I've spotted a little bug in how asyncio.CancelledError() exception is propagated inside an asyncio.Task.

Since python 3.9 the asyncio.Task.cancel() method has a new 'msg' parameter, that will create an asyncio.CancelledError(msg) exception incorporating that message.

The exception is successfully propagated to the coroutine the asyncio.Task is running, so the coroutine successfully gets raised an asyncio.CancelledError(msg) with the specified message in asyncio.Task.cancel(msg) method.

But, once the asyncio.Task is cancelled, is impossible to retrieve that original asyncio.CancelledError(msg) exception with the message, because it seems that *a new* asyncio.CancelledError() [without the message] is raised when asyncio.Task.result() or asyncio.Task.exception() methods are called.

I have the feeling that this is just wrong, and that the original message specified in asyncio.Task.cancel(msg) should be propagated even also asyncio.Task.result() is called.

I'm including a little snippet of code that clearly shows this bug.

I'm using python 3.9.6, in particular:
Python 3.9.6 (default, Aug 21 2021, 09:02:49) 
[GCC 10.2.1 20210110] on linux

This seems to be present in both the Python implementation as well as the accelerated C _asyncio module.

It looks like that when a Task awaits a cancelled future,
the task itself is cancelled but the cancellation message is not propagated to the task.
https://github.com/python/cpython/blob/main/Lib/asyncio/tasks.py#L242

> But, once the asyncio.Task is cancelled, is impossible to retrieve that original asyncio.CancelledError(msg) exception with the message, because it seems that *a new* asyncio.CancelledError() [without the message] is raised when asyncio.Task.result() or asyncio.Task.exception() methods are called.

You say it's "impossible", but isn't the message accessible via the exception chain (and visible in the traceback)? One benefit of not duplicating the message on the internal call to cancel() is that it makes it easier to pinpoint which CancelledError object is associated with the user's call to cancel(), and which is associated with the call done by asyncio internals, which is a different cancellation. Another benefit is that it prevents info from being duplicated in the traceback.

afaik this is intentional https://bugs.python.org/issue31033

OK, I see your point.
But I still can't understand one thing and I think it's very confusing:

1) if you see my example, inside the job() coroutine, I get correctly
cancelled with an `asyncio.CancelledError` exception containing my message.
2) Now: if I re-raise the asyncio.CancelledError as-is, I lose the message,
if I call the `asyncio.Task.exception()` function.
3) If I raise a *new* asyncio.CancelledError with a new message, inside the
job() coroutine's `except asyncio.CancelledError:` block, I still lose the
message if I call `asyncio.Task.exception()`.
4) But if I raise another exception, say `raise ValueError("TEST")`, always
from the `except asyncio.CancelledError:` block of the job() coroutine, I
*get* the message!
I get `ValueError("TEST")` by calling `asyncio.Task.exception()`, while I
don't with the `asyncio.CancelledError()` one.

Is this really wanted? Sorry, but I still find this a lot confusing.
Shouldn't it be better to return from the `asyncio.Task.exception()` the
old one (containing the message) ?
Or, otherwise, create a new instance of the exception for *ALL* the
exception classes?

Thank you for your time,
My Best Regards,

M.

On Thu, Oct 7, 2021 at 10:25 AM Thomas Grainger <report@bugs.python.org>
wrote:

>
> Thomas Grainger <tagrain@gmail.com> added the comment:
>
> afaik this is intentional https://bugs.python.org/issue31033
>
> ----------
> nosy: +graingert
>
> _______________________________________
> Python tracker <report@bugs.python.org>
> <https://bugs.python.org/issue45390>
> _______________________________________
>

> 2) Now: if I re-raise the asyncio.CancelledError as-is, I lose the message,
if I call the `asyncio.Task.exception()` function.

Re-raise asyncio.CancelledError where? (And what do you mean by "re-raise"?) Call asyncio.Task.exception() where? This isn't part of your example, so it's not clear what you mean exactly.

Chris,
I'm attaching to this e-mail the code I'm referring to.
As you can see, in line 10, I re-raise the asyncio.CancelledError exception
with a message "TEST".
That message is lost, due to the reasons we've talked about.

My point is that, if we substitute that line 10, with the commented line
11, and we comment the line 10, so we raise a ValueError("TEST") exception,
as you can see, the message "TEST" is NOT LOST.
I just find this counter-intuitive, and error-prone.

AT LEAST should be very well specified in the docs.

Regards,
M.

On Sat, Oct 9, 2021 at 2:51 PM Chris Jerdonek <report@bugs.python.org>
wrote:

>
> Chris Jerdonek <chris.jerdonek@gmail.com> added the comment:
>
> > 2) Now: if I re-raise the asyncio.CancelledError as-is, I lose the
> message,
> if I call the `asyncio.Task.exception()` function.
>
> Re-raise asyncio.CancelledError where? (And what do you mean by
> "re-raise"?) Call asyncio.Task.exception() where? This isn't part of your
> example, so it's not clear what you mean exactly.
>
> ----------
>
> _______________________________________
> Python tracker <report@bugs.python.org>
> <https://bugs.python.org/issue45390>
> _______________________________________
>

I still don't see you calling asyncio.Task.exception() in your new attachment...

Chris,
ok, I have modified the snippet of code to better show what I mean.
Still here, the message of the CancelledError exception is lost, but if I
comment line 10, and uncomment line 11, so I throw a ValueError("TEST"),
that "TEST" string will be printed, so the message is not lost.
Again, I just find this behavior very counter-intuitive, and should be VERY
WELL documented in the docs.

Thanks,
M.

On Sat, Oct 9, 2021 at 3:06 PM Chris Jerdonek <report@bugs.python.org>
wrote:

>
> Chris Jerdonek <chris.jerdonek@gmail.com> added the comment:
>
> I still don't see you calling asyncio.Task.exception() in your new
> attachment...
>
> ----------
>
> _______________________________________
> Python tracker <report@bugs.python.org>
> <https://bugs.python.org/issue45390>
> _______________________________________
>

Here's a simplification of Marco's snippet to focus the discussion.

import asyncio

async def job():
    # raise RuntimeError('error!')
    await asyncio.sleep(5)

async def main():
    task = asyncio.create_task(job())
    await asyncio.sleep(1)
    task.cancel('cancel job')
    await task

if __name__=="__main__":
    asyncio.run(main())

----
Running this pre-Python 3.9 gives something like this--

Traceback (most recent call last):
  File "test.py", line 15, in <module>
    asyncio.run(main())
  File "/.../python3.7/asyncio/runners.py", line 43, in run
    return loop.run_until_complete(main)
  File "/.../python3.7/asyncio/base_events.py", line 579, in run_until_complete
    return future.result()
concurrent.futures._base.CancelledError

----
Running this with Python 3.9+ gives something like the following. The difference is that the traceback now starts at the sleep() call:

Traceback (most recent call last):
  File "/.../test.py", line 6, in job
    await asyncio.sleep(5)
  File "/.../python3.9/asyncio/tasks.py", line 654, in sleep
    return await future
asyncio.exceptions.CancelledError: cancel job

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/.../test.py", line 12, in main
    await task
asyncio.exceptions.CancelledError

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/.../test.py", line 15, in <module>
    asyncio.run(main())
  File "/.../python3.9/asyncio/runners.py", line 44, in run
    return loop.run_until_complete(main)
  File "/.../python3.9/asyncio/base_events.py", line 642, in run_until_complete
    return future.result()
asyncio.exceptions.CancelledError

----
Uncommenting the RuntimeError turns it into this--

Traceback (most recent call last):
  File "/.../test.py", line 15, in <module>
    asyncio.run(main())
  File "/.../python3.9/asyncio/runners.py", line 44, in run
    return loop.run_until_complete(main)
  File "/.../python3.9/asyncio/base_events.py", line 642, in run_until_complete
    return future.result()
  File "/.../test.py", line 12, in main
    await task
  File "/.../test.py", line 5, in job
    raise RuntimeError('error!')
RuntimeError: error!

----
I agree it would be a lot nicer if the original CancelledError('cancel job') could bubble up just like the RuntimeError does, instead of creating a new CancelledError at each await and chaining it to the previous CancelledError. asyncio's creation of a new CancelledError at each stage predates the PR that added the chaining, so this could be viewed as an evolution of the change that added the chaining.

I haven't checked to be sure, but the difference in behavior between CancelledError and other exceptions might be explained by the following lines:
https://github.com/python/cpython/blob/3d1ca867ed0e3ae343166806f8ddd9739e568ab4/Lib/asyncio/tasks.py#L242-L250
You can see that for exceptions other than CancelledError, the exception is propagated by calling super().set_exception(exc), whereas with CancelledError, it is propagated by calling super().cancel() again.

Maybe this would even be an easy change to make. Instead of asyncio creating a new CancelledError and chaining it to the previous, asyncio can just raise the existing one. For the pure Python implementation at least, it may be as simple as making a change here, inside _make_cancelled_error():
https://github.com/python/cpython/blob/3d1ca867ed0e3ae343166806f8ddd9739e568ab4/Lib/asyncio/futures.py#L135-L142

I have a pull request for the issue.
It doesn't use `Future.set_exception()` but creates a new CancelledError() with propagated message.
The result is the same, except raised exceptions are not comparable by `is` check.
As a benefit, `_cancelled_exc` works with the patch, exc.__context__ is correctly set.

The patch is not backported because it changes existing behavior a little. I'd like to avoid a situation when third-party code works with Python 3.11+, 3.10.3+, and 3.9.11+ only.

Andrew,
many thanks for your time, solving this issue.
I think your solution is the best to fix this little problem and I agree
with you on backporting.

My Best Regards,
and thanks again.

Marco

On Thu, Feb 17, 2022 at 10:29 AM Andrew Svetlov <report@bugs.python.org>
wrote:

>
> Andrew Svetlov <andrew.svetlov@gmail.com> added the comment:
>
> I have a pull request for the issue.
> It doesn't use `Future.set_exception()` but creates a new CancelledError()
> with propagated message.
> The result is the same, except raised exceptions are not comparable by
> `is` check.
> As a benefit, `_cancelled_exc` works with the patch, exc.__context__ is
> correctly set.
>
> The patch is not backported because it changes existing behavior a little.
> I'd like to avoid a situation when third-party code works with Python
> 3.11+, 3.10.3+, and 3.9.11+ only.
>
> ----------
>
> _______________________________________
> Python tracker <report@bugs.python.org>
> <https://bugs.python.org/issue45390>
> _______________________________________
>

Andrew, the approach I described would I feel be much better. It would result in more concise, less verbose tracebacks, as opposed to more verbose -- not just because the message won't be repeated, but also because it eliminates the unneeded creation of intermediate exceptions. It would also cause is checks to work, which is what we want since behaviorally it should be the same exception.

New changeset 4140bcb1cd76dec5cf8d398f4d0e86c438c987d0 by Andrew Svetlov in branch 'main':
bpo-45390: Propagate CancelledError's message from cancelled task to its awaiter (GH-31383)
https://github.com/python/cpython/commit/4140bcb1cd76dec5cf8d398f4d0e86c438c987d0

Seems a CancelledError message can be lost also in Condition.wait().

For future reference, with Andrew's change merged above, the traceback for the example snippet in my message above:
https://bugs.python.org/issue45390#msg403570
is now the following. Observe that (1) the call to sleep() continues to be present, but (2) without introducing two new intermediate CancelledErrors, which increase the verbosity of the traceback:

Traceback (most recent call last):
  File "/home/andrew/projects/cpython/exc_traceback.py", line 14, in <module>
    asyncio.run(main())
    ^^^^^^^^^^^^^^^^^^^
  File "/home/andrew/projects/cpython/Lib/asyncio/runners.py", line 44, in run
    return loop.run_until_complete(main)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/andrew/projects/cpython/Lib/asyncio/base_events.py", line 640, in run_until_complete
    return future.result()
           ^^^^^^^^^^^^^^^
  File "/home/andrew/projects/cpython/exc_traceback.py", line 11, in main
    await task
    ^^^^^^^^^^
  File "/home/andrew/projects/cpython/exc_traceback.py", line 5, in job
    await asyncio.sleep(5)
    ^^^^^^^^^^^^^^^^^^^^^^
  File "/home/andrew/projects/cpython/Lib/asyncio/tasks.py", line 619, in sleep
    return await future
           ^^^^^^^^^^^^
asyncio.exceptions.CancelledError: cancel job

(This is copied from Andrew's comment in the PR here:
https://github.com/python/cpython/pull/31383#issuecomment-1046822899 )

Serhiy, can you provide a sample snippet for your case with output, like I did in my message linked above?

Serhiy is right, Condition.wait() has the following code:

        finally:
            # Must reacquire lock even if wait is cancelled
            cancelled = False
            while True:
                try:
                    await self.acquire()
                    break
                except exceptions.CancelledError:
                    cancelled = True

            if cancelled:
                raise exceptions.CancelledError

It swallows CancelledError exceptions from waiters and re-raises CancelledError without the cancellation message.

Also Future.result() and Future.exception() can raise a CancelledError. So a CancelledError raised in a task may not contain a message passed to Task.cancel().

import asyncio
import random

async def main():
    fut = asyncio.Future()
    fut.cancel()
    async def job():
        if random.random() < 0.5:
            await asyncio.sleep(2)
        fut.result()
        await asyncio.sleep(5)
    task = asyncio.create_task(job())
    await asyncio.sleep(1)
    task.cancel("cancel task")
    await task

asyncio.run(main())

You need to catch a CancelledError raised in a coroutine and re-raise a new CancelledError with the specified cancel message if the task was cancelled.

there could be multiple messages here

perhaps it could be:

```
        finally:
            # Must reacquire lock even if wait is cancelled
            cancelled = []
            while True:
                try:
                    await self.acquire()
                    break
                except exceptions.CancelledError as e:
                    cancelled.append(e)

            if len(cancelled) > 1:
                raise ExceptionGroup("Cancelled", cancelled)
            if cancelled:
                raise cancelled[0]
```

We should really stop appending to a closed issue.

Anyway, raising ExceptionGroup is backwards incompatible, since "except CancelledError" wouldn't cancel the group.