asyncio.Task doesn't propagate CancelledError() exception correctly.

@gvanrossum

BPO	45390
Nosy	@gvanrossum, @asvetlov, @cjerdonek, @serhiy-storchaka, @1st1, @graingert, @bensimner
PRs	bpo-45390: Propagate CancelledError's message from cancelled task to its awaiter #31383
Files	task_bug.py task_bug.py task_bug.py

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = <Date 2022-02-22.09:46:40.916>
created_at = <Date 2021-10-06.11:30:42.256>
labels = ['3.11', 'type-bug', 'expert-asyncio']
title = "asyncio.Task doesn't propagate CancelledError() exception correctly."
updated_at = <Date 2022-02-23.18:04:42.655>
user = 'https://bugs.python.org/pagliariccim'

bugs.python.org fields:

activity = <Date 2022-02-23.18:04:42.655>
actor = 'gvanrossum'
assignee = 'none'
closed = True
closed_date = <Date 2022-02-22.09:46:40.916>
closer = 'asvetlov'
components = ['asyncio']
creation = <Date 2021-10-06.11:30:42.256>
creator = 'pagliaricci.m'
dependencies = []
files = ['50328', '50333', '50334']
hgrepos = []
issue_num = 45390
keywords = ['patch']
message_count = 20.0
messages = ['403294', '403297', '403350', '403360', '403521', '403530', '403531', '403532', '403533', '403570', '413392', '413395', '413397', '413679', '413779', '413794', '413808', '413830', '413836', '413838']
nosy_count = 8.0
nosy_names = ['gvanrossum', 'asvetlov', 'chris.jerdonek', 'serhiy.storchaka', 'yselivanov', 'graingert', 'bjs', 'pagliaricci.m']
pr_nums = ['31383']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue45390'
versions = ['Python 3.11']

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:

if you see my example, inside the job() coroutine, I get correctly
cancelled with an `asyncio.CancelledError` exception containing my message.
Now: if I re-raise the asyncio.CancelledError as-is, I lose the message,
if I call the `asyncio.Task.exception()` function.
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()`.
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\>

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:

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\>

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 4140bcb by Andrew Svetlov in branch 'main':
bpo-45390: Propagate CancelledError's message from cancelled task to its awaiter (GH-31383)
4140bcb

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:
#31383 (comment) )

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.

Add workaround for <python/cpython#89553>.

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asyncio.Task doesn't propagate CancelledError() exception correctly. #89553

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asyncio.Task doesn't propagate CancelledError() exception correctly. #89553

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pagliariccim commented on Oct 6, 2021

bensimner commented on Oct 6, 2021

cjerdonek commented on Oct 7, 2021

graingert commented on Oct 7, 2021

pagliariccim commented on Oct 9, 2021

cjerdonek commented on Oct 9, 2021

pagliariccim commented on Oct 9, 2021

cjerdonek commented on Oct 9, 2021

pagliariccim commented on Oct 9, 2021

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asvetlov commented on Feb 17, 2022

pagliariccim commented on Feb 17, 2022

cjerdonek commented on Feb 17, 2022

asvetlov commented on Feb 21, 2022

serhiy-storchaka commented on Feb 23, 2022

cjerdonek commented on Feb 23, 2022

asvetlov commented on Feb 23, 2022

serhiy-storchaka commented on Feb 23, 2022

graingert commented on Feb 23, 2022

gvanrossum commented on Feb 23, 2022

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