test_forkserver_sigkill: join() can return before the child sets the event
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評估
研究方向
閱讀Lib/test/_test_multiprocessing.py，尤其是check_forkserver_death()和test_forkserver_sigkill；問題在於 forkserver 被終止後，子行程設定事件之前join()就可能返回。更新事件檢查，讓它等待並設定逾時時間，然後執行針對 forkserver 的測試。完成標準是：測試給子行程留出設定事件的時間，並繼續檢查預期的結束碼。
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描述
Bug report
Bug description:
test_forkserver_sigkill failed once during a full ./python.exe -m test -j0 run on my machine (macOS 27.0.1, Apple M3, pydebug build of main at 1705e512b30). The load average was around 65 at the time. It passes when run on its own.
test test.test_multiprocessing_forkserver.test_processes failed -- Traceback (most recent call last):
File "Lib/test/_test_multiprocessing.py", line 1007, in test_forkserver_sigkill
self.check_forkserver_death(signal.SIGKILL)
File "Lib/test/_test_multiprocessing.py", line 997, in check_forkserver_death
self.assertTrue(evt.is_set())
AssertionError: False is not true
(paths shortened)
This is the same failure as gh-78221 (bpo-34040). It was worked around in 2018 by shortening the child's delay relative to the parent's sleep (07888e1cce), but the race is still there.
check_forkserver_death() kills the forkserver right after starting the first process. After a SIGKILL, the forkserver can no longer report that child's exit status: proc.join() gets EOF on the status pipe, and Popen.poll() in Lib/multiprocessing/popen_forkserver.py sets returncode to 255 immediately. The child may still be running at that point. The test then checks evt.is_set(), which only passes if the child has set the event by then (within the 1 second sleep plus the time proc2 takes). The next line already accepts exit code 255 for the case where the forkserver was killed.
SIGINT is not affected: the forkserver ignores it, stays alive and reports the real exit code.
To reproduce it reliably, stop the first child with SIGSTOP for a few seconds after killing the forkserver:
Reproducer
import multiprocessing as mp, os, signal, threading, time
from multiprocessing.forkserver import _forkserver
def sleep_and_set(evt, delay=0.0):
time.sleep(delay)
evt.set()
if __name__ == '__main__':
mp.set_start_method('forkserver')
_forkserver.ensure_running()
evt = mp.Event()
proc = mp.Process(target=sleep_and_set, args=(evt, 0.5))
proc.start()
os.kill(_forkserver._forkserver_pid, signal.SIGKILL)
# Simulate the child not getting CPU time for 3 seconds.
os.kill(proc.pid, signal.SIGSTOP)
threading.Timer(3.0, os.kill, (proc.pid, signal.SIGCONT)).start()
time.sleep(1.0)
proc.join()
print(proc.exitcode, evt.is_set()) # 255 False
print(evt.wait(60)) # True: the child did run, later
Waiting for the event with a timeout, instead of checking it right after join(), fixes it.
The same code is in 3.13, 3.14 and 3.15.
CPython versions tested on:
CPython main branch
Operating systems tested on:
macOS, Linux
Linked PRs
- gh-160296
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