gh-129195: use future_add_to_awaited_by/future_discard_from_awaited_by in asyncio.staggered.staggered_race

[graingert]

• Issue: asyncio staggered race missing calls to asyncio.future_add_to_awaited_by() and asyncio.future_discard_from_awaited_by() #129195

[ZeroIntensity]

What happens if we're not running in a task? Should we assert that this is non-None?

[graingert]

one possible outcome is that there's no task or running loop:

import asyncio.staggered

async def main():
    async def asyncfn():
        pass

    await asyncio.staggered.staggered_race([asyncfn, asyncfn], delay=None)

main().__await__().send(None)

we get a nice traceback, saying that:

./python demo.py
Traceback (most recent call last):
  File "/home/graingert/projects/cpython/demo.py", line 9, in <module>
    main().__await__().send(None)
    ~~~~~~~~~~~~~~~~~~~~~~~^^^^^^
  File "/home/graingert/projects/cpython/demo.py", line 7, in main
    await asyncio.staggered.staggered_race([asyncfn, asyncfn], delay=None)
  File "/home/graingert/projects/cpython/Lib/asyncio/staggered.py", line 66, in staggered_race
    loop = loop or events.get_running_loop()
                   ~~~~~~~~~~~~~~~~~~~~~~~^^
RuntimeError: no running event loop

[graingert]

if you manually pass a loop, and step the coroutine outside of a task:

import asyncio.staggered

async def main(loop):
    async def asyncfn():
        print("hello")

    await asyncio.staggered.staggered_race([asyncfn, asyncfn], delay=None, loop=loop)
    return "world"

loop = asyncio.EventLoop()
coro = main(loop).__await__()
f = coro.send(None)
loop.run_until_complete(f)
try:
    coro.send(None)
except StopIteration as e:
    print(e.value)

by some miracle it all works, this is because future_add_to_awaited_by and future_discard_from_awaited_by is noop if any arg is not a future (eg is None) and we don't have any further use of the parent task

./python demo.py
hello
world

so we should probably leave it like that.

[graingert]

I don't think it's worth testing this usecase though

[ZeroIntensity]

Above this--for cancelling all the coroutines, future_discard_from_awaited_by should get called, right?

[graingert]

Yeah add_done_callback callbacks are called on cancellation, so it will get called

[ZeroIntensity]

LGTM as well.