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gh-148318: Fix multiprocessing exitcode=None after join() in multithreaded programs #150360

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gh-148318: Fix multiprocessing exitcode=None after join() in multithreaded programs #150360

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31 changes: 22 additions & 9 deletions Lib/multiprocessing/popen_fork.py
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Original file line number Diff line number Diff line change
@@ -1,6 +1,7 @@
import atexit
import os
import signal
import threading

from . import util

Expand All @@ -13,6 +14,13 @@
class Popen(object):
method = 'fork'

# Lock to protect os.waitpid() calls from concurrent threads.
# Without this lock, a thread can reap a child process that
# another thread is also trying to wait on, causing the second
# thread's waitpid() to raise OSError(ECHILD) and leaving
# the Process.exitcode stuck at None. See gh-148318.
_waitpid_lock = threading.Lock()

def __init__(self, process_obj):
util._flush_std_streams()
self.returncode = None
Expand All @@ -24,15 +32,20 @@ def duplicate_for_child(self, fd):

def poll(self, flag=os.WNOHANG):
if self.returncode is None:
try:
pid, sts = os.waitpid(self.pid, flag)
except OSError:
# Child process not yet created. See #1731717
# e.errno == errno.ECHILD == 10
return None
if pid == self.pid:
self.returncode = os.waitstatus_to_exitcode(sts)
return self.returncode
with self._waitpid_lock:
# Another thread may have set returncode while we
# waited for the lock.
if self.returncode is not None:
return self.returncode
try:
pid, sts = os.waitpid(self.pid, flag)
except OSError:
# Child process not yet created. See #1731717
# e.errno == errno.ECHILD == 10
return None
if pid == self.pid:
self.returncode = os.waitstatus_to_exitcode(sts)
return self.returncode

def wait(self, timeout=None):
if self.returncode is None:
Expand Down