asyncio - how to stop background task cleanly
Frank Millman
frank at chagford.com
Sun Feb 7 02:10:36 EST 2016
"Frank Millman" wrote in message news:n96kjr$mvl$1 at ger.gmane.org...
>
> "Marko Rauhamaa" wrote in message
> news:8737t5shhp.fsf at elektro.pacujo.net...
>
> > Actually, cancellation is specially supported in asyncio (<URL:
> > https://docs.python.org/3/library/asyncio-task.html#asyncio.Task.cancel>)
> > so this should do:
> >
> > async def background_task():
> > while True:
> > await perform_task()
> > await asyncio.sleep(10)
> >
>
> That's exactly what I needed - thanks, Marko
>
> async def background_task()
> try:
> while True:
> await perform_task()
> await asyncio.sleep(10)
> except asyncio.CancelledError:
> await perform_cleanup()
>
> At startup -
>
> task = asyncio.ensure_future(background_task())
>
> At shutdown -
>
> task.cancel()
> await asyncio.wait([task])
>
> Works perfectly - thanks again.
>
Alas, I spoke too soon.
I tried to simulate what would happen if the background task was busy with a
task when it was cancelled -
async def background_task()
try:
while True:
print('start')
time.sleep(2)
print('done')
await asyncio.sleep(10)
except asyncio.CancelledError:
print('cleanup')
print('DONE')
If I cancel after a pair of 'start/done' appear, the background task is in
the 'asyncio.sleep' stage. The words 'cleanup' and 'DONE' appear instantly,
and the program halts.
If I cancel after 'start', but before 'done', the background task is
executing a task. There is a delay of up to 2 seconds, then the words
'done', 'cleanup', and 'DONE' appear, but the program hangs. If I press
Ctrl+C, I get a traceback from the threading module -
line 1288, in _shutdown
t.join()
line 1054, in join
self._wait_for_tstate_lock()
line 1070, in _wait_for_tstate_lock
KeyboardInterrupt
So it is waiting for join() to complete. I will continue investigating, but
will report it here to see if anyone can come up with an
explanation/solution.
Thanks
Frank
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