gh-111506: Implement Py_REFCNT() as opaque function call

In the limited C API version 3.13, the Py_REFCNT() function is now
implemented as an opaque function call.

Author: vstinner

Include/object.h

@@ -290,8 +290,14 @@ _Py_IsOwnedByCurrentThread(PyObject *ob)
 }
 #endif
 
+PyAPI_FUNC(Py_ssize_t) _Py_GetRefcnt(PyObject *ob);
+
 static inline Py_ssize_t Py_REFCNT(PyObject *ob) {
-#if !defined(Py_GIL_DISABLED)
+#if defined(Py_LIMITED_API) && Py_LIMITED_API+0 >= 0x030d0000
+    // Stable ABI implements Py_SET_REFCNT() as a function call
+    // on limited C API version 3.13 and newer.
+    return _Py_GetRefcnt(ob);
+#elif !defined(Py_GIL_DISABLED)
     return ob->ob_refcnt;
 #else
     uint32_t local = _Py_atomic_load_uint32_relaxed(&ob->ob_ref_local);

Misc/NEWS.d/next/C API/2023-12-05-10-39-06.gh-issue-111506.gz90Ft.rst

@@ -0,0 +1,2 @@
+In the limited C API version 3.13, the :c:func:`Py_REFCNT` function is now
+implemented as an opaque function call. Patch by Victor Stinner.

Objects/object.c

@@ -2932,6 +2932,14 @@ int Py_IsFalse(PyObject *x)
 }
 
 
+// Py_REFCNT() implementation for stable ABI
+Py_ssize_t
+_Py_GetRefcnt(PyObject *ob)
+{
+    return Py_REFCNT(ob);
+}
+
+
 // Py_SET_REFCNT() implementation for stable ABI
 void
 _Py_SetRefcnt(PyObject *ob, Py_ssize_t refcnt)