I've realised the error of my ways: because Task separates the scheduling from the response handling, you cannot know if an exception is unhandled until the task is deleted. So in my example the reference means the task is not deleted, so the exception is not yet unhandled.
This is in contrast to APIs like call_soon(callable, success_callback, error_callback) where there the possibility of delayed error handling is not present. In that case the loop can reliably crash if either callback raises an exception. So, the 'solution' to this use-case is to always attach error handers to Tasks. A catch-all solution cannot catch every error case. On Tue., 26 Feb. 2019, 6:06 am Josh Quigley, <[email protected]> wrote: > 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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