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

Lib/test/test_concurrent_futures/test_thread_pool.py にある、引用されている再利用テストと、Lib/concurrent/futures/thread.py にあるワーカーのライフサイクルコードから始め、対応する ProcessPoolExecutor のパスも調べます。この競合状態に、より強いテスト同期が必要なのか、それとも executor の変更が必要なのかを判断し、その後 concurrent-futures テストスイートを実行して、GIL が有効な実行と GIL が無効な実行の両方でワーカーの再利用が確実に機能することを確認します。