cancellation error in extmod/uasyncio

Hi!

I was used to using the uasyncio implementation from micropython-lib. I saw you're moving to an implementation within extmod, so I thought I start using that instead. My issue is I don't know the proper way to cancel a task.

If I call cancel() on a uasyncio.Task instance, I get the following error:

RuntimeError: cannot cancel self

Thanks for the help!

Zsolt

The error says that a task cannot cancel itself.
If you think that that's not what your code is attempting, please post a test case.

I'm not trying to cancel a task from within itself, rather I'm calling task.cancel()

The code is:

import uasyncio

_loop = uasyncio.get_event_loop()
_tasks = {}

def create_task(name, coro, *args):
    if str(name) not in _tasks:
        _tasks[name] = _loop.create_task(coro(*args))
        return True
    return False


def destroy_task(name):
    if str(name) in _tasks:
        _tasks[name].cancel()  # this is the line raising the error
        del(_tasks[name])
        return True
    return False

Assume there's a task created with create_task('mytask', mycoro()) and I'm calling destroy_task('mytask') when the error is raised.

In the future please post complete test cases that a reviewer can run!

Here's my test case:

import uasyncio

_loop = uasyncio.get_event_loop()
_tasks = {}

def create_task(name, coro, *args):
    if str(name) not in _tasks:
        _tasks[name] = _loop.create_task(coro(*args))
        return True
    return False


def destroy_task(name):
    if str(name) in _tasks:
        _tasks[name].cancel()  # this is the line raising the error
        del(_tasks[name])
        return True
    return False

async def coro1():
    print("coro1 start")
    await uasyncio.sleep(2)
    print("coro1 done")
async def main():
    create_task("coro1", coro1)
    await uasyncio.sleep(0.2)
    print("destroy 1")
    destroy_task("coro1")
    await uasyncio.sleep(3)
    print("main exiting")

uasyncio.run(main())

Transcript:

> ../../tools/pyboard.py  --device /dev/ttyUSB1 foo.py
coro1 start
destroy 1
main exiting
>

Problem?

Sorry for not showing the whole test case. I have tried yours, which works, but the fact that it works, doesn't solve the error that I get. I will try to strip down my case to the relevant parts and share it. Hopefully I'll find out the cause of the error.

Try it with:

import uasyncio


_loop = uasyncio.get_event_loop()
_tasks = {}


async def dummy_task():
    while True:
        print('dummy')
        await uasyncio.sleep(0)


def create_task(name, coro, *args):
    if str(name) not in _tasks:
        _tasks[name] = _loop.create_task(coro(*args))
        return True
    return False


def destroy_task(name):
    if str(name) in _tasks:
        _tasks[name].cancel()  # this is the line raising the error
        del(_tasks[name])
        return True
    return False


def stop():
    for task_name in _tasks:
        print('destroy', task_name)
        destroy_task(task_name)


def run():
    create_task('dummy', dummy_task)
    _loop.run_forever()


try:
    run()
except KeyboardInterrupt:
    stop()

(it will spam a lot of 'dummy' 😄 )

I suspect the problem is that the KeyboardInterrupt is raised within the coroutine itself, therefore it is considered to be still executing, and thus can not be cancelled. Is this a desired behavior?

I have not read up on how exceptions like this mesh with tasks. I also don't know what is still executing by the time you hit your call to stop()... Might want to read the spec... How does CPython behave?

This code doesn't even run on CPython correctly because stop() iterates over a dictionary whose size gets changed:

Traceback (most recent call last):
  File "R:/test.py", line 43, in <module>
    stop()
  File "R:/test.py", line 30, in stop
    for task_name in _tasks:
RuntimeError: dictionary changed size during iteration

But,I now know where the "problem" is.
uasyncio.cur_task is being set in

micropython/extmod/uasyncio/core.py

Line 174 in 6c77d5c

cur_task = t

and when a task is cancelled, it checks if it is uasyncio.cur_task and if so, throws a runtime error. If you exit the loop by interrupt, that variable will still stay the same although no task is running anymore because uasyncio exited.

However, this is not a problem because what @antariandel is trying to do, doesn't even work on CPython. You simply can't cancel tasks after exiting the asyncio loop because the CancelledErrors won't be raised to the tasks anymore becauase asyncio is not running anymore.

See this minimal testcase and run it on Cpython, you'll see that the 2nd task will never catch any exception (or ever even see that it got cancelled):

try:
    import asyncio as uasyncio
except:
    import uasyncio


_loop = uasyncio.get_event_loop()
_tasks=[]


async def dummy_task():
    while True:
        try:
            print('dummy')
            await uasyncio.sleep(0)
        except uasyncio.CancelledError as e:
            print("Task cancelled",e)
            raise
        except Exception as e:
            print("Task exception",e)
            raise
        except BaseException as e:
            print("Task base exception",e)
            raise


def create_task(name, coro, *args):
    if str(name) not in _tasks:
        _tasks[name] = _loop.create_task(coro(*args))
        return True
    return False

def run():
    global _task
    loop=uasyncio.get_event_loop()
    _tasks.append(loop.create_task(dummy_task()))
    _tasks.append(loop.create_task(dummy_task()))
    try:
        loop.run_forever()
    except Exception as e:
        print("Run exception",e)
        raise
    except BaseException as e:
        print("Run base exception",e)
        raise

def stop():
    for t in _tasks:
        t.cancel()

try:
    run()
except KeyboardInterrupt:
    stop()

It would possible to live without this, if there was a way to clean up the loop and start fresh. In CPython this isn't a problem, because you can have more loops, but in uasyncio, the event loop is a singleton.

if there was a way to clean up the loop and start fresh

To clean up the event loop would need at least one of the following:

before run_until_complete returns make sure it clears the task and IO queues
add a Loop.close() method which clears the task and IO queues

@antariandel you could try the following to see if it works for you:

def loop_close():
    from uasyncio import core
    core._task_queue = core.TaskQueue()
    core._io_queue = core.IOQueue()

@dpgeorge

@dpgeorge thanks, this works for me!

@dpgeorge

@dpgeorge I see uasyncio.core.Loop.close() is currently just a pass.
Should your suggested close function above be added there?

Should your suggested close function above be added there?

Yes that's probably a good solution for now.

This sounds like a very worthwhile improvement.

The functionality to reset the event loop is added in PR #5890 (using new_event_loop() rather than close()).

Merge pull request micropython#5820 from CharString/waveshare_rp2040_…

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