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

先執行提供的 MWE，並追蹤 pool 終止後 Pool.exit 與非同步結果的 .get() 呼叫之間的互動。將行為與 issue #79659 進行比較，並為文件中所述的預期結果新增覆蓋：完成或發生例外，而不是卡住。