Question

(I’m using the pyprocessing module in this example, but replacing processing with multiprocessing should probably work if you run python 2.6 or use the multiprocessing backport)

I currently have a program that listens to a unix socket (using a processing.connection.Listener), accept connections and spawns a thread handling the request. At a certain point I want to quit the process gracefully, but since the accept()-call is blocking and I see no way of cancelling it in a nice way. I have one way that works here (OS X) at least, setting a signal handler and signalling the process from another thread like so:

import processing from processing.connection import Listener import threading import time import os import signal import socket import errno  # This is actually called by the connection handler. def closeme():     time.sleep(1)     print 'Closing socket...'     listener.close()     os.kill(processing.currentProcess().getPid(), signal.SIGPIPE)  oldsig = signal.signal(signal.SIGPIPE, lambda s, f: None)  listener = Listener('/tmp/asdf', 'AF_UNIX') # This is a thread that handles one already accepted connection, left out for brevity threading.Thread(target=closeme).start() print 'Accepting...' try:     listener.accept() except socket.error, e:     if e.args[0] != errno.EINTR:         raise # Cleanup here... print 'Done...' 

The only other way I’ve thought about is reaching deep into the connection (listener._listener._socket) and setting the non-blocking option…but that probably has some side effects and is generally really scary.

Does anyone have a more elegant (and perhaps even correct!) way of accomplishing this? It needs to be portable to OS X, Linux and BSD, but Windows portability etc is not necessary.

Clarification: Thanks all! As usual, ambiguities in my original question are revealed 🙂

• I need to perform cleanup after I have cancelled the listening, and I don’t always want to actually exit that process.
• I need to be able to access this process from other processes not spawned from the same parent, which makes Queues unwieldy
• The reasons for threads are that:
  • They access a shared state. Actually more or less a common in-memory database, so I suppose it could be done differently.
  • I must be able to have several connections accepted at the same time, but the actual threads are blocking for something most of the time. Each accepted connection spawns a new thread; this in order to not block all clients on I/O ops.

Regarding threads vs. processes, I use threads for making my blocking ops non-blocking and processes to enable multiprocessing.

Answer 1

Isnt that what select is for??

Only call accept on the socket if the select indicates it will not block…

The select has a timeout, so you can break out occasionally occasionally to check if its time to shut down….