gh-155561: Use PySlot API in Modules/_testlimitedcapi.c

[vstinner]

• Issue: C API: Enable limited C API tests on Free Threading #155561

[vstinner]

I hesitated to keep PyMethodDef and PyModuleDef code path when the Py_GIL_DISABLED macro is not defined, but this API is already tested by Modules/xxlimited_3_13.c. IMO it's better and simpler to use PySlot in both case (if Py_GIL_DISABLED is defined or not).

cc @encukou

[vstinner]

Oh. The build fails on WASI because Modules/config.c refers to PyModExport__testlimitedcapi() which doesn't exist.

wasm-ld: error: Modules/config.o: undefined symbol: PyInit__testlimitedcapi

and

error: undefined symbol: PyInit__testlimitedcapi (referenced by root reference (e.g. compiled C/C++ code))

Modules/config.c is generated Modules/config.c.in by Modules/makesetup.

I tried adding PyInit__testlimitedcapi() by calling PyModule_FromSlotsAndSpec() with NULL spec, but this function does crash. The spec argument must be non-NULL.

// Provide a PyInit function for Modules/config.c when _testlimitedcapi 
// extension module is built as a built-in module (ex: on WASI and Emscripten).
PyMODINIT_FUNC      
PyInit__testlimitedcapi(void)
{      
    return PyModule_FromSlotsAndSpec(_testlimitedcapimodule_slots, NULL);
}

[vstinner]

I modified PyModule_FromSlotsAndSpec() to get the module name from slots, but this approach doesn't work. Later, the Tools/build/check_extension_modules.py scripts raises SystemError: initialization of _testlimitedcapi did not return a valid extension module error because _testlimitedcapi has no "def": _PyModule_GetDefOrNull() returns NULL.

[vstinner]

Another approach would be to add a variant of _PyRuntime.imports.inittab for "export functions" (PyMODEXPORT_FUNC). We should add a struct _inittab variant where the initfunc returns PySlot*.

[encukou]

Let's keep PyModuleDef, but switch to multi-phase init? See https://github.com/python/cpython/compare/main...encukou:cpython:limited_pyslot?expand=1

We should add a struct _inittab variant where the initfunc returns PySlot*.

That's might be PEP-sized :)

[vstinner]

Let's keep PyModuleDef, but switch to multi-phase init? See https://github.com/python/cpython/compare/main...encukou:cpython:limited_pyslot?expand=1

Please go ahead and propose a PR if you think that it's the right approach. I will be on holiday for next 2 weeks anyway.