Occasional incorrect chaining of CancelledError when calling 'cancel' on result of 'asyncio.gather'

import asyncio

async def f1():
    await asyncio.sleep(1)

async def f2():
    return 42

async def main():
    # case 1
    gfut = asyncio.gather(f1(), f2(), return_exceptions=True)
    # case 2
    #gfut = asyncio.gather(f2(), f1(), return_exceptions=True)
    await asyncio.sleep(0.1)
    gfut.cancel("my message")
    await gfut

asyncio.run(main())

# case 1

#Traceback (most recent call last):
#  File "/Users/vladima/Sources/gather.py", line 14, in main
#    print(await gfut)
#asyncio.exceptions.CancelledError
#
#During handling of the above exception, another exception occurred:
#
#Traceback (most recent call last):
#  File "/Users/vladima/Sources/gather.py", line 16, in <module>
#    asyncio.run(main())
#  File "/Library/Frameworks/Python.framework/Versions/3.10/lib/python3.10/asyncio/runners.py", line 44, in run
#    return loop.run_until_complete(main)
#  File "/Library/Frameworks/Python.framework/Versions/3.10/lib/python3.10/asyncio/base_events.py", line 646, in run_until_complete
#    return future.result()
#asyncio.exceptions.CancelledError

# case 2
#Traceback (most recent call last):
#  File "/Library/Frameworks/Python.framework/Versions/3.10/lib/python3.10/asyncio/tasks.py", line 605, in sleep
#    return await future
#asyncio.exceptions.CancelledError: my message

#During handling of the above exception, another exception occurred:

#Traceback (most recent call last):
#  File "/Users/vladima/Sources/gather.py", line 14, in main
#    print(await gfut)
#asyncio.exceptions.CancelledError
#
#During handling of the above exception, another exception occurred:
#
#Traceback (most recent call last):
#  File "/Users/vladima/Sources/gather.py", line 16, in <module>
#    asyncio.run(main())
#  File "/Library/Frameworks/Python.framework/Versions/3.10/lib/python3.10/asyncio/runners.py", line 44, in run
#    return loop.run_until_complete(main)
#  File "/Library/Frameworks/Python.framework/Versions/3.10/lib/python3.10/asyncio/base_events.py", line 646, in run_until_complete
#    return future.result()
#asyncio.exceptions.CancelledError

Currently traceback and description parts of CancelledError depend upon the order of arguments that were passed into asyncio.gather. The reason I believe is this: here fut would be bound to the last item in children which might or might not correspond to the cancelled task and instead it should probably be one of tasks that were actually cancelled (which is still not ideal since there might be many of them but I guess with gather it is as good as it can be).

Linked PRs

• gh-133243

まず issue にある 2 つの asyncio.gather ケースを実行し、次に Lib/asyncio/tasks.py の 812 行付近を調べてください。そこでは、issue が順序に依存する future の選択箇所を特定しています。完了条件は、キャンセルの traceback と description の動作が gather 引数の順序に依存しなくなることです。開始前に、リンクされている PR gh-133243 を確認してください。