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bpo-36533: Reinit logging.Handler locks on fork(). (GH-12704)

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Instead of attempting to acquire and release them all across fork
which was leading to deadlocks in some applications that had chained
their own handlers while holding multiple locks.
This commit is contained in:
Gregory P. Smith 2019-05-07 12:18:20 -04:00 • committed by GitHub
parent e85ef7a7ea
commit 64aa6d2000
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3 changed files with 57 additions and 39 deletions
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31
Lib/test/test_logging.py
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@ -668,10 +668,28 @@ class HandlerTest(BaseTest):
# register_at_fork mechanism is also present and used.
@unittest.skipIf(not hasattr(os, 'fork'), 'Test requires os.fork().')
def test_post_fork_child_no_deadlock(self):
"""Ensure forked child logging locks are not held; bpo-6721."""
refed_h = logging.Handler()
"""Ensure child logging locks are not held; bpo-6721 & bpo-36533."""
class _OurHandler(logging.Handler):
def __init__(self):
super().__init__()
self.sub_handler = logging.StreamHandler(
stream=open('/dev/null', 'wt'))
def emit(self, record):
self.sub_handler.acquire()
try:
self.sub_handler.emit(record)
finally:
self.sub_handler.release()
self.assertEqual(len(logging._handlers), 0)
refed_h = _OurHandler()
refed_h.name = 'because we need at least one for this test'
self.assertGreater(len(logging._handlers), 0)
self.assertGreater(len(logging._at_fork_reinit_lock_weakset), 1)
test_logger = logging.getLogger('test_post_fork_child_no_deadlock')
test_logger.addHandler(refed_h)
test_logger.setLevel(logging.DEBUG)
locks_held__ready_to_fork = threading.Event()
fork_happened__release_locks_and_end_thread = threading.Event()
@ -709,19 +727,24 @@ class HandlerTest(BaseTest):
locks_held__ready_to_fork.wait()
pid = os.fork()
if pid == 0: # Child.
logging.error(r'Child process did not deadlock. \o/')
os._exit(0)
try:
test_logger.info(r'Child process did not deadlock. \o/')
finally:
os._exit(0)
else: # Parent.
test_logger.info(r'Parent process returned from fork. \o/')
fork_happened__release_locks_and_end_thread.set()
lock_holder_thread.join()
start_time = time.monotonic()
while True:
test_logger.debug('Waiting for child process.')
waited_pid, status = os.waitpid(pid, os.WNOHANG)
if waited_pid == pid:
break # child process exited.
if time.monotonic() - start_time > 7:
break # so long? implies child deadlock.
time.sleep(0.05)
test_logger.debug('Done waiting.')
if waited_pid != pid:
os.kill(pid, signal.SIGKILL)
waited_pid, status = os.waitpid(pid, 0)