How to implement redis's pubsub timeout feature?

Question

I want to use Redis's pubsub feature to implement comet, but pubsub doesn't have timeout, so if I use ps.listen(), it will block, even if client closes browser.

Greenlet has a timeout feature when spawn process, but I don't know how to combine them.

Flask's pseudo code:

@app.route('/')
def comet():
    rc = redis.Redis()
    ps = rc.pubsub()
    ps.subscribe('foo')
    for item in ps.listen():
        if item['type'] == 'message':
            return item['data']
    # ps.listen() will block, so how to make it timeout after 30 s?

Answer 1

Because you're not threading (and I'm assuming this is intentional and in some cases wise) you must use a type of interrupt. Signals are a type of interrupt on Unix systems to allow you to return to a callback during a call that could block.

This example of a file open which will never return is in line with what you want to do. It's taken from http://docs.python.org/library/signal.html#module-signal

But a warning. Because Python uses a Global Interpreter Lock to perform OS signal handling it is subject to some stability problems. These problems should be rare normally though.

import signal, os

def handler(signum, frame):
    print 'Signal handler called with signal', signum
    raise IOError("Couldn't open device!")

# Set the signal handler and a 5-second alarm
signal.signal(signal.SIGALRM, handler)
signal.alarm(5)

# This open() may hang indefinitely
fd = os.open('/dev/ttyS0', os.O_RDWR)

signal.alarm(0)          # Disable the alarm