gh-98724: Fix Py_CLEAR() macro side effects (#99100)

The Py_CLEAR(), Py_SETREF() and Py_XSETREF() macros now only evaluate
their argument once. If an argument has side effects, these side
effects are no longer duplicated.

Add test_py_clear() and test_py_setref() unit tests to _testcapi.

1 files changed, 24 insertions, 20 deletions

diff --git a/Include/cpython/object.h b/Include/cpython/object.h
index fa0cfb2..d07cb3b 100644
--- a/Include/cpython/object.h
+++ b/Include/cpython/object.h
@@ -305,37 +305,41 @@ _PyObject_GenericSetAttrWithDict(PyObject *, PyObject *,
 
 PyAPI_FUNC(PyObject *) _PyObject_FunctionStr(PyObject *);
 
-/* Safely decref `op` and set `op` to `op2`.
+/* Safely decref `dst` and set `dst` to `src`.
  *
  * As in case of Py_CLEAR "the obvious" code can be deadly:
  *
- *     Py_DECREF(op);
- *     op = op2;
+ *     Py_DECREF(dst);
+ *     dst = src;
  *
  * The safe way is:
  *
- *      Py_SETREF(op, op2);
+ *      Py_SETREF(dst, src);
  *
- * That arranges to set `op` to `op2` _before_ decref'ing, so that any code
- * triggered as a side-effect of `op` getting torn down no longer believes
- * `op` points to a valid object.
+ * 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.
  *
- * Py_XSETREF is a variant of Py_SETREF that uses Py_XDECREF instead of
- * Py_DECREF.
+ * gh-98724: Use the _tmp_dst_ptr variable to evaluate the 'dst' macro argument
+ * exactly once, to prevent the duplication of side effects in this macro.
  */
-
-#define Py_SETREF(op, op2)                      \
-    do {                                        \
-        PyObject *_py_tmp = _PyObject_CAST(op); \
-        (op) = (op2);                           \
-        Py_DECREF(_py_tmp);                     \
+#define Py_SETREF(dst, src)                                     \
+    do {                                                        \
+        PyObject **_tmp_dst_ptr = _Py_CAST(PyObject**, &(dst)); \
+        PyObject *_tmp_dst = (*_tmp_dst_ptr);                   \
+        *_tmp_dst_ptr = _PyObject_CAST(src);                    \
+        Py_DECREF(_tmp_dst);                                    \
     } while (0)
 
-#define Py_XSETREF(op, op2)                     \
-    do {                                        \
-        PyObject *_py_tmp = _PyObject_CAST(op); \
-        (op) = (op2);                           \
-        Py_XDECREF(_py_tmp);                    \
+/* Py_XSETREF() is a variant of Py_SETREF() that uses Py_XDECREF() instead of
+ * Py_DECREF().
+ */
+#define Py_XSETREF(dst, src)                                    \
+    do {                                                        \
+        PyObject **_tmp_dst_ptr = _Py_CAST(PyObject**, &(dst)); \
+        PyObject *_tmp_dst = (*_tmp_dst_ptr);                   \
+        *_tmp_dst_ptr = _PyObject_CAST(src);                    \
+        Py_XDECREF(_tmp_dst);                                   \
     } while (0)