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PyType_AddWatcher and friends are not thread safe in free-threading #131544

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PyType_AddWatcher and friends are not thread safe in free-threading#131544
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interpreter-core(Objects, Python, Grammar, and Parser dirs)topic-free-threadingtype-bugAn unexpected behavior, bug, or error
@kumaraditya303

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@kumaraditya303
kumaraditya303
opened on Mar 21, 2025

PyType_AddWatcher and PyType_ClearWatcher are not thread safe as it modifies the interp's type watchers non-atomically.

PyType_AddWatcher:

cpython/Objects/typeobject.c

Lines 949 to 963 in d3f6063

PyType_AddWatcher(PyType_WatchCallback callback)
{
PyInterpreterState *interp = _PyInterpreterState_GET();
// start at 1, 0 is reserved for cpython optimizer
for (int i = 1; i < TYPE_MAX_WATCHERS; i++) {
if (!interp->type_watchers[i]) {
interp->type_watchers[i] = callback;
return i;
}
}
PyErr_SetString(PyExc_RuntimeError, "no more type watcher IDs available");
return -1;
}

PyType_ClearWatcher:

cpython/Objects/typeobject.c

Lines 980 to 989 in d3f6063

PyType_ClearWatcher(int watcher_id)
{
PyInterpreterState *interp = _PyInterpreterState_GET();
if (validate_watcher_id(interp, watcher_id) < 0) {
return -1;
}
interp->type_watchers[watcher_id] = NULL;
return 0;
}

I think adding and removing of type watchers is a rare event so maybe instead of adding atomics or locks it would be better to change them to use stop-the-world pause event.

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    interpreter-core(Objects, Python, Grammar, and Parser dirs)topic-free-threadingtype-bugAn unexpected behavior, bug, or error

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