On Sat, Aug 5, 2017 at 9:41 PM, Chris Jerdonek <chris.jerdo...@gmail.com> wrote:
> I want to share a pattern I came up with for handling interrupt > signals in asyncio to see if you had any feedback (ways to make it > easier, similar approaches, etc). > Just after sending this email, I learned that approaches like this can't work in general since they can't interrupt a "tight loop," and that changes to asyncio are needed. There are some discussions about this on GitHub: The signal solution is indeed nicer, because it ensures that interrupts are > treated as regular asyncio events, but it means you can't interrupt code > that's stuck in a tight CPU loop (e.g. while True: pass), and it requires > more sophistication from users. (from: https://github.com/python/asyncio/pull/305#issuecomment-168541045 ) This is a big no-no. In the first version of uvloop I did exactly this -- > handle SIGINT and let the loop to handle it asynchronously. It was > completely unusable. Turns out people write tight loops quite frequently, > and inability to stop your Python program with Ctrl-C is something they > aren't prepared to handle at all. (from: https://github.com/python/asyncio/issues/341#issuecomment-236443331 ) The current open issue is here: https://github.com/python/asyncio/issues/341 --Chris > I wanted something that was easy to check and reason about. I'm > already familiar with some of the pitfalls in handling signals, for > example as described in Nathaniel's Control-C blog post announced > here: > https://mail.python.org/pipermail/async-sig/2017-April/thread.html > > The basic idea is to create a Future to run alongside the main > coroutine whose only purpose is to "catch" the signal. And then call-- > > asyncio.wait(futures, return_when=asyncio.FIRST_COMPLETED) > > When a signal is received, both tasks stop, and then you have access > to the main task (which will be pending) for things like cleanup and > inspection. > > One advantage of this approach is that it lets you put all your > cleanup logic in the main program instead of putting some of it in the > signal handler. You also don't need to worry about things like > handling KeyboardInterrupt at arbitrary points in your code. > > I'm including the code at bottom. > > On the topic of asyncio.run() that I mentioned in an earlier email > [1], it doesn't look like the run() API posted in PR #465 [2] has > hooks to support what I'm describing (but I could be wrong). So maybe > this is another use case that the future API should contemplate. > > --Chris > > [1] https://mail.python.org/pipermail/async-sig/2017-August/000373.html > [2] https://github.com/python/asyncio/pull/465 > > > import asyncio > import io > import signal > > def _cleanup(loop): > try: > loop.run_until_complete(loop.shutdown_asyncgens()) > finally: > loop.close() > > def handle_sigint(future): > future.set_result(signal.SIGINT) > > async def run(): > print('running...') > await asyncio.sleep(1000000) > > def get_message(sig, task): > stream = io.StringIO() > task.print_stack(file=stream) > traceback = stream.getvalue() > return f'interrupted by {sig.name}:\n{traceback}' > > def main(coro): > loop = asyncio.new_event_loop() > > try: > # This is made truthy if the loop is interrupted by a signal. > interrupted = [] > > future = asyncio.Future(loop=loop) > future.add_done_callback(lambda future: interrupted.append(1)) > > loop.add_signal_handler(signal.SIGINT, handle_sigint, future) > > futures = [future, coro] > future = asyncio.wait(futures, return_when=asyncio.FIRST_ > COMPLETED) > done, pending = loop.run_until_complete(future) > > if interrupted: > # Do whatever cleanup you want here and/or get the stacktrace > # of the interrupted main task. > sig = done.pop().result() > task = pending.pop() > msg = get_message(sig, task) > > task.cancel() > raise KeyboardInterrupt(msg) > > finally: > _cleanup(loop) > > main(run()) > > > Below is what the code above outputs if you run it and then press > Control-C: > > running... > ^CTraceback (most recent call last): > File "test-signal.py", line 54, in <module> > main(run()) > File "test-signal.py", line 49, in main > raise KeyboardInterrupt(msg) > KeyboardInterrupt: interrupted by SIGINT: > Stack for <Task pending coro=<run() running at test-signal.py:17> > wait_for=<Future pending cb=[<TaskWakeupMethWrapper object at > 0x10fe9b9a8>()]>> (most recent call last): > File "test-signal.py", line 17, in run > await asyncio.sleep(1000000) >
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