with the following demo script I can get a IndexError: pop from empty list

```
import itertools
import asyncio
import concurrent.futures
import sys
import threading

threads = 200

def test_all_tasks_threading() -> None:
    async def foo() -> None:
        await asyncio.sleep(0)

    async def create_tasks() -> None:
        for i in range(1000):
            asyncio.create_task(foo())

        await asyncio.sleep(0)

    results = []
    with concurrent.futures.ThreadPoolExecutor(threads) as tpe:
        for f in concurrent.futures.as_completed(
            tpe.submit(asyncio.run, create_tasks()) for i in range(threads)
        ):
            results.append(f.result())
    assert results == [None] * threads


def main():
    for i in itertools.count():
        test_all_tasks_threading()
        print(f"worked {i}")
    return 0


if __name__ == "__main__":
    sys.exit(main())
```



```
worked 0
worked 1
worked 2
worked 3
worked 4
worked 5
worked 6
worked 7
worked 8
worked 9
worked 10
worked 11
worked 12
worked 13
worked 14
worked 15
worked 16
worked 17
worked 18
Traceback (most recent call last):
  File "/home/graingert/projects/asyncio-demo/demo.py", line 36, in <module>
    sys.exit(main())
  File "/home/graingert/projects/asyncio-demo/demo.py", line 30, in main
    test_all_tasks_threading()
  File "/home/graingert/projects/asyncio-demo/demo.py", line 24, in test_all_tasks_threading
    results.append(f.result())
  File "/usr/lib/python3.9/concurrent/futures/_base.py", line 438, in result
    return self.__get_result()
  File "/usr/lib/python3.9/concurrent/futures/_base.py", line 390, in __get_result
    raise self._exception
  File "/usr/lib/python3.9/concurrent/futures/thread.py", line 52, in run
    result = self.fn(*self.args, **self.kwargs)
  File "/usr/lib/python3.9/asyncio/runners.py", line 48, in run
    loop.run_until_complete(loop.shutdown_asyncgens())
  File "/usr/lib/python3.9/asyncio/base_events.py", line 621, in run_until_complete
    future = tasks.ensure_future(future, loop=self)
  File "/usr/lib/python3.9/asyncio/tasks.py", line 667, in ensure_future
    task = loop.create_task(coro_or_future)
  File "/usr/lib/python3.9/asyncio/base_events.py", line 433, in create_task
    task = tasks.Task(coro, loop=self, name=name)
  File "/usr/lib/python3.9/_weakrefset.py", line 84, in add
    self._commit_removals()
  File "/usr/lib/python3.9/_weakrefset.py", line 57, in _commit_removals
    discard(l.pop())
IndexError: pop from empty list
sys:1: RuntimeWarning: coroutine 'BaseEventLoop.shutdown_asyncgens' was never awaited
Task was destroyed but it is pending!
task: <Task pending name='Task-74178' coro=<BaseEventLoop.shutdown_asyncgens() running at /usr/lib/python3.9/asyncio/base_events.py:530>>
```

here's a live demo on github actions: https://github.com/graingert/asyncio-backport/runs/3380502247#step:5:90

still happens on 3.10 even though there's an `with _IterationGuard`

```
worked 0
Traceback (most recent call last):
  File "/home/graingert/projects/asyncio-demo/demo.py", line 36, in <module>
    sys.exit(main())
  File "/home/graingert/projects/asyncio-demo/demo.py", line 30, in main
    test_all_tasks_threading()
  File "/home/graingert/projects/asyncio-demo/demo.py", line 24, in test_all_tasks_threading
    results.append(f.result())
  File "/usr/lib/python3.10/concurrent/futures/_base.py", line 438, in result
    return self.__get_result()
  File "/usr/lib/python3.10/concurrent/futures/_base.py", line 390, in __get_result
    raise self._exception
  File "/usr/lib/python3.10/concurrent/futures/thread.py", line 52, in run
    result = self.fn(*self.args, **self.kwargs)
  File "/usr/lib/python3.10/asyncio/runners.py", line 47, in run
    _cancel_all_tasks(loop)
  File "/usr/lib/python3.10/asyncio/runners.py", line 56, in _cancel_all_tasks
    to_cancel = tasks.all_tasks(loop)
  File "/usr/lib/python3.10/asyncio/tasks.py", line 53, in all_tasks
    tasks = list(_all_tasks)
  File "/usr/lib/python3.10/_weakrefset.py", line 60, in __iter__
    with _IterationGuard(self):
  File "/usr/lib/python3.10/_weakrefset.py", line 33, in __exit__
    w._commit_removals()
  File "/usr/lib/python3.10/_weakrefset.py", line 57, in _commit_removals
    discard(l.pop())
IndexError: pop from empty list
```

interestingly 3.10 didn't show:

sys:1: RuntimeWarning: coroutine 'BaseEventLoop.shutdown_asyncgens' was never awaited

I can reproduce on 3.9.6

A little digging and it seems asyncio imports Task from _asyncio
and _asyncio's implementation (in asynciomodule.c) of Task has an __init__ which adds the task to the `all_tasks` weakref.WeakSet
which appears to be implemented in Python (in Lib/_weakrefset.py)

weakref.WeakSet is not thread-safe,  which means concurrent create_task's in different threads (even on separate event loops) is not safe.

> weakref.WeakSet is not thread-safe,  which means concurrent create_task's in different threads (even on separate event loops) is not safe.

actually it looks like WeakSet is *supposed* to be thread-safe


```
import patchy

patchy.patch(
    "weakref:WeakSet._commit_removals",
    """\
    @@ -1,5 +1,10 @@
     def _commit_removals(self):
    -    l = self._pending_removals
    +    pop = self._pending_removals.pop
         discard = self.data.discard
    -    while l:
    -        discard(l.pop())
    +    while True:
    +        try:
    +            item = pop()
    +        except IndexError:
    +            return
    +        else:
    +            discard(item)
    """
)

import itertools
import asyncio
import concurrent.futures
import sys
import threading

threads = 200

def test_all_tasks_threading() -> None:
    async def foo() -> None:
        await asyncio.sleep(0)

    async def create_tasks() -> None:
        for i in range(1000):
            asyncio.create_task(foo())

        await asyncio.sleep(0)

    results = []
    with concurrent.futures.ThreadPoolExecutor(threads) as tpe:
        for f in concurrent.futures.as_completed(
            tpe.submit(asyncio.run, create_tasks()) for i in range(threads)
        ):
            results.append(f.result())
    assert results == [None] * threads


def main():
    for i in itertools.count():
        test_all_tasks_threading()
        print(f"worked {i}")
    return 0


if __name__ == "__main__":
    sys.exit(main())
```

New changeset 206b21ed9f64fedff67bfea7cf73e423e3e32393 by Thomas Grainger in branch 'main':
bpo-44962: Fix a race in WeakKeyDict, WeakValueDict and WeakSet when two threads attempt to commit the last pending removal (GH-27921)
https://github.com/python/cpython/commit/206b21ed9f64fedff67bfea7cf73e423e3e32393

New changeset 8aa64cc45bff516a6db1f3a3c037cbcce9417fea by Miss Islington (bot) in branch '3.10':
bpo-44962: Fix a race in WeakKeyDict, WeakValueDict and WeakSet when two threads attempt to commit the last pending removal (GH-27921)
https://github.com/python/cpython/commit/8aa64cc45bff516a6db1f3a3c037cbcce9417fea

New changeset 166ad706066a2aad84d0ae5b1594c88904fbb939 by Miss Islington (bot) in branch '3.9':
bpo-44962: Fix a race in WeakKeyDict, WeakValueDict and WeakSet when two threads attempt to commit the last pending removal (GH-27921) (GH-28014)
https://github.com/python/cpython/commit/166ad706066a2aad84d0ae5b1594c88904fbb939

Thanks, Thomas! ✨ 🍰 ✨