Forward a terminal with window size changes across a unix domain socket

pipewinch

#!/usr/bin/python3
#
# pipewinch: Forward a terminal with window size changes across a unix
#            domain socket.
#
# Usage:
#   pipewinch <uds-action> <uds-path> exec ARGS...
#   pipewinch <uds-action> <uds-path> term
# where uds-action is "listen" or "connect"
#
# A pair of `pipewinch` instances, one invoked with `exec` and the other
# with `term`, forwards the input and output of a program across a unix
# domain socket to a terminal, including window size changes.  This can be
# used to interact with programs running in containers or as other users.
#
# The `exec` instance runs the command line `ARGS...` in a pseudo-terminal
# (pty).  It sends the program's output to the socket and forwards input
# from the socket to the program.  It also resizes the pty when it
# receives a window size changed message from the socket.
#
# The `term` instance acts as a terminal across the socket, displaying the
# output and forwarding the input.  If the terminal's window size changes,
# it sends a window size changed message.
#
# If the uds-action `listen` is used then `pipewinch` will act as a
# server, creating and listening on a unix domain socket at
# the location <uds-path>; or if `connect` is used then `pipewinch` will
# act as a client, connecting to the socket at <uds-path>.
#
# It does not matter whether the `exec` or the `term` instance does the
# `listen` or `connect`, but the instance doing `listen` must start first
# and then the other must do `connect`.
#
# Example:
#
#   pipewinch listen /tmp/uds exec /bin/bash -l
# 
#   pipewinch connect /tmp/uds term
#
# Background:  `pipewinch` can be used as an alternative to `socat`, for
# example the commands below which forward the terminal but not the window
# size changed information.  This means that programs such as `vi` and
# `less` (or even command-line editing in the shell) do not display
# correctly after you resize the window.
#
#   rm -f /tmp/uds ; \
#     socat EXEC:"/bin/bash -l",pty,setsid,setpgid,stderr,ctty,echo=0,sane \
#           UNIX-LISTEN:/tmp/uds
# 
#   socat UNIX-CONNECT:/tmp/uds STDIO,raw,echo=0
# 
# Instead, `pipewinch` in the second session sends the window size changed
# information encoded as an in-band message, and `pipewinch` in the first
# session decodes it and sets the terminal size.
#
# (Similarly, `telnet` sends window size changed information as an in-band
# message.  `ssh` is able to send an out-of-band message because its protocol
# has multiple channels.  A script similar to `pipewinch` called `pty` from
# https://github.com/wheelcomplex/misc uses a separate connection to forward
# the information.  It would be useful if `socat` could also do forward window
# size changed information - presumably in-band - but it currently does not.)
#
# Security:  Any user who can access `uds_path` will be able to attach to
# the program.  The socket is created using `umask()` such that only the
# current user can access it.  (It would be possible to allow other users
# to access the socket if it, or its containing directory, is created with
# the appropriate group and permissions; but this is not implemented here.)
#
#
# For more information, see:
#    https://unix.stackexchange.com/questions/207782/
#    https://stackoverflow.com/questions/63230001/
#    http://www.dest-unreach.org/socat/
#    https://stackoverflow.com/questions/28776502/unix-domain-socket-securing-receiver
#    https://unix.stackexchange.com/questions/117981/what-are-the-responsibilities-of-each-pseudo-terminal-pty-component-software
#
# Ben Cohen, August 2022.
#

import argparse
import fcntl
import os
import pty
import select
import signal
import socket
import struct
import subprocess
import sys
import termios
import threading

# The window size changed message has the format
#   "\xFFWS=" + [8 byte TIOCGWINSZ result converted to network byte order]
#             + "\xFF"
# If the stream contains the bytes "\xFFWS" then they must be escaped using
#   "\xFFWS\xFF"
# It is an error for the stream to contain the bytes "\xFFWS" and the following
# character to be anything other than '=' or '\xFF'.

MAGIC_0 = b"\xFF"
MAGIC_1 = b"W"
MAGIC_2 = b"S"
MAGIC_3 = b"="

def winch_handler(signum, stack_frame):
    global window_size_changed
    window_size_changed = True

def get_window_size_string():
    arg = struct.pack('HHHH', 0, 0, 0, 0)
    winszle = fcntl.ioctl(sys.stdout.fileno(), termios.TIOCGWINSZ, arg)
    # Convert to network byte order
    return struct.pack('!HHHH', *struct.unpack('HHHH', winszle))

def set_window_size(msg):
    assert(len(msg) == 8)
    # Convert from network byte order
    winsz = struct.pack('!HHHH', *struct.unpack('HHHH', msg))
    fcntl.ioctl(mfd, termios.TIOCSWINSZ, winsz)

def check_winch(input_buffer, copy_behaviour):
    # COPY_TO_FD_SEND_WINCH: escape "\xFFWS".
    # COPY_FROM_FD_CHECK_WINCH: handle escape for "\xFFWS" and set window size.
    global check_winch_state, check_winch_buffer
    assert(type(input_buffer) == bytes)
    result = b""

    def reset_state():
        global check_winch_state, check_winch_buffer
        check_winch_state = 0
        check_winch_buffer = b""

    def advance_state():
        global check_winch_state, check_winch_buffer
        check_winch_state += 1
        check_winch_buffer += nextchar

    for nextchar in input_buffer:
        nextchar = bytes([nextchar])
        if check_winch_state == 0:    # '\xFF'
            if nextchar == MAGIC_0:
                advance_state()
            else:
                result += nextchar
        elif check_winch_state == 1:  # 'W'
            if nextchar == MAGIC_1:
                advance_state()
            else:
                result += check_winch_buffer + nextchar
                reset_state()
        elif check_winch_state == 2:  # 'S'
            if nextchar == MAGIC_2:
                if copy_behaviour == COPY_TO_FD_SEND_WINCH:
                    result += check_winch_buffer + nextchar + MAGIC_0
                    reset_state()
                else:
                    advance_state()
            else:
                result += check_winch_buffer + nextchar
                reset_state()
        elif check_winch_state == 3:  # '=' or '\xFF'
            assert(copy_behaviour != COPY_TO_FD_SEND_WINCH)
            if nextchar == MAGIC_0:
                result += check_winch_buffer
                reset_state()
            elif nextchar == MAGIC_3:
                advance_state()
            else:
                assert(False)
        elif 4 <= check_winch_state <= 11:  # TIOCGWINSZ result in network byte order
            assert(copy_behaviour != COPY_TO_FD_SEND_WINCH)
            advance_state()
        elif check_winch_state == 12: # '\xFF'
            assert(copy_behaviour != COPY_TO_FD_SEND_WINCH)
            assert(nextchar == MAGIC_0)
            set_window_size(check_winch_buffer[4:12])
            reset_state()
        else:
            assert(False)

    assert(type(result) == bytes)
    return result
    
COPY_NOOP                = 0
COPY_FROM_FD_CHECK_WINCH = 1
COPY_TO_FD_SEND_WINCH    = 2

def do_copy(from_fd, to_fd, copy_behaviour = COPY_NOOP):
    global running, window_size_changed
    while True:
        # TODO: Improve error handling!
        try:
            rfds, wfds, xfds = select.select([from_fd], [], [], 0.001)
            if len(rfds) == 0 and not running:
                # No more input and we got EOF on another stream so give up
                break
            if from_fd in rfds:
                data = os.read(from_fd, 1024)
                if len(data) > 0:
                    if copy_behaviour != COPY_NOOP:
                        data = check_winch(data, copy_behaviour)
                    os.write(to_fd, data)
                else:
                    # EOF
                    running = False
                    break
            if copy_behaviour == COPY_TO_FD_SEND_WINCH and window_size_changed:
                os.write(to_fd, MAGIC_0 + MAGIC_1 + MAGIC_2 + MAGIC_3 + get_window_size_string() + MAGIC_0)
                window_size_changed = False
        except:
            running = False
            break

def usage(message = None):
    if message is not None:
        print(f"{sys.argv[0]}: error: " + message)
        print()
    print("Usage:")
    print(f"  {sys.argv[0]} <uds-action> <uds-path> exec [ARGS...]")
    print(f"  {sys.argv[0]} <uds-action> <uds-path> term ")
    print('where uds-action is "listen" or "connect"')
    sys.exit(1)

if len(sys.argv) < 4:
    usage("too few arguments")

uds_action = sys.argv[1]
uds_path = sys.argv[2]
main_action = sys.argv[3]
subprocessargs = sys.argv[4:]

check_winch_state = 0
check_winch_buffer = b""
running = True
window_size_changed = True
uds_conn = None
signal.signal(signal.SIGWINCH, winch_handler)

if main_action not in ['exec', 'term']:
    usage(f"unknown mode '{main_action}'")

if uds_action == 'listen':
    # UDS server
    if os.path.exists(uds_path):
        os.remove(uds_path)
    old_umask = os.umask(0o077)
    uds_server = socket.socket(socket.AF_UNIX, socket.SOCK_STREAM)
    uds_server.bind(uds_path)
    os.umask(old_umask)
    uds_server.listen()
    (uds_conn, addr) = uds_server.accept()
    os.remove(uds_path)
    uds_server.close()
elif uds_action == 'connect':
    # UDS client
    if not os.path.exists(uds_path):
        print(f"Path {uds_path} does not exist")
        sys.exit(1)
    uds_conn = socket.socket(socket.AF_UNIX, socket.SOCK_STREAM)
    uds_conn.connect(uds_path)
else:
    usage(f"unknown socket action '{uds_action}'")

if main_action == 'exec':
    if len(subprocessargs) == 0:
        subprocessargs = ["/bin/sh"]

    # Set up pty and terminal.  (See "man 7 pty".)
    mfd, sfd = pty.openpty()

    # Start subprocess...
    child_pid = os.fork()
    if child_pid == 0:
        # ... Child continues here
        for fd in [0, 1, 2]:
            if fd != sfd:
                os.close(fd)
            os.dup2(sfd, fd)
        os.close(mfd)
        os.close(sfd)

        os.setsid()
        fcntl.ioctl(sys.stdin, termios.TIOCSCTTY, 0)

        # Set up terminal
        os.system("stty sane")
        os.execvpe(subprocessargs[0], subprocessargs, os.environ)

    # ... Parent continues here
    assert(child_pid)
    os.close(sfd)

    # Start threads to copy data
    stdinthr = threading.Thread(target=do_copy, args=(uds_conn.fileno(), mfd, COPY_FROM_FD_CHECK_WINCH))
    stdoutthr = threading.Thread(target=do_copy, args=(mfd, uds_conn.fileno()))
    stdinthr.start()
    stdoutthr.start()

    # Wait for everything to finish
    rc = os.waitpid(child_pid, 0) # Could send `rc` to the other end at this point
    uds_conn.shutdown(socket.SHUT_RDWR)
    uds_conn.close()
    os.close(mfd)

    stdinthr.join()
    stdoutthr.join()

elif main_action == 'term':
    # Set up terminal
    if (os.isatty(sys.stdin.fileno())):
        old_attrs = termios.tcgetattr(sys.stdin)
        os.system("stty raw -echo")

    # Start threads to copy data
    stdinthr = threading.Thread(target=do_copy, args=(sys.stdin.fileno(), uds_conn.fileno(), COPY_TO_FD_SEND_WINCH))
    stdoutthr = threading.Thread(target=do_copy, args=(uds_conn.fileno(), sys.stdout.fileno()))
    stdinthr.start()
    stdoutthr.start()

    # Wait for everything to finish
    stdinthr.join()
    stdoutthr.join()

    uds_conn.shutdown(socket.SHUT_RDWR)
    uds_conn.close()

    # Reset terminal
    if (os.isatty(sys.stdin.fileno())):
        termios.tcsetattr(sys.stdin, termios.TCSANOW, old_attrs)

else:
    assert(False)