gh-111506: Implement Py_SET_REFCNT() as opaque function in limited C API

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

Add _Py_SetRefcnt() to the stable ABI.

Author: vstinner

Include/object.h

@@ -329,15 +329,23 @@ static inline int Py_IS_TYPE(PyObject *ob, PyTypeObject *type) {
 #endif
 
 
+// Py_SET_REFCNT() implementation for stable ABI
+PyAPI_FUNC(void) _Py_SetRefcnt(PyObject *ob, Py_ssize_t refcnt);
+
 static inline void Py_SET_REFCNT(PyObject *ob, Py_ssize_t refcnt) {
+#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.
+    _Py_SetRefcnt(ob, refcnt);
+#else
     // This immortal check is for code that is unaware of immortal objects.
     // The runtime tracks these objects and we should avoid as much
     // as possible having extensions inadvertently change the refcnt
     // of an immortalized object.
     if (_Py_IsImmortal(ob)) {
         return;
     }
-#if !defined(Py_NOGIL)
+#ifndef Py_NOGIL
     ob->ob_refcnt = refcnt;
 #else
     if (_Py_IsOwnedByCurrentThread(ob)) {
@@ -354,7 +362,8 @@ static inline void Py_SET_REFCNT(PyObject *ob, Py_ssize_t refcnt) {
         ob->ob_ref_local = 0;
         ob->ob_ref_shared = _Py_REF_SHARED(refcnt, _Py_REF_MERGED);
     }
-#endif
+#endif  // Py_NOGIL
+#endif  // Py_LIMITED_API+0 < 0x030d0000
 }
 #if !defined(Py_LIMITED_API) || Py_LIMITED_API+0 < 0x030b0000
 #  define Py_SET_REFCNT(ob, refcnt) Py_SET_REFCNT(_PyObject_CAST(ob), (refcnt))

Misc/NEWS.d/next/C API/2023-10-30-18-13-01.gh-issue-111506.EUdO22.rst

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

Misc/stable_abi.toml

@@ -2480,3 +2480,5 @@
     added = '3.13'
 [function.PyUnicode_AsUTF8]
     added = '3.13'
+[function._Py_SetRefcnt]
+    added = '3.13'

Objects/object.c

@@ -2931,3 +2931,11 @@ int Py_IsFalse(PyObject *x)
 {
     return Py_Is(x, Py_False);
 }
+
+
+// Py_SET_REFCNT() implementation for stable ABI
+void
+_Py_SetRefcnt(PyObject *ob, Py_ssize_t refcnt)
+{
+    Py_SET_REFCNT(ob, refcnt);
+}