gh-155561: Use multi-phase init for Modules/_testlimitedcapi.c

[encukou]

This is a continuation of #155693

We can't use PyModExport for modules that might be in inittab, but we can move to multi-phase init.

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

[vstinner]

According to CIs, building _testlimitedcapi fails on Free Threading:

./Modules/_testlimitedcapi.c:103:5: error: use of undeclared identifier 'PyModuleDef_HEAD_INIT'
  103 |     PyModuleDef_HEAD_INIT,
      |     ^~~~~~~~~~~~~~~~~~~~~
./Modules/_testlimitedcapi.c:102:27: error: variable has incomplete type 'struct PyModuleDef'
  102 | static struct PyModuleDef _testlimitedcapimodule_def = {
      |                           ^
./Include/pytypedefs.h:12:16: note: forward declaration of 'struct PyModuleDef'
   12 | typedef struct PyModuleDef PyModuleDef;
      |                ^
2 errors generated.

[vstinner]

PySlot is not used. I suppose that this comment is outdated.

[bedevere-bot]

🤖 New build scheduled with the buildbot fleet by @encukou for commit 009ce74 🤖

Results will be shown at:

https://buildbot.python.org/all/#/grid?branch=refs%2Fpull%2F156052%2Fmerge

If you want to schedule another build, you need to add the 🔨 test-with-buildbots label again.

[vstinner]

The Alpine Linux buildbot failure looks like a legit bug, but it's unrelated to this change:

  /usr/lib/ccache/bin/g++ -fno-omit-frame-pointer -mno-omit-leaf-frame-pointer -fno-strict-overflow -Wsign-compare -g -Og -Wall -fPIC -I/buildbot/buildarea/pull_request.ware-alpine.nogil/build/build/test_python_92877æ/tempcwd/env/include -I/buildbot/buildarea/pull_request.ware-alpine.nogil/build/Include -I/buildbot/buildarea/pull_request.ware-alpine.nogil/build -c extension.cpp -o build/temp.linux-x86_64-cpython-316t-pydebug/extension.o -Werror -DMODULE_NAME=_testcppext_internal -DTEST_INTERNAL_C_API=1
  In file included from /buildbot/buildarea/pull_request.ware-alpine.nogil/build/Include/internal/pycore_object.h:13,
                   from /buildbot/buildarea/pull_request.ware-alpine.nogil/build/Include/internal/pycore_cell.h:5,
                   from extension.cpp:29:
  /buildbot/buildarea/pull_request.ware-alpine.nogil/build/Include/internal/pycore_interpframe.h: In function 'PyFrameObject* _PyFrame_GetFrameObject(_PyInterpreterFrame*)':
  /buildbot/buildarea/pull_request.ware-alpine.nogil/build/Include/internal/pycore_pyatomic_ft_wrappers.h:33:32: error: invalid conversion from 'void*' to 'PyFrameObject*' {aka '_frame*'} [-fpermissive]
     33 |     _Py_atomic_load_ptr_acquire(&value)
        |     ~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~
        |                                |
        |                                void*
  /buildbot/buildarea/pull_request.ware-alpine.nogil/build/Include/internal/pycore_interpframe.h:348:26: note: in expansion of macro 'FT_ATOMIC_LOAD_PTR_ACQUIRE'
    348 |     PyFrameObject *res = FT_ATOMIC_LOAD_PTR_ACQUIRE(frame->frame_obj);
        |                          ^~~~~~~~~~~~~~~~~~~~~~~~~~
  error: command '/usr/lib/ccache/bin/g++' failed with exit code 1

[vstinner]

The Alpine Linux buildbot failure looks like a legit bug, but it's unrelated to this change:

Ah, in fact the AMD64 Alpine Linux NoGIL 3.x buildbot already fails on the main branch: I wrote PR gh-156893 to fix the issue.

[vstinner]

The "buildbot/AMD64 CentOS9 NoGIL PR" failure is the same as the Alpine Linux issue: test_cppext fails, so PR #156893 will fix it.

[vstinner]

LGTM.

This change doesn't build Modules/_testlimitedcapi.c with the limited C API on Free Threading yet, but it converts the module to multi-phase init which is already a good step forwards!