I'm still climbing up the asyncio learning curve. I'm building an
application on top of asyncio, and one of my goals is that when the
application is interrupted (e.g. SIGINT), it should carefully clean up all
running tasks before closing the event loop. Some tasks should be cancelled
right away, but other tasks should be allowed to complete. As an example of
the latter, I may have several file download tasks that I want to finish
before closing the event loop. (If I get a 2nd SIGINT, then I close the
loop immediately and exit.)
I have a hard time debugging the "Task was destroyed..." errors. For
example:
Task was destroyed but it is pending!
task: <Task pending coro=<sleep() done, defined at
/usr/lib/python3.5/asyncio/tasks.py:502>
wait_for=<Future pending cb=[Task._wakeup()]>
cb=[_wait.<locals>._on_completion() at
/usr/lib/python3.5/asyncio/tasks.py:414]>
I'm pretty sure that I understand why I get this error: this is a task
(e.g. created with asyncio.ensure_future()) that was never awaited. To
prevent this error, I should await the task (or call its result() method,
right?). But how do I figure out where the task was created? I have
PYTHONASYNCIODEBUG=1 in my environment, and I see the "defined at" text,
which is helpful if the coroutine is in my own code, but for
asyncio.sleep(), it's not very helpful.
Is it possible to see where this task was created, e.g. the line containing
asyncio.ensure_future(asyncio.sleep(...)) ? If not, is this a capability
that could be added to debug mode? E.g. ensure_future would add its own
stack trace to the task object so that it could be retrieved later when a
pending task is destroyed.
In my current scenario, I have few enough tasks that there's only one place
I think this particular error could come from, but as my application grows,
this will get harder and harder to debug.
Thoughts?
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