How can coroutines be used in signal handlers?

[jinn...@openmailbox.org]

I am developing an application that uses asyncio from python3.4 for networking. When this application shuts down cleanly, a node needs to "disconnect" from the hub.
This disconnect is an active process that requires a network connection so the loop needs to wait for this to complete before shutting down.

My issue is that using a coroutine as a signal handler will result in the application not shutting down. Please consider the following example:
   
    import asyncio
    import functools
    import os
    import signal
   
    @asyncio.coroutine
    def ask_exit(signame):
        print("got signal %s: exit" % signame)
        yield from asyncio.sleep(10.0)
        loop.stop()
   
    loop = asyncio.get_event_loop()
    for signame in ('SIGINT', 'SIGTERM'):
        loop.add_signal_handler(getattr(signal, signame),
                                            functools.partial(ask_exit, signame))
   
    print("Event loop running forever, press CTRL+c to interrupt.")
    print("pid %s: send SIGINT or SIGTERM to exit." % os.getpid())
    loop.run_forever()

If you run this example and then press Ctrl+C, nothing will happen.
The question is, how do I make this behavior happen with siganls and coroutines?


PS: If you want to earn points, I have also asked this on stackoverflow:
https://stackoverflow.com/questions/23313720/asyncio-how-can-coroutines-be-used-in-signal-handlers

[Luca Sbardella]

Hi,

On 27 April 2014 11:20, <jinn...@openmailbox.org> wrote:

I am developing an application that uses asyncio from python3.4 for networking. When this application shuts down cleanly, a node needs to "disconnect" from the hub.
This disconnect is an active process that requires a network connection so the loop needs to wait for this to complete before shutting down.

My issue is that using a coroutine as a signal handler will result in the application not shutting down. Please consider the following example:
   
    import asyncio
    import functools
    import os
    import signal
   
    @asyncio.coroutine
    def ask_exit(signame):
        print("got signal %s: exit" % signame)
        yield from asyncio.sleep(10.0)
        loop.stop()
   
    loop = asyncio.get_event_loop()
    for signame in ('SIGINT', 'SIGTERM'):
        loop.add_signal_handler(getattr(signal, signame),
                                            functools.partial(ask_exit, signame))
   
    print("Event loop running forever, press CTRL+c to interrupt.")
    print("pid %s: send SIGINT or SIGTERM to exit." % os.getpid())
    loop.run_forever()

If you run this example and then press Ctrl+C, nothing will happen.
The question is, how do I make this behavior happen with siganls and coroutines?

The signal handler cannot return a coroutine.

If you replace:

loop.add_signal_handler(getattr(signal, signame),
                                            functools.partial(ask_exit, signame))

with

loop.add_signal_handler(getattr(signal, signame), lambda: asyncio.async(ask_exit(signame)))

your example will work fine.

The async function runs your coroutine in the event loop.

 

PS: If you want to earn points, I have also asked this on stackoverflow:
https://stackoverflow.com/questions/23313720/asyncio-how-can-coroutines-be-used-in-signal-handlers

--

http://lucasbardella.com

[jinn...@openmailbox.org]

Hi Luca,

Am Sonntag, 27. April 2014 11:01:31 UTC schrieb Luca Sbardella:

The signal handler cannot return a coroutine.

If you replace:

loop.add_signal_handler(getattr(signal, signame),
                                            functools.partial(ask_exit, signame))

with

loop.add_signal_handler(getattr(signal, signame), lambda: asyncio.async(ask_exit(signame)))

your example will work fine.

The async function runs your coroutine in the event loop.

This does indeed work. Thank you very much.

[Victor Stinner]

Your question is a frequent question. We should enhance the documentation to explain how to use coroutines in protocols, signal handlers and all other places using callbacks, maybe with a few simple examples.

Victor