asyncio.staggered.staggered_race can log NameError: cannot access free variable 'exceptions' where it is not associated with a value in enclosing scope

[graingert]

Bug report

Bug description:

https://github.com/Chia-Network/chia-blockchain/actions/runs/12586550910/job/35084132471#step:16:1813

______ test_long_reorg_nodes[ConsensusMode.HARD_FORK_2_0-2-500-100-True] _______
[gw1] darwin -- Python 3.12.8 /Users/runner/work/chia-blockchain/chia-blockchain/.venv/bin/python
/Library/Frameworks/Python.framework/Versions/3.12/lib/python3.12/asyncio/staggered.py:104: in run_one_coro
    result = await coro_fn()
.venv/lib/python3.12/site-packages/aiohappyeyeballs/impl.py:166: in _connect_sock
    await loop.sock_connect(sock, address)
/Library/Frameworks/Python.framework/Versions/3.12/lib/python3.12/asyncio/selector_events.py:651: in sock_connect
    return await fut
E   asyncio.exceptions.CancelledError

During handling of the above exception, another exception occurred:
.venv/lib/python3.12/site-packages/anyio/pytest_plugin.py:160: in pytest_pyfunc_call
    runner.run_test(pyfuncitem.obj, testargs)
.venv/lib/python3.12/site-packages/anyio/_backends/_asyncio.py:2316: in run_test
    self._raise_async_exceptions()
.venv/lib/python3.12/site-packages/anyio/_backends/_asyncio.py:2220: in _raise_async_exceptions
    raise exceptions[0]
/Library/Frameworks/Python.framework/Versions/3.12/lib/python3.12/asyncio/staggered.py:108: in run_one_coro
    exceptions[this_index] = e
E   NameError: cannot access free variable 'exceptions' where it is not associated with a value in enclosing scope

This occurs when created run_one_coro tasks outlive the staggered_race task

The fix is to repeatedly wait for a future that fires when all tasks are completed - and propagate any outside cancellation to the child tasks.

CPython versions tested on:

3.12

Operating systems tested on:

Linux

Linked PRs

• gh-128479: fix asyncio staggered race leaking tasks, and logging unhandled exception.append exception #128475
• [3.13] gh-128479: fix asyncio staggered race leaking tasks, and logging unhandled exception.append exception (GH-128475) #129227
• [3.12] gh-128479: fix asyncio staggered race leaking tasks, and logging unhandled exception.append exception (GH-128475) #129228