<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def fileno(self):
 """ Returns an OS-level file descriptor which can be used for polling, but but not for reading or writing. This is primarily to allow Python's ``select`` module to work. The first time ``fileno`` is called on a channel, a pipe is created to simulate real OS-level file descriptor (FD) behavior. Because of this, two OS-level FDs are created, which will use up FDs faster than normal. (You won't notice this effect unless you have hundreds of channels open at the same time.) :return: an OS-level file descriptor (`int`) .. warning:: This method causes channel reads to be slightly less efficient. """

        self.lock.acquire()
        try:
            if self._pipe is not None:
                return self._pipe.fileno()
            # create the pipe and feed in any existing data
            self._pipe = pipe.make_pipe()
            p1, p2 = pipe.make_or_pipe(self._pipe)
            self.in_buffer.set_event(p1)
            self.in_stderr_buffer.set_event(p2)
            return self._pipe.fileno()
        finally:
            self.lock.release()