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gh-87135: threading.Lock: Raise rather than hang on Python finalization #135991

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Jul 1, 2025
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5 changes: 5 additions & 0 deletions Include/internal/pycore_lock.h
Original file line number Diff line number Diff line change
Expand Up @@ -51,6 +51,11 @@ typedef enum _PyLockFlags {

// Fail if interrupted by a signal while waiting on the lock.
_PY_FAIL_IF_INTERRUPTED = 4,

// Locking & unlocking this lock requires attached thread state.
// If locking returns PY_LOCK_FAILURE, a Python exception *may* be raised.
// Implies _PY_LOCK_HANDLE_SIGNALS and _PY_LOCK_DETACH.
_PY_LOCK_PYTHONLOCK = 8 | 2 | 1,
} _PyLockFlags;

// Lock a mutex with an optional timeout and additional options. See
Expand Down
45 changes: 45 additions & 0 deletions Lib/test/test_threading.py
Original file line number Diff line number Diff line change
Expand Up @@ -1247,6 +1247,51 @@ def __del__(self):
self.assertEqual(err, b"")
self.assertIn(b"all clear", out)

def test_acquire_daemon_thread_lock_in_finalization(self):
# gh-123940: Py_Finalize() prevents other threads from running Python
# code (and so, releasing locks), so acquiring a locked lock can not
# succeed.
# We raise an exception rather than hang.
for timeout in (None, 10):
with self.subTest(timeout=timeout):
code = textwrap.dedent(f"""
import threading
import time

thread_started_event = threading.Event()

lock = threading.Lock()
def loop():
with lock:
thread_started_event.set()
while True:
time.sleep(1)

class Cycle:
def __init__(self):
self.self_ref = self
self.thr = threading.Thread(
target=loop, daemon=True)
self.thr.start()
thread_started_event.wait()

def __del__(self):
assert self.thr.is_alive()
try:
lock.acquire()
except PythonFinalizationError:
assert self.thr.is_alive()
print('got the correct exception!')

# Cycle holds a reference to itself, which ensures it is
# cleaned up during the GC that runs after daemon threads
# have been forced to exit during finalization.
Cycle()
""")
rc, out, err = assert_python_ok("-c", code)
self.assertEqual(err, b"")
self.assertIn(b"got the correct exception", out)

def test_start_new_thread_failed(self):
# gh-109746: if Python fails to start newly created thread
# due to failure of underlying PyThread_start_new_thread() call,
Expand Down
12 changes: 10 additions & 2 deletions Modules/_threadmodule.c
Original file line number Diff line number Diff line change
Expand Up @@ -835,8 +835,12 @@ lock_PyThread_acquire_lock(PyObject *op, PyObject *args, PyObject *kwds)
}

PyLockStatus r = _PyMutex_LockTimed(&self->lock, timeout,
_PY_LOCK_HANDLE_SIGNALS | _PY_LOCK_DETACH);
_PY_LOCK_PYTHONLOCK);
if (r == PY_LOCK_INTR) {
assert(PyErr_Occurred());
return NULL;
}
if (r == PY_LOCK_FAILURE && PyErr_Occurred()) {
return NULL;
}

Expand Down Expand Up @@ -1055,8 +1059,12 @@ rlock_acquire(PyObject *op, PyObject *args, PyObject *kwds)
}

PyLockStatus r = _PyRecursiveMutex_LockTimed(&self->lock, timeout,
_PY_LOCK_HANDLE_SIGNALS | _PY_LOCK_DETACH);
_PY_LOCK_PYTHONLOCK);
if (r == PY_LOCK_INTR) {
assert(PyErr_Occurred());
return NULL;
}
if (r == PY_LOCK_FAILURE && PyErr_Occurred()) {
return NULL;
}

Expand Down
12 changes: 12 additions & 0 deletions Python/lock.c
Original file line number Diff line number Diff line change
Expand Up @@ -95,6 +95,18 @@ _PyMutex_LockTimed(PyMutex *m, PyTime_t timeout, _PyLockFlags flags)
if (timeout == 0) {
return PY_LOCK_FAILURE;
}
if ((flags & _PY_LOCK_PYTHONLOCK) && Py_IsFinalizing()) {
// At this phase of runtime shutdown, only the finalization thread
// can have attached thread state; others hang if they try
// attaching. And since operations on this lock requires attached
// thread state (_PY_LOCK_PYTHONLOCK), the finalization thread is
// running this code, and no other thread can unlock.
// Raise rather than hang. (_PY_LOCK_PYTHONLOCK allows raising
// exceptons.)
PyErr_SetString(PyExc_PythonFinalizationError,
"cannot acquire lock at interpreter finalization");
return PY_LOCK_FAILURE;
}

uint8_t newv = v;
if (!(v & _Py_HAS_PARKED)) {
Expand Down
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