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gh-151475: Fix data race in faulthandler watchdog on free-threaded builds #152123

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gh-151475: Fix data race in faulthandler watchdog on free-threaded builds #152123

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2 changes: 2 additions & 0 deletions Include/internal/pycore_faulthandler.h
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Original file line number Diff line number Diff line change
Expand Up @@ -48,6 +48,8 @@ struct faulthandler_user_signal {


struct _faulthandler_runtime_state {
PyMutex mutex;

struct {
int enabled;
PyObject *file;
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4 changes: 4 additions & 0 deletions Misc/NEWS.d/next/Library/2026-06-24-14-00-00.gh-issue-151475.FhTrSf.rst
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@@ -0,0 +1,4 @@
Fix data race in :mod:`faulthandler` where concurrent calls to
:func:`~faulthandler.dump_traceback_later` and
:func:`~faulthandler.cancel_dump_traceback_later` could corrupt the
watchdog lock handshake on free-threaded builds.
27 changes: 19 additions & 8 deletions Modules/faulthandler.c
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Original file line number Diff line number Diff line change
Expand Up @@ -56,6 +56,7 @@ typedef struct {
int all_threads;
} fault_handler_t;

#define faulthandler_mutex _PyRuntime.faulthandler.mutex
#define fatal_error _PyRuntime.faulthandler.fatal_error
#define thread _PyRuntime.faulthandler.thread

Expand Down Expand Up @@ -836,17 +837,31 @@ faulthandler_dump_traceback_later_impl(PyObject *module,
return NULL;
}

/* format the timeout before acquiring the lock (no shared state) */
header = format_timeout(timeout_us);
if (header == NULL) {
Py_XDECREF(file);
return PyErr_NoMemory();
}
header_len = strlen(header);

PyMutex_Lock(&faulthandler_mutex);

if (!thread.running) {
thread.running = PyThread_allocate_lock();
if (!thread.running) {
PyMutex_Unlock(&faulthandler_mutex);
Py_XDECREF(file);
PyMem_Free(header);
return PyErr_NoMemory();
}
}
if (!thread.cancel_event) {
thread.cancel_event = PyThread_allocate_lock();
if (!thread.cancel_event || !thread.running) {
PyMutex_Unlock(&faulthandler_mutex);
Py_XDECREF(file);
PyMem_Free(header);
return PyErr_NoMemory();
}

Expand All @@ -855,14 +870,6 @@ faulthandler_dump_traceback_later_impl(PyObject *module,
PyThread_acquire_lock(thread.cancel_event, 1);
}

/* format the timeout */
header = format_timeout(timeout_us);
if (header == NULL) {
Py_XDECREF(file);
return PyErr_NoMemory();
}
header_len = strlen(header);

/* Cancel previous thread, if running */
cancel_dump_traceback_later();

Expand All @@ -885,11 +892,13 @@ faulthandler_dump_traceback_later_impl(PyObject *module,
Py_CLEAR(thread.file);
PyMem_Free(header);
thread.header = NULL;
PyMutex_Unlock(&faulthandler_mutex);
PyErr_SetString(PyExc_RuntimeError,
"unable to start watchdog thread");
return NULL;
}

PyMutex_Unlock(&faulthandler_mutex);
Py_RETURN_NONE;
}

Expand All @@ -904,7 +913,9 @@ static PyObject *
faulthandler_cancel_dump_traceback_later_py_impl(PyObject *module)
/*[clinic end generated code: output=2cf303015d39c926 input=51ad64b6ca8412a4]*/
{
PyMutex_Lock(&faulthandler_mutex);
cancel_dump_traceback_later();
PyMutex_Unlock(&faulthandler_mutex);
Py_RETURN_NONE;
}

Expand Down
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