asyncio event loop hangs if an eager task fails to start

[deadlovelll]

Bug report

Bug description:

Repro 1 - context not a context:

import asyncio

async def child(): 
    return 1

async def main():
    asyncio.Task(child(), loop=asyncio.get_running_loop(),
                 eager_start=True, context=2.14)

asyncio.run(main())

Actually the repro will hang, and the displayed exception is not readable and informative about what was wrong:

➜  cpython git:(main) ✗ ./python.exe repro.py
Exception in callback <_asyncio.TaskStepMethWrapper object at 0x104911ba0>()
handle: <Handle <_asyncio.TaskStepMethWrapper object at 0x104911ba0>()>
Traceback (most recent call last):
  File "/Users/timofeiivankov/cpython/Lib/asyncio/events.py", line 91, in _run
    self._context.run(self._callback, *self._args)
    ~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
RuntimeError: Invalid attempt to leave task <Task finished name='Task-1' coro=<main() done, defined at /Users/timofeiivankov/cpython/repro.py:6> exception=TypeError('an instance of Context was expected')> while task <Task pending name='Task-2' coro=<child() running at /Users/timofeiivankov/cpython/repro.py:3>> is entered.

Repro 2 - a valid Context that is already entered

import asyncio, contextvars

async def child():
    return 1

async def main():
    loop = asyncio.get_running_loop()
    ctx = contextvars.copy_context()
    ctx.run(lambda: asyncio.Task(child(), loop=loop, eager_start=True, context=ctx))

asyncio.run(main())

Also hangs - type check alone doesn't cover this path

Expected behaviour: the constructor raises and the loop stays usable

TypeError: a contextvars.Context was expected, got float

Proposed fix: add type check for context, and restore the current task in the _PyContext_Enter failure branch of task_eager_start.

I have fix for this already

CPython versions tested on:

CPython main branch

Operating systems tested on:

macOS

Linked PRs

• gh-157301: Fix asyncio event loop hanging on a failed eager task start #157302