bpo-40241: Add pycore_gc.h header file (GH-19494)

Move the PyGC_Head structure and the following private macros to the
internal C API:

* _PyGCHead_FINALIZED()
* _PyGCHead_NEXT()
* _PyGCHead_PREV()
* _PyGCHead_SET_FINALIZED()
* _PyGCHead_SET_NEXT()
* _PyGCHead_SET_PREV()
* _PyGC_FINALIZED()
* _PyGC_PREV_MASK
* _PyGC_PREV_MASK_COLLECTING
* _PyGC_PREV_MASK_FINALIZED
* _PyGC_PREV_SHIFT
* _PyGC_SET_FINALIZED()
* _PyObject_GC_IS_TRACKED()
* _PyObject_GC_MAY_BE_TRACKED()
* _Py_AS_GC(o)

Keep the private _PyGC_FINALIZED() macro in the public C API for
backward compatibility with Python 3.8: make it an alias to the new
PyObject_GC_IsFinalized() function.

Move the SIZEOF_PYGC_HEAD constant from _testcapi module to
_testinternalcapi module.

2 files changed, 16 insertions, 2 deletions

diff --git a/Modules/_testcapimodule.c b/Modules/_testcapimodule.c
index 0a30fea..c97cbe8 100644
--- a/Modules/_testcapimodule.c
+++ b/Modules/_testcapimodule.c
@@ -6716,7 +6716,6 @@ PyInit__testcapi(void)
     PyModule_AddObject(m, "ULLONG_MAX", PyLong_FromUnsignedLongLong(ULLONG_MAX));
     PyModule_AddObject(m, "PY_SSIZE_T_MAX", PyLong_FromSsize_t(PY_SSIZE_T_MAX));
     PyModule_AddObject(m, "PY_SSIZE_T_MIN", PyLong_FromSsize_t(PY_SSIZE_T_MIN));
-    PyModule_AddObject(m, "SIZEOF_PYGC_HEAD", PyLong_FromSsize_t(sizeof(PyGC_Head)));
     PyModule_AddObject(m, "SIZEOF_TIME_T", PyLong_FromSsize_t(sizeof(time_t)));
     Py_INCREF(&PyInstanceMethod_Type);
     PyModule_AddObject(m, "instancemethod", (PyObject *)&PyInstanceMethod_Type);
diff --git a/Modules/_testinternalcapi.c b/Modules/_testinternalcapi.c
index 394b870..8352f6e 100644
--- a/Modules/_testinternalcapi.c
+++ b/Modules/_testinternalcapi.c
@@ -10,6 +10,7 @@
 
 #include "Python.h"
 #include "pycore_initconfig.h"   // _Py_GetConfigsAsDict()
+#include "pycore_gc.h"           // PyGC_Head
 
 
 static PyObject *
@@ -52,5 +53,19 @@ static struct PyModuleDef _testcapimodule = {
 PyMODINIT_FUNC
 PyInit__testinternalcapi(void)
 {
-    return PyModule_Create(&_testcapimodule);
+    PyObject *module = PyModule_Create(&_testcapimodule);
+    if (module == NULL) {
+        return NULL;
+    }
+
+    if (PyModule_AddObject(module, "SIZEOF_PYGC_HEAD",
+                           PyLong_FromSsize_t(sizeof(PyGC_Head))) < 0) {
+        goto error;
+    }
+
+    return module;
+
+error:
+    Py_DECREF(module);
+    return NULL;
 }