Prototype a workaround for unix domain socket length limit of 107.

nc-chdir.py

#!/usr/bin/env python

# this is a tiny little clone of `nc -U -l path`
# but it creates the socket *after chdir()ing in a subprocess* to avoid ENAMETOOLONG: 
#
# To test the difference, get openbsd-netcat and run:
# 
# mkdir -p /tmp/abcdefhijklmnopqrstuvwxyz/abcdefhijklmnopqrstuvwxyz/abcdefhijklmnopqrstuvwxyz/abcdefhijklmnopqrstuvwxyz
# nc -v -U -l /tmp/abcdefhijklmnopqrstuvwxyz/abcdefhijklmnopqrstuvwxyz/abcdefhijklmnopqrstuvwxyz/abcdefhijklmnopqrstuvwxyz/ipc.sock
#
# This will fail with "nc: File name too long", but
#
# ./nc-chdir /tmp/abcdefhijklmnopqrstuvwxyz/abcdefhijklmnopqrstuvwxyz/abcdefhijklmnopqrstuvwxyz/abcdefhijklmnopqrstuvwxyz/ipc.sock
#
# starts, and can be communicated with by
#
# (cd /tmp/abcdefhijklmnopqrstuvwxyz/abcdefhijklmnopqrstuvwxyz/abcdefhijklmnopqrstuvwxyz/abcdefhijklmnopqrstuvwxyz/ && nc -v -U ipc.sock)

import sys, os
import socket, select
import traceback

def reliable_bind(socket, path):
    """
    This is a wrapper for socket.socket.bind(), with the same API.

    But it works for paths of (almost) any length rather than being
    limited to about 100: https://unix.stackexchange.com/questions/367008/why-is-socket-path-length-limited-to-a-hundred-chars

    (this will still fail if len(basename(path)) > 100, or thereabouts; just don't do that)
    """
    o = socket.get_inheritable()
    try:
        socket.set_inheritable(True) # this is a pythonism, to ensure the parent and child can share file descriptors
        child = os.fork()
        if child == 0:
            # child process

            try:
                # in the subprocess, chdir() to the desired path
                # so that we only need to bind() on basename(path);
                # so long as the actual filename is not too long,
                # this should succeed.
                #
                # The reason for using a child process is so that we can call chdir() -- which sets a global variable -- without having to worry about concurrency,
                # or undoing the chdir() afterwards -- when the child process dies the chdir() is forgotten, but the socket is left bound correctly.
                os.chdir(os.path.dirname(path))
                # On Linux at least(?) this bind() is picked up correctly on the matching file descriptor in the parent.
                socket.bind(os.path.basename(path))
            except OSError as exc:
                # assume that most of the errors are OSErrors
                # and pass them back via the process's exit code
                raise SystemExit(exc.errno) # error
            except Exception as exc:
                # but in really exceptional circumstances (low memory? corrupt process code?)
                # encode that as code 255, which should be greater than any defined errno
                traceback.print_exc()
                raise SystemExit(255)
            raise SystemExit(0) # success

        else:
            # parent process
            
            _, status = os.waitpid(child, 0)
            errno = os.waitstatus_to_exitcode(status)
            if errno != 0:
                if errno == 255:
                    raise Exception("Unknown error occurred in child")
                raise OSError(errno, os.strerror(errno))
    finally:
        socket.set_inheritable(o)

#reliable_bind = socket.socket.bind # no subdir

def main():
    addr = sys.argv[1]

    s = socket.socket(socket.AF_UNIX)
    reliable_bind(s, addr)
    print("Bound on", s.getsockname())
    try:
        s.listen()
        print("Listening on", s.getsockname())
        S, remote = s.accept()
        print("Connection received on ", S.getsockname())
          # XXX 'remote' is empty; on linux, to find out who is talking to you, you have to use recvmsg() and look at the "acnillary data":  https://stackoverflow.com/questions/8104904/identify-program-that-connects-to-a-unix-domain-socket
        while True:
           readable, _, _ = select.select([S, sys.stdin], [], []) 
           for fd in readable:
               if fd == S:
                   buf = S.recv(512)
                   if not buf:
                       # socket disconnected
                       return
                   sys.stdout.buffer.raw.write(buf) # TODO: .buffer.raw?? should I just use os.write(sys.stdout.fileno())??
               elif fd == sys.stdin:
                   buf = sys.stdin.buffer.raw.read(512) # TODO: ditto
                   if not buf:
                       return
                   S.send(buf)
    except KeyboardInterrupt:
        pass
    finally:
        os.unlink(addr) # TODO: is there a 'with:' for unix domain sockets?

if __name__ == '__main__':
    main()