https://github.com/python/cpython/commit/fe119a08177feea27611450b0bb3901e330a2d58
commit: fe119a08177feea27611450b0bb3901e330a2d58
branch: main
author: Petr Viktorin <encu...@gmail.com>
committer: encukou <encu...@gmail.com>
date: 2025-07-01T10:57:42+02:00
summary:
gh-87135: threading.Lock: Raise rather than hang on Python finalization
(GH-135991)
After Python finalization gets to the point where no other thread
can attach thread state, attempting to acquire a Python lock must hang.
Raise PythonFinalizationError instead of hanging.
files:
A Misc/NEWS.d/next/Library/2025-06-27-09-26-04.gh-issue-87135.33z0UW.rst
M Doc/library/exceptions.rst
M Include/internal/pycore_lock.h
M Lib/test/test_threading.py
M Modules/_threadmodule.c
M Python/lock.c
diff --git a/Doc/library/exceptions.rst b/Doc/library/exceptions.rst
index 9806ae80905ca0..c09e1615a5b733 100644
--- a/Doc/library/exceptions.rst
+++ b/Doc/library/exceptions.rst
@@ -429,7 +429,9 @@ The following exceptions are the exceptions that are
usually raised.
* Creating a new Python thread.
* :meth:`Joining <threading.Thread.join>` a running daemon thread.
- * :func:`os.fork`.
+ * :func:`os.fork`,
+ * acquiring a lock such as :class:`threading.Lock`, when it is known that
+ the operation would otherwise deadlock.
See also the :func:`sys.is_finalizing` function.
@@ -440,6 +442,11 @@ The following exceptions are the exceptions that are
usually raised.
:meth:`threading.Thread.join` can now raise this exception.
+ .. versionchanged:: next
+
+ This exception may be raised when acquiring :meth:`threading.Lock`
+ or :meth:`threading.RLock`.
+
.. exception:: RecursionError
This exception is derived from :exc:`RuntimeError`. It is raised when the
diff --git a/Include/internal/pycore_lock.h b/Include/internal/pycore_lock.h
index 32b60cc33a21f1..bd6011b60ac09a 100644
--- a/Include/internal/pycore_lock.h
+++ b/Include/internal/pycore_lock.h
@@ -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.
+ // (Intended for use with _PY_LOCK_HANDLE_SIGNALS and _PY_LOCK_DETACH.)
+ _PY_LOCK_PYTHONLOCK = 8,
} _PyLockFlags;
// Lock a mutex with an optional timeout and additional options. See
diff --git a/Lib/test/test_threading.py b/Lib/test/test_threading.py
index 13b55d0f0a2e73..00a3037c3e1e01 100644
--- a/Lib/test/test_threading.py
+++ b/Lib/test/test_threading.py
@@ -1247,6 +1247,61 @@ def __del__(self):
self.assertEqual(err, b"")
self.assertIn(b"all clear", out)
+ @support.subTests('lock_class_name', ['Lock', 'RLock'])
+ def test_acquire_daemon_thread_lock_in_finalization(self, lock_class_name):
+ # 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.
+ code = textwrap.dedent(f"""
+ import threading
+ import time
+
+ thread_started_event = threading.Event()
+
+ lock = threading.{lock_class_name}()
+ def loop():
+ if {lock_class_name!r} == 'RLock':
+ lock.acquire()
+ with lock:
+ thread_started_event.set()
+ while True:
+ time.sleep(1)
+
+ uncontested_lock = threading.{lock_class_name}()
+
+ 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()
+
+ # We *can* acquire an unlocked lock
+ uncontested_lock.acquire()
+ if {lock_class_name!r} == 'RLock':
+ uncontested_lock.acquire()
+
+ # Acquiring a locked one fails
+ 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,
diff --git
a/Misc/NEWS.d/next/Library/2025-06-27-09-26-04.gh-issue-87135.33z0UW.rst
b/Misc/NEWS.d/next/Library/2025-06-27-09-26-04.gh-issue-87135.33z0UW.rst
new file mode 100644
index 00000000000000..4b6bc74cad87a4
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2025-06-27-09-26-04.gh-issue-87135.33z0UW.rst
@@ -0,0 +1,3 @@
+Acquiring a :class:`threading.Lock` or :class:`threading.RLock` at interpreter
+shutdown will raise :exc:`PythonFinalizationError` if Python can determine
+that it would otherwise deadlock.
diff --git a/Modules/_threadmodule.c b/Modules/_threadmodule.c
index b6ccca943f2085..8886a9d6bd0c8d 100644
--- a/Modules/_threadmodule.c
+++ b/Modules/_threadmodule.c
@@ -834,9 +834,14 @@ lock_PyThread_acquire_lock(PyObject *op, PyObject *args,
PyObject *kwds)
return NULL;
}
- PyLockStatus r = _PyMutex_LockTimed(&self->lock, timeout,
- _PY_LOCK_HANDLE_SIGNALS |
_PY_LOCK_DETACH);
+ PyLockStatus r = _PyMutex_LockTimed(
+ &self->lock, timeout,
+ _PY_LOCK_PYTHONLOCK | _PY_LOCK_HANDLE_SIGNALS | _PY_LOCK_DETACH);
if (r == PY_LOCK_INTR) {
+ assert(PyErr_Occurred());
+ return NULL;
+ }
+ if (r == PY_LOCK_FAILURE && PyErr_Occurred()) {
return NULL;
}
@@ -1054,9 +1059,14 @@ rlock_acquire(PyObject *op, PyObject *args, PyObject
*kwds)
return NULL;
}
- PyLockStatus r = _PyRecursiveMutex_LockTimed(&self->lock, timeout,
- _PY_LOCK_HANDLE_SIGNALS |
_PY_LOCK_DETACH);
+ PyLockStatus r = _PyRecursiveMutex_LockTimed(
+ &self->lock, timeout,
+ _PY_LOCK_PYTHONLOCK | _PY_LOCK_HANDLE_SIGNALS | _PY_LOCK_DETACH);
if (r == PY_LOCK_INTR) {
+ assert(PyErr_Occurred());
+ return NULL;
+ }
+ if (r == PY_LOCK_FAILURE && PyErr_Occurred()) {
return NULL;
}
diff --git a/Python/lock.c b/Python/lock.c
index ea6ac00bfeccb4..eb09019e0a236d 100644
--- a/Python/lock.c
+++ b/Python/lock.c
@@ -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)) {
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