Worker management race conditions in `ThreadPoolExecutor` and `ProcessPoolExecutor`

We have tests that ThreadPoolExecutor and ProcessPoolExecutor reuse workers. The test wait for the previous task to complete before submitting the next task.

https://github.com/python/cpython/blob/29829b58a8328a7c2ccacaa74c1d7d120a5e5ca5/Lib/test/test_concurrent_futures/test_thread_pool.py#L44-L50

The problem is that the tasks are potentially completed before the thread or process is marked as idle. Calling Future.result() doesn't guarantee that there is an idle worker. This happens much more frequently with the GIL disabled, but it can happen with the GIL enabled too if you have an unlucky GIL switch.

https://github.com/python/cpython/blob/29829b58a8328a7c2ccacaa74c1d7d120a5e5ca5/Lib/concurrent/futures/thread.py#L81-L95

For example, you can reliably trigger the problem with the GIL enabled by adding a short time.sleep(0.01) immediately after the work_item.run() call in:
https://github.com/python/cpython/blob/29829b58a8328a7c2ccacaa74c1d7d120a5e5ca5/Lib/concurrent/futures/thread.py#L92

It's not clear to me if this is just a problem with the unit tests or an actual bug with the executor implementation.

Linked PRs

• gh-117355

Comienza con las pruebas de reutilización citadas en Lib/test/test_concurrent_futures/test_thread_pool.py y el código del ciclo de vida de los workers en Lib/concurrent/futures/thread.py; inspecciona también las rutas correspondientes de ProcessPoolExecutor. Determina si la condición de carrera requiere una sincronización más estricta en las pruebas o un cambio en el executor, luego ejecuta la suite de pruebas de concurrent-futures y asegúrate de que la reutilización de workers sea fiable tanto en la ejecución con el GIL habilitado como en la ejecución con el GIL deshabilitado.