Mailman 3 February 2019 - Async-sig

Reliably make unhandled exceptions crash the event loop
by Josh Quigley 26 Feb '19

26 Feb '19

Hi, I have been trying to make unhandled exceptions reliably crash the event loop (eg replicated behaviour of boost::asio, for those familiar with that C++ library). I'm aiming to have any exception bubble up from run_forever or run_until_complete style functions. I had thought I had a perfectly acceptable solution and then hit a strange case that threw my understanding of the way the loop worked. In 'test_b' below, the only difference is we keep a reference to the created task. This test hangs - the exception is raised, but the custom exception handler is never called. I'd be very interested to understand exactly why this happens. I'd also appreciate any feedback on the best way to reliably crash the event loop on unhandled exceptions (my next attempt will be to replace AbstractEventLoop.call_exception and see what happens). # example.py import asyncio class CustomException(RuntimeError): pass class UnhandledExceptionError(RuntimeError): pass def run_until_unhandled_exception(*, loop=None): """Run the event until there is an unhandled error in a callback This function sets the exception handler on the loop """ loop = loop if loop is not None else asyncio.get_event_loop() ex = [] def handler(loop, context): print('handler') loop.default_exception_handler(context) loop.stop() ex.append(context.get('exception')) loop.set_exception_handler(handler) loop.run_forever() if len(ex) > 0: raise UnhandledExceptionError('Unhandled exception in loop') from ex[0] async def fail_after(delay): await asyncio.sleep(delay) print('raise CustomException(...)') raise CustomException(f'fail_after(delay={delay})') async def finish_after(delay): await asyncio.sleep(delay) return delay def test_a(event_loop): event_loop.create_task(fail_after(0.01)) run_until_unhandled_exception(loop=event_loop) def test_b(event_loop): task = event_loop.create_task(fail_after(0.01)) run_until_unhandled_exception(loop=event_loop) def run_test(test): try: test(asyncio.get_event_loop()) except Exception as ex: print(ex) if __name__ == '__main__': run_test(test_a) run_test(test_b) # This hangs

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killing tasks that won't cancel
by Chris Jerdonek 19 Feb '19

19 Feb '19

I have an asyncio question. In Python 3.7, is there a way to reliably end a task after having already tried calling cancel() on it and waiting for it to end? In Python 3.6, I did this with task.set_exception(), but in 3.7 that method was removed. --Chris

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set_blocking_signal_threshold equivalent for asyncio?
by Yusuke Tsutsumi 13 Feb '19

13 Feb '19

Hi, In tornado, there's a really nice feature called set_blocking_signal_threshold, which sets a signal that fires if a coroutine has been running for too long without returning control back to the main loop: https://www.tornadoweb.org/en/stable/ioloop.html?highlight=signal#tornado.i… In tornado this will then log the traceback of the coroutine in question. This has been a very valuable tool when a developer accidentally introduces code that blocks the event loop for way too long of a time. Is there an equivalent in asyncio? I have a sketch in my head of how to implement that, but wanted to see if it existed somewhere first. Thanks! -Yusuke

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AI_V4MAPPED on Windows
by Jan Seeger 03 Feb '19

03 Feb '19

Greetings! I'm working with aiocoap, which uses the AI_V4MAPPED flag to use IPv4-mapped addresses for dual-stack support. When trying to run on Windows, creating a connection fails, because the socket option IPV6_V6ONLY is set to true per default on Windows, whereas the value is configurable on Linux. I've attached a file that should reproduce the error. A possible fix would be calling socket.setsockopt(socket.IPPROTO_IPV6, socket.IPV6_V6ONLY, False) when V4-mapped addresses have been requested (this bug can also appear on Linux when /proc/sys/net/ipv6/bindv6only contains 1). If you require any more information, feel free to contact me! Best Regards, Jan Seeger PS: I am not a subscriber of this list, so please leave my address on any replies you send.

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