gh-99574: Add Py_SETREF() to the limited C API

Add Py_SETREF() and Py_XSETREF() macros to the limited C API.
python · vstinner · Nov 18, 2022 · Dec 7, 2022 · bf7dc148841deb22099a2ea55968a7a0e10dde8b
commit bf7dc148841deb22099a2ea55968a7a0e10dde8b
diff --git a/Doc/data/stable_abi.dat b/Doc/data/stable_abi.dat
@@ -798,6 +798,10 @@ New Features
   get a frame variable by its name.
   (Contributed by Victor Stinner in :gh:`91248`.)
 
+* Add :c:macro:`Py_SETREF` and :c:macro:`Py_XSETREF` macros to the limited C
+  API.
+  (Contributed by Victor Stinner in :gh:`99574`.)
+
 Porting to Python 3.12
 ----------------------
 

diff --git a/Include/cpython/object.h b/Include/cpython/object.h
@@ -305,70 +305,6 @@ _PyObject_GenericSetAttrWithDict(PyObject *, PyObject *,
 
 PyAPI_FUNC(PyObject *) _PyObject_FunctionStr(PyObject *);
 
-/* Safely decref `dst` and set `dst` to `src`.
- *
- * As in case of Py_CLEAR "the obvious" code can be deadly:
- *
- *     Py_DECREF(dst);
- *     dst = src;
- *
- * The safe way is:
- *
- *      Py_SETREF(dst, src);
- *
- * That arranges to set `dst` to `src` _before_ decref'ing, so that any code
- * triggered as a side-effect of `dst` getting torn down no longer believes
- * `dst` points to a valid object.
- *
- * Temporary variables are used to only evalutate macro arguments once and so
- * avoid the duplication of side effects. _Py_TYPEOF() or memcpy() is used to
- * avoid a miscompilation caused by type punning. See Py_CLEAR() comment for
- * implementation details about type punning.
- *
- * The memcpy() implementation does not emit a compiler warning if 'src' has
- * not the same type than 'src': any pointer type is accepted for 'src'.
- */
-#ifdef _Py_TYPEOF
-#define Py_SETREF(dst, src) \
-    do { \
-        _Py_TYPEOF(dst)* _tmp_dst_ptr = &(dst); \
-        _Py_TYPEOF(dst) _tmp_old_dst = (*_tmp_dst_ptr); \
-        *_tmp_dst_ptr = (src); \
-        Py_DECREF(_tmp_old_dst); \
-    } while (0)
-#else
-#define Py_SETREF(dst, src) \
-    do { \
-        PyObject **_tmp_dst_ptr = _Py_CAST(PyObject**, &(dst)); \
-        PyObject *_tmp_old_dst = (*_tmp_dst_ptr); \
-        PyObject *_tmp_src = _PyObject_CAST(src); \
-        memcpy(_tmp_dst_ptr, &_tmp_src, sizeof(PyObject*)); \
-        Py_DECREF(_tmp_old_dst); \
-    } while (0)
-#endif
-
-/* Py_XSETREF() is a variant of Py_SETREF() that uses Py_XDECREF() instead of
- * Py_DECREF().
- */
-#ifdef _Py_TYPEOF
-#define Py_XSETREF(dst, src) \
-    do { \
-        _Py_TYPEOF(dst)* _tmp_dst_ptr = &(dst); \
-        _Py_TYPEOF(dst) _tmp_old_dst = (*_tmp_dst_ptr); \
-        *_tmp_dst_ptr = (src); \
-        Py_XDECREF(_tmp_old_dst); \
-    } while (0)
-#else
-#define Py_XSETREF(dst, src) \
-    do { \
-        PyObject **_tmp_dst_ptr = _Py_CAST(PyObject**, &(dst)); \
-        PyObject *_tmp_old_dst = (*_tmp_dst_ptr); \
-        PyObject *_tmp_src = _PyObject_CAST(src); \
-        memcpy(_tmp_dst_ptr, &_tmp_src, sizeof(PyObject*)); \
-        Py_XDECREF(_tmp_old_dst); \
-    } while (0)
-#endif
-
 
 PyAPI_DATA(PyTypeObject) _PyNone_Type;
 PyAPI_DATA(PyTypeObject) _PyNotImplemented_Type;

diff --git a/Include/object.h b/Include/object.h
@@ -637,6 +637,75 @@ static inline void Py_DECREF(PyObject *op)
 #endif
 
 
+#if !defined(Py_LIMITED_API) || Py_LIMITED_API+0 >= 0x030C0000
+
+/* Safely decref `dst` and set `dst` to `src`.
+ *
+ * As in case of Py_CLEAR "the obvious" code can be deadly:
+ *
+ *     Py_DECREF(dst);
+ *     dst = src;
+ *
+ * The safe way is:
+ *
+ *      Py_SETREF(dst, src);
+ *
+ * That arranges to set `dst` to `src` _before_ decref'ing, so that any code
+ * triggered as a side-effect of `dst` getting torn down no longer believes
+ * `dst` points to a valid object.
+ *
+ * Temporary variables are used to only evalutate macro arguments once and so
+ * avoid the duplication of side effects. _Py_TYPEOF() or memcpy() is used to
+ * avoid a miscompilation caused by type punning. See Py_CLEAR() comment for
+ * implementation details about type punning.
+ *
+ * The memcpy() implementation does not emit a compiler warning if 'src' has
+ * not the same type than 'src': any pointer type is accepted for 'src'.
+ */
+#ifdef _Py_TYPEOF
+#define Py_SETREF(dst, src) \
+    do { \
+        _Py_TYPEOF(dst)* _tmp_dst_ptr = &(dst); \
+        _Py_TYPEOF(dst) _tmp_old_dst = (*_tmp_dst_ptr); \
+        *_tmp_dst_ptr = (src); \
+        Py_DECREF(_tmp_old_dst); \
+    } while (0)
+#else
+#define Py_SETREF(dst, src) \
+    do { \
+        PyObject **_tmp_dst_ptr = _Py_CAST(PyObject**, &(dst)); \
+        PyObject *_tmp_old_dst = (*_tmp_dst_ptr); \
+        PyObject *_tmp_src = _PyObject_CAST(src); \
+        memcpy(_tmp_dst_ptr, &_tmp_src, sizeof(PyObject*)); \
+        Py_DECREF(_tmp_old_dst); \
+    } while (0)
+#endif
+
+/* Py_XSETREF() is a variant of Py_SETREF() that uses Py_XDECREF() instead of
+ * Py_DECREF().
+ */
+#ifdef _Py_TYPEOF
+#define Py_XSETREF(dst, src) \
+    do { \
+        _Py_TYPEOF(dst)* _tmp_dst_ptr = &(dst); \
+        _Py_TYPEOF(dst) _tmp_old_dst = (*_tmp_dst_ptr); \
+        *_tmp_dst_ptr = (src); \
+        Py_XDECREF(_tmp_old_dst); \
+    } while (0)
+#else
+#define Py_XSETREF(dst, src) \
+    do { \
+        PyObject **_tmp_dst_ptr = _Py_CAST(PyObject**, &(dst)); \
+        PyObject *_tmp_old_dst = (*_tmp_dst_ptr); \
+        PyObject *_tmp_src = _PyObject_CAST(src); \
+        memcpy(_tmp_dst_ptr, &_tmp_src, sizeof(PyObject*)); \
+        Py_XDECREF(_tmp_old_dst); \
+    } while (0)
+#endif
+
+#endif  // Py_LIMITED_API
+
+
 /* Function to use in case the object pointer can be NULL: */
 static inline void Py_XINCREF(PyObject *op)
 {

diff --git a/Misc/NEWS.d/next/C API/2022-11-18-13-22-31.gh-issue-99574.kmYSuk.rst b/Misc/NEWS.d/next/C API/2022-11-18-13-22-31.gh-issue-99574.kmYSuk.rst
@@ -0,0 +1,2 @@
+Add :c:macro:`Py_SETREF()` and :c:macro:`Py_XSETREF()` macros to the limited
+C API. Patch by Victor Stinner.
@@ -2386,3 +2386,8 @@
     added = '3.12'  # Before 3.12, available in "structmember.h" w/o Py_ prefix
 [const.Py_AUDIT_READ]
     added = '3.12'  # Before 3.12, available in "structmember.h"
+
+[macro.Py_SETREF]
+    added = '3.12'
+[macro.Py_XSETREF]
+    added = '3.12'
Original file line number	Diff line number	Diff line change
		@@ -0,0 +1,2 @@
		Add :c:macro:`Py_SETREF()` and :c:macro:`Py_XSETREF()` macros to the limited
		C API. Patch by Victor Stinner.