Multiprocessing exitcode=None after join

Bug report

Bug description:

I sometimes observe exitcode=None after joining a process.

import multiprocessing as mp
import sys
import threading

sys.setswitchinterval(1e-6)
ctx = mp.get_context("fork")
failures = []

def lifecycle(n_iters=100, n_procs=4):
    for _ in range(n_iters):
        ps = [ctx.Process(target=int, daemon=True) for _ in range(n_procs)]
        for p in ps: 
            p.start()
        for p in ps:
            p.join()
            if p.exitcode is None: 
                failures.append(p.pid)

ts = [threading.Thread(target=lifecycle) for _ in range(2)]
for t in ts: 
    t.start()
for t in ts: 
    t.join()
print(f"{len(failures)}/{2*100*4} had exitcode=None after join()")

CPython versions tested on:

3.13

Operating systems tested on:

Linux

Linked PRs

• gh-150360

首先執行提供的 Linux Python 3.13 reproducer，重點關注使用 fork context 的 multiprocessing 以及並行的生命週期呼叫。檢查連結的 PR gh-150360，並驗證 join 一個程序始終會得到非 None 的 exitcode，同時不會導致 threaded case 出現回歸。