diff --git a/Doc/library/exceptions.rst b/Doc/library/exceptions.rst index bb72032891ea98..e35a06749ea686 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 ` 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 150a266b521736..a13a4784fde015 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)) {