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It seems that PyOS_AfterFork_Child() needs to do something like
PyThreadState *tstate = PyThreadState_Get();
PyObject *wr = _PyObject_CAST(tstate->on_delete_data);
PyObject *obj = PyWeakref_GET_OBJECT(wr);
lockobject *lock;
if (obj != Py_None) {
 lock = (lockobject *) obj;
 if (lock->locked) {
 /* Leak memory on purpose. */
 lock->locked = 0;
 }
}
before the call to _PyEval_ReInitThreads.
Maybe encapsulate that as PyThread_ReInitThreads().
The locks in the threads in the child need to be cleared before _PyThreadState_DeleteExcept() so that Python does not try to release the locks in the child.