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At commit ac46eb4ad6662cf6d771b20d8963658b2186c48c ("bpo-38113: Update the Python-ast.c generator to PEP384 (gh-15957)"):
---------------
static struct PyModuleDef _astmodule = {
 PyModuleDef_HEAD_INIT,
 "_ast",
 NULL,
 sizeof(astmodulestate),
 NULL,
 NULL,
 astmodule_traverse,
 astmodule_clear,
 astmodule_free,
};
#define astmodulestate_global ((astmodulestate *)PyModule_GetState(PyState_FindModule(&_astmodule)))
PyMODINIT_FUNC
PyInit__ast(void)
{
 PyObject *m;
 if (!init_types()) return NULL;
 m = PyState_FindModule(&_astmodule);
 ...
 return m;
}
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=> I investigated bpo-41194 crash again. It seems like the root issue is more than PyInit__ast() returns a *borrowed* reference, rather than a strong reference.
=> The _ast module can only be loaded once, astmodule_free() and astmodule_clear() are not called at Python exit.
At commit 91e1bc18bd467a13bceb62e16fbc435b33381c82 ("bpo-41194: The _ast module cannot be loaded more than once (GH-21290)"):
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static astmodulestate global_ast_state;
static astmodulestate *
get_ast_state(PyObject *Py_UNUSED(module))
{
 return &global_ast_state;
}
#define get_global_ast_state() (&global_ast_state)
static struct PyModuleDef _astmodule = {
 PyModuleDef_HEAD_INIT,
 .m_name = "_ast",
 .m_size = -1,
 .m_traverse = astmodule_traverse,
 .m_clear = astmodule_clear,
 .m_free = astmodule_free,
};
PyMODINIT_FUNC
PyInit__ast(void)
{
 PyObject *m = PyModule_Create(&_astmodule);
 if (!m) {
 return NULL;
 }
 astmodulestate *state = get_ast_state(m);
 ...
}
---------------
* bpo-41194 => ok, PyInit__ast() is fine
* bpo-41631 => ok: PyAST_Check() doesn't import the module, it uses a global state.
* bpo-41261 => ERROR: This implementation allows to create two instances of the _ast module, but also to unload one instance (call astmodule_free), because _astmodule.m_size = -1.
At commit b1cc6ba73a51d5cc3aeb113b5e7378fb50a0e20a ("bpo-41194: Convert _ast extension to PEP 489 (GH-21293)"):
---------------
static astmodulestate*
get_global_ast_state(void)
{
 ...
 PyObject *module = PyImport_GetModule(name);
 if (module == NULL) {
 if (PyErr_Occurred()) {
 return NULL;
 }
 module = PyImport_Import(name);
 if (module == NULL) {
 return NULL;
 }
 }
 ...
 astmodulestate *state = get_ast_state(module);
 Py_DECREF(module);
 return state;
}
static struct PyModuleDef _astmodule = {
 PyModuleDef_HEAD_INIT,
 .m_name = "_ast",
 .m_size = sizeof(astmodulestate),
 .m_slots = astmodule_slots,
 .m_traverse = astmodule_traverse,
 .m_clear = astmodule_clear,
 .m_free = astmodule_free,
};
PyMODINIT_FUNC
PyInit__ast(void)
{
 return PyModuleDef_Init(&_astmodule);
}
---------------
* bpo-41631 => ERROR: PyAST_Check() imports "_ast" module