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

先运行 issue 中的两个 asyncio.gather 用例，然后检查 Lib/asyncio/tasks.py 第 812 行附近，issue 在此处指出了依赖顺序的 future 选择。完成的标准是：取消 traceback 和描述的行为不再取决于 gather 参数的顺序；开始前请查看链接的 PR gh-133243。