
        with term.cbreak():
            inp = term.inkey(timeout=3)
            os.write(sys.__stdout__.fileno(), inp.name.encode('utf-8'))
            sys.stdout.flush()
        if cov is not None:
            cov.stop()
            cov.save()
        os._exit(0)

    with echo_off(master_fd):
        read_until_semaphore(master_fd)
        stime = time.time()
        time.sleep(1)
        os.write(master_fd, u'\x1b[C'.encode('ascii'))
        output = read_until_eof(master_fd)

    pid, status = os.waitpid(pid, 0)
    assert output == u'KEY_RIGHT'
    assert os.WEXITSTATUS(status) == 0
    assert math.floor(time.time() - stime) == 1.0 

def __init__(self, cmd, cwd):
        self._cmd_return_code = 0
        self._cmd_kill_signal = 0
        self._shell_pid, self._master_fd = pty.fork()
        if self._shell_pid == pty.CHILD:
            os.environ["TERM"] = "linux"
            os.chdir(cwd)
            os.execv(cmd[0], cmd) 

def test_esc_delay_cbreak_timout_0():
    """esc_delay still in effect with timeout of 0 ("nonblocking")."""
    pid, master_fd = pty.fork()
    if pid is 0:  # child
        try:
            cov = __import__('cov_core_init').init()
        except ImportError:
            cov = None
        term = TestTerminal()
        os.write(sys.__stdout__.fileno(), SEMAPHORE)
        with term.cbreak():
            stime = time.time()
            inp = term.inkey(timeout=0)
            measured_time = (time.time() - stime) * 100
            os.write(sys.__stdout__.fileno(), (
                '%s %i' % (inp.name, measured_time,)).encode('ascii'))
            sys.stdout.flush()
        if cov is not None:
            cov.stop()
            cov.save()
        os._exit(0)

    with echo_off(master_fd):
        os.write(master_fd, u'\x1b'.encode('ascii'))
        read_until_semaphore(master_fd)
        stime = time.time()
        key_name, duration_ms = read_until_eof(master_fd).split()

    pid, status = os.waitpid(pid, 0)
    assert key_name == u'KEY_ESCAPE'
    assert os.WEXITSTATUS(status) == 0
    assert math.floor(time.time() - stime) == 0.0
    assert 35 <= int(duration_ms) <= 45, int(duration_ms) 

def remote_exec(hostname, username, password, cmd):
    """Remote execution on specified host"""
    global subproc_flag
    pid, fd = pty.fork()
    if pid == 0:
        try:
            os.execv("/usr/bin/ssh", ["/usr/bin/ssh", "-l",
                                      username, hostname, cmd])
        except OSError as err:
            print("OSError: " + str(err))
            return -1
    else:
        signal.signal(signal.SIGCHLD, subproc)
        try:
            timeout = 50
            i = 0
            while i <= timeout:
                time.sleep(1)
                output = os.read(fd, 10240)
                if re.search(r'(yes\/no)', output):
                    os.write(fd, "yes\r")
                elif re.search('password:', output):
                    os.write(fd, password + "\r")
                elif subproc_flag == 1:
                    ret = output.decode().strip()
                    break
                elif i == timeout:
                    print("TIMEOUT!!!!")
                    return -1

                i = i+1

            subproc_flag = 0
            return ret
        except Exception as err:
            print(err)
            subproc_flag = 0
            return -1 

def restart(self, cmd):
		if not self.completed:
			self.process_input("\n\033[01;31mProcess killed.\033[00m\r\n")
			os.kill(self.pid, signal.SIGHUP)
			thread = self.thread
			thread.stop()
			while thread.alive:
				QCoreApplication.processEvents()

		self.exit_pipe, child_pipe = os.pipe()
		pid, fd = pty.fork()
		if pid == 0:
			try:
				os.environ["TERM"] = "xterm-256color"
				retval = subprocess.call(cmd, close_fds=True)
				os.write(2, "\033[01;34mProcess has completed.\033[00m\n")
				os.write(child_pipe, "t")
				os._exit(retval)
			except:
				pass
			os.write(2, "\033[01;31mCommand '" + cmd[0] + "' failed to execute.\033[00m\n")
			os.write(child_pipe, "f")
			os._exit(1)

		os.close(child_pipe)
		self.process_input("\033[01;34mStarted process with PID %d.\033[00m\r\n" % pid)

		self.pid = pid
		self.data_pipe = fd
		self.completed = False

		# Initialize terminal settings
		fcntl.ioctl(self.data_pipe, termios.TIOCSWINSZ, struct.pack("hhhh", self.rows, self.cols, 0, 0))
		attribute = termios.tcgetattr(self.data_pipe)
		termios.tcsetattr(self.data_pipe, termios.TCSAFLUSH, attribute)

		self.thread = TerminalUpdateThread(self, self.data_pipe, self.exit_pipe)
		self.thread.start() 

def pty_fork(*args):
    """Runs a subprocess with a PTY attached via fork and exec.
    The output from the PTY is streamed through log_to_client.
    This should not be necessary for most subprocesses, we
    built this to handle Compose up which only streams pull
    progress if it is attached to a TTY."""

    updated_env = copy(os.environ)
    updated_env.update(get_docker_env())
    args += (updated_env,)
    executable = args[0]
    demote_fn = demote_to_user(get_config_value(constants.CONFIG_MAC_USERNAME_KEY))

    child_pid, pty_fd = pty.fork()
    if child_pid == 0:
        demote_fn()
        os.execle(_executable_path(executable), *args)
    else:
        child_process = psutil.Process(child_pid)
        terminal = os.fdopen(pty_fd, 'r', 0)
        with streaming_to_client():
            while child_process.status() == 'running':
                output = terminal.read(1)
                log_to_client(output)
        _, exit_code = os.waitpid(child_pid, 0)
        if exit_code != 0:
            raise subprocess.CalledProcessError(exit_code, ' '.join(args[:-1])) 

def handle(self):
        """
            Creates a child process that is fully controlled by this
            request handler, and serves data to and from it via the 
            protocol handler.
        """
        pid, fd = pty.fork()
        if pid:
            protocol = TelnetServerProtocolHandler(self.request, fd)
            protocol.handle()
        else:
            self.execute() 

def test_esc_delay_cbreak_135():
    "esc_delay=1.35 will cause a single ESC (\\x1b) to delay for 1.35."
    pid, master_fd = pty.fork()
    if pid is 0:  # child
        try:
            cov = __import__('cov_core_init').init()
        except ImportError:
            cov = None
        term = TestTerminal()
        os.write(sys.__stdout__.fileno(), SEMAPHORE)
        with term.cbreak():
            stime = time.time()
            inp = term.inkey(timeout=5, esc_delay=1.35)
            measured_time = (time.time() - stime) * 100
            os.write(sys.__stdout__.fileno(), (
                '%s %i' % (inp.name, measured_time,)).encode('ascii'))
            sys.stdout.flush()
        if cov is not None:
            cov.stop()
            cov.save()
        os._exit(0)

    with echo_off(master_fd):
        read_until_semaphore(master_fd)
        stime = time.time()
        os.write(master_fd, u'\x1b'.encode('ascii'))
        key_name, duration_ms = read_until_eof(master_fd).split()

    pid, status = os.waitpid(pid, 0)
    assert key_name == u'KEY_ESCAPE'
    assert os.WEXITSTATUS(status) == 0
    assert math.floor(time.time() - stime) == 1.0
    assert 135 <= int(duration_ms) <= 145, int(duration_ms) 

def handle(self):
        """
            Creates a child process that is fully controlled by this
            request handler, and serves data to and from it via the
            protocol handler.
        """
        pid, fd = pty.fork()
        if pid:
            protocol = TelnetServerProtocolHandler(self.request, fd)
            protocol.handle()
        else:
            self.execute() 

def connect():
    """new client connected"""

    if app.config["child_pid"]:
        # already started child process, don't start another
        return

    # create child process attached to a pty we can read from and write to
    (child_pid, fd) = pty.fork()
    if child_pid == 0:
        # this is the child process fork.
        # anything printed here will show up in the pty, including the output
        # of this subprocess
        subprocess.run(app.config["cmd"])
    else:
        # this is the parent process fork.
        # store child fd and pid
        app.config["fd"] = fd
        app.config["child_pid"] = child_pid
        set_winsize(fd, 50, 50)
        cmd = " ".join(shlex.quote(c) for c in app.config["cmd"])
        print("child pid is", child_pid)
        print(
            f"starting background task with command `{cmd}` to continously read "
            "and forward pty output to client"
        )
        socketio.start_background_task(target=read_and_forward_pty_output)
        print("task started") 

def __init__(self, hostname: str, port: str) -> None:
        if port != "22":
            port = "-p " + port
        else:
            port = ''

        self.pid, fd = pty.fork()
        if self.pid == 0:
            # Child
            self.launch_ssh(hostname, port)
            sys.exit(1)

        # Parent
        super().__init__(fd)
        self.temporary = False
        self.hostname = hostname
        self.port = port
        self.debug = options.debug
        self.enabled = True  # shells can be enabled and disabled
        self.state = STATE_NOT_STARTED
        self.term_size = (-1, -1)
        self.display_name = None  # type: Optional[str]
        self.change_name(self.hostname.encode())
        self.init_string = self.configure_tty() + self.set_prompt()
        self.init_string_sent = False
        self.read_in_state_not_started = b''
        self.command = options.command
        self.last_printed_line = b''
        self.color_code = None
        if sys.stdout.isatty() and not options.disable_color:
            COLORS.insert(0, COLORS.pop())  # Rotate the colors
            self.color_code = COLORS[0] 

def alloc_pty(ctx, f, *args, **kwargs):
    pid, fd = pty.fork()

    if pid == pty.CHILD:
        tty = os.ttyname(0)

        sys.stdin = open(tty, 'r')
        sys.stdout = open(tty, 'w')
        sys.stderr = open(tty, 'w')

        # alternative way of doing ^ is to do:
        # kwargs["stdout"] = open(tty, 'w')
        # kwargs["stderr"] = open(tty, 'w')
        # kwargs["stdin"] = open(tty, 'r')

        # Create a new client for the child process to avoid concurrency issues
        client = get_client()
        f(client, *args, **kwargs)
        sys.exit(0)
    else:
        ctx.pty = fd
        util.set_pty_size(
            ctx.pty,
            (ctx.rows, ctx.cols)
        )
        ctx.pid = pid
        util.wait(ctx.pty, timeout=5)
    time.sleep(1)  # give the terminal some time to print prompt

    # util.exit_code can be called only once
    ctx.exit_code = util.exit_code(ctx.pid, timeout=5)
    if ctx.exit_code != 0:
        raise Exception("child process did not finish correctly") 

def __fork_pty(self):

        """This implements a substitute for the forkpty system call. This
        should be more portable than the pty.fork() function. Specifically,
        this should work on Solaris.

        Modified 10.06.05 by Geoff Marshall: Implemented __fork_pty() method to
        resolve the issue with Python's pty.fork() not supporting Solaris,
        particularly ssh. Based on patch to posixmodule.c authored by Noah
        Spurrier::

            http://mail.python.org/pipermail/python-dev/2003-May/035281.html

        """

        parent_fd, child_fd = os.openpty()
        if parent_fd < 0 or child_fd < 0:
            raise ExceptionPexpect, "Error! Could not open pty with os.openpty()."

        pid = os.fork()
        if pid < 0:
            raise ExceptionPexpect, "Error! Failed os.fork()."
        elif pid == 0:
            # Child.
            os.close(parent_fd)
            self.__pty_make_controlling_tty(child_fd)

            os.dup2(child_fd, 0)
            os.dup2(child_fd, 1)
            os.dup2(child_fd, 2)

            if child_fd > 2:
                os.close(child_fd)
        else:
            # Parent.
            os.close(child_fd)

        return pid, parent_fd 

def test_tty():
    """Ensure processes under dumb-init can write successfully, given a tty."""
    pid, fd = pty.fork()
    if pid == 0:
        os.execvp('dumb-init', ('dumb-init', 'tac'))
    else:
        # write to tac via the pty and verify its output
        ttyflags(fd)
        assert os.write(fd, b'1\n2\n3\n') == 6
        assert os.write(fd, EOF * 2) == 2
        output = readall(fd)
        assert os.waitpid(pid, 0) == (pid, 0)

        assert output == b'3\n2\n1\n', repr(output) 

def test_sighup_sigcont_ignored_if_was_session_leader():
    """The first SIGHUP/SIGCONT should be ignored if dumb-init is the session leader.

    Due to TTY quirks (#136), when dumb-init is the session leader and forks,
    it needs to avoid forwarding the first SIGHUP and SIGCONT to the child.
    Otherwise, the child might receive the SIGHUP post-exec and terminate
    itself.

    You can "force" this race by adding a `sleep(1)` before the signal handling
    loop in dumb-init's code, but it's hard to reproduce the race reliably in a
    test otherwise. Because of this, we're stuck just asserting debug messages.
    """
    pid, fd = pty.fork()
    if pid == 0:
        # child
        os.execvp('dumb-init', ('dumb-init', '-v', 'sleep', '20'))
    else:
        # parent
        ttyflags(fd)

        # send another SIGCONT to make sure only the first is ignored
        time.sleep(0.5)
        os.kill(pid, signal.SIGHUP)

        output = readall(fd).decode('UTF-8')

        assert 'Ignoring tty hand-off signal {}.'.format(signal.SIGHUP) in output
        assert 'Ignoring tty hand-off signal {}.'.format(signal.SIGCONT) in output

        assert '[dumb-init] Forwarded signal {} to children.'.format(signal.SIGHUP) in output
        assert '[dumb-init] Forwarded signal {} to children.'.format(signal.SIGCONT) not in output 

def test_inkey_cbreak_input_slowly():
    "0-second inkey with input; Keypress should be immediately returned."
    pid, master_fd = pty.fork()
    if pid is 0:
        try:
            cov = __import__('cov_core_init').init()
        except ImportError:
            cov = None
        # child pauses, writes semaphore and begins awaiting input
        term = TestTerminal()
        read_until_semaphore(sys.__stdin__.fileno(), semaphore=SEMAPHORE)
        os.write(sys.__stdout__.fileno(), SEMAPHORE)
        with term.cbreak():
            while True:
                inp = term.inkey(timeout=0.5)
                os.write(sys.__stdout__.fileno(), inp.encode('utf-8'))
                if inp == 'X':
                    break
        if cov is not None:
            cov.stop()
            cov.save()
        os._exit(0)

    with echo_off(master_fd):
        os.write(master_fd, SEND_SEMAPHORE)
        os.write(master_fd, u'a'.encode('ascii'))
        time.sleep(0.1)
        os.write(master_fd, u'b'.encode('ascii'))
        time.sleep(0.1)
        os.write(master_fd, u'cdefgh'.encode('ascii'))
        time.sleep(0.1)
        os.write(master_fd, u'X'.encode('ascii'))
        read_until_semaphore(master_fd)
        stime = time.time()
        output = read_until_eof(master_fd)

    pid, status = os.waitpid(pid, 0)
    assert output == u'abcdefghX'
    assert os.WEXITSTATUS(status) == 0
    assert math.floor(time.time() - stime) == 0.0 

def __fork_pty(self):

        """This implements a substitute for the forkpty system call. This
        should be more portable than the pty.fork() function. Specifically,
        this should work on Solaris.

        Modified 10.06.05 by Geoff Marshall: Implemented __fork_pty() method to
        resolve the issue with Python's pty.fork() not supporting Solaris,
        particularly ssh. Based on patch to posixmodule.c authored by Noah
        Spurrier::

            http://mail.python.org/pipermail/python-dev/2003-May/035281.html

        """

        parent_fd, child_fd = os.openpty()
        if parent_fd < 0 or child_fd < 0:
            raise ExceptionPexpect, "Error! Could not open pty with os.openpty()."

        pid = os.fork()
        if pid < 0:
            raise ExceptionPexpect, "Error! Failed os.fork()."
        elif pid == 0:
            # Child.
            os.close(parent_fd)
            self.__pty_make_controlling_tty(child_fd)

            os.dup2(child_fd, 0)
            os.dup2(child_fd, 1)
            os.dup2(child_fd, 2)

            if child_fd > 2:
                os.close(child_fd)
        else:
            # Parent.
            os.close(child_fd)

        return pid, parent_fd 

def __fork_pty(self):
        '''This implements a substitute for the forkpty system call. This
        should be more portable than the pty.fork() function. Specifically,
        this should work on Solaris.

        Modified 10.06.05 by Geoff Marshall: Implemented __fork_pty() method to
        resolve the issue with Python's pty.fork() not supporting Solaris,
        particularly ssh. Based on patch to posixmodule.c authored by Noah
        Spurrier::

            http://mail.python.org/pipermail/python-dev/2003-May/035281.html

        '''

        parent_fd, child_fd = os.openpty()
        if parent_fd < 0 or child_fd < 0:
            raise ExceptionPexpect("Could not open with os.openpty().")

        pid = os.fork()
        if pid == pty.CHILD:
            # Child.
            os.close(parent_fd)
            self.__pty_make_controlling_tty(child_fd)

            os.dup2(child_fd, self.STDIN_FILENO)
            os.dup2(child_fd, self.STDOUT_FILENO)
            os.dup2(child_fd, self.STDERR_FILENO)

        else:
            # Parent.
            os.close(child_fd)

        return pid, parent_fd 

def __pty_make_controlling_tty(self, tty_fd):
        '''This makes the pseudo-terminal the controlling tty. This should be
        more portable than the pty.fork() function. Specifically, this should
        work on Solaris. '''

        child_name = os.ttyname(tty_fd)

        # Disconnect from controlling tty, if any.  Raises OSError of ENXIO
        # if there was no controlling tty to begin with, such as when
        # executed by a cron(1) job.
        try:
            fd = os.open("/dev/tty", os.O_RDWR | os.O_NOCTTY)
            os.close(fd)
        except OSError as err:
            if err.errno != errno.ENXIO:
                raise

        os.setsid()

        # Verify we are disconnected from controlling tty by attempting to open
        # it again.  We expect that OSError of ENXIO should always be raised.
        try:
            fd = os.open("/dev/tty", os.O_RDWR | os.O_NOCTTY)
            os.close(fd)
            raise ExceptionPexpect("OSError of errno.ENXIO should be raised.")
        except OSError as err:
            if err.errno != errno.ENXIO:
                raise

        # Verify we can open child pty.
        fd = os.open(child_name, os.O_RDWR)
        os.close(fd)

        # Verify we now have a controlling tty.
        fd = os.open("/dev/tty", os.O_WRONLY)
        os.close(fd) 

def read_until_semaphore(fd, semaphore=RECV_SEMAPHORE,
                         encoding='utf8', timeout=10):
    """Read file descriptor ``fd`` until ``semaphore`` is found.

    Used to ensure the child process is awake and ready. For timing
    tests; without a semaphore, the time to fork() would be (incorrectly)
    included in the duration of the test, which can be very length on
    continuous integration servers (such as Travis-CI).
    """
    # note that when a child process writes xyz\\n, the parent
    # process will read xyz\\r\\n -- this is how pseudo terminals
    # behave; a virtual terminal requires both carriage return and
    # line feed, it is only for convenience that \\n does both.
    outp = six.text_type()
    decoder = codecs.getincrementaldecoder(encoding)()
    semaphore = semaphore.decode('ascii')
    while not outp.startswith(semaphore):
        try:
            _exc = os.read(fd, 1)
        except OSError:  # Linux EOF
            break
        if not _exc:     # BSD EOF
            break
        outp += decoder.decode(_exc, final=False)
    assert outp.startswith(semaphore), (
        'Semaphore not recv before EOF '
        '(expected: %r, got: %r)' % (semaphore, outp,))
    return outp[len(semaphore):] 

def _spawn(shell, master_read):
    """Create a spawned process.

    Modified version of pty.spawn with terminal size support.

    """
    pid, master_fd = pty.fork()

    if pid == pty.CHILD:
        os.execlp(shell, shell)

    try:
        mode = tty.tcgetattr(pty.STDIN_FILENO)
        tty.setraw(pty.STDIN_FILENO)
        restore = True
    except tty.error:    # This is the same as termios.error
        restore = False

    _set_pty_size(master_fd)
    signal.signal(signal.SIGWINCH, lambda *_: _set_pty_size(master_fd))

    try:
        pty._copy(master_fd, master_read, pty._read)
    except OSError:
        if restore:
            tty.tcsetattr(pty.STDIN_FILENO, tty.TCSAFLUSH, mode)

    os.close(master_fd)
    return os.waitpid(pid, 0)[1] 

def read_until_semaphore(fd, semaphore=RECV_SEMAPHORE,
                         encoding='utf8', timeout=10):
    """Read file descriptor ``fd`` until ``semaphore`` is found.

    Used to ensure the child process is awake and ready. For timing
    tests; without a semaphore, the time to fork() would be (incorrectly)
    included in the duration of the test, which can be very length on
    continuous integration servers (such as Travis-CI).
    """
    # note that when a child process writes xyz\\n, the parent
    # process will read xyz\\r\\n -- this is how pseudo terminals
    # behave; a virtual terminal requires both carriage return and
    # line feed, it is only for convenience that \\n does both.
    outp = unicode()
    decoder = codecs.getincrementaldecoder(encoding)()
    semaphore = semaphore.decode('ascii')
    while not outp.startswith(semaphore):
        try:
            _exc = os.read(fd, 1)
        except OSError:  # Linux EOF
            break
        if not _exc:     # BSD EOF
            break
        outp += decoder.decode(_exc, final=False)
    assert outp.startswith(semaphore), (
        'Semaphore not recv before EOF '
        '(expected: %r, got: %r)' % (semaphore, outp,))
    return outp[len(semaphore):]