code: def allocate_tcp_port():
      
      # We want to bind() the socket but not listen(). Twisted (in
      # tcp.Port.createInternetSocket) would do several other things:
      # non-blocking, close-on-exec, and SO_REUSEADDR. We don't need
      # non-blocking because we never listen on it, and we don't need
      # close-on-exec because we close it right away. So just add SO_REUSEADDR.
      s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
      if platformType == "posix" and sys.platform != "cygwin":
          s.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)
      s.bind(("127.0.0.1", 0))
      port = s.getsockname()[1]
      s.close()
      return port
docs: Return an (integer) available TCP port on localhost. This briefly
      listens on the port in question, then closes it right away.