multiprocessing async .get() hangs if the pool is terminated

Bug report

Bug description:

MWE

Steps to reproduce:

import multiprocessing
from time import sleep

with multiprocessing.Pool() as p:
    result = p.apply_async(sleep, (1,))
result.get()

Expected behavior:

Either:

• result is returned. (Pool().__exit__ waits for every async task to finish); or
• some kind of exception is raised when .get() is called outside with Pool()

Comments

This is the same as #79659. That was marked as completed in 2021. However it seems the issue is still present.

CPython versions tested on:

3.12, 3.13, 3.14

Operating systems tested on:

Linux

まず、提供された MWE を実行し、pool の終了後における Pool.exit と非同期結果の .get() 呼び出しの相互作用を追跡します。issue #79659 と動作を比較し、ハングではなく完了または例外になるという、文書化された期待結果をカバレッジに追加します。