gh-98724: Fix Py_CLEAR() macro side effects (#99100) (#100070)

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

Use temporary variables to avoid duplicating side effects of macro
arguments. If available, use _Py_TYPEOF() to avoid type punning.
Otherwise, use memcpy() for the assignment to prevent a
miscompilation with strict aliasing caused by type punning.

Add _Py_TYPEOF() macro: __typeof__() on GCC and clang.

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

Modules/_testcapimodule.c

@@ -2589,6 +2589,91 @@ test_set_type_size(PyObject *self, PyObject *Py_UNUSED(ignored))
 }
 
 
+// Test Py_CLEAR() macro
+static PyObject*
+test_py_clear(PyObject *self, PyObject *Py_UNUSED(ignored))
+{
+    // simple case with a variable
+    PyObject *obj = PyList_New(0);
+    if (obj == NULL) {
+        return NULL;
+    }
+    Py_CLEAR(obj);
+    assert(obj == NULL);
+
+    // gh-98724: complex case, Py_CLEAR() argument has a side effect
+    PyObject* array[1];
+    array[0] = PyList_New(0);
+    if (array[0] == NULL) {
+        return NULL;
+    }
+
+    PyObject **p = array;
+    Py_CLEAR(*p++);
+    assert(array[0] == NULL);
+    assert(p == array + 1);
+
+    Py_RETURN_NONE;
+}
+
+
+// Test Py_SETREF() and Py_XSETREF() macros, similar to test_py_clear()
+static PyObject*
+test_py_setref(PyObject *self, PyObject *Py_UNUSED(ignored))
+{
+    // Py_SETREF() simple case with a variable
+    PyObject *obj = PyList_New(0);
+    if (obj == NULL) {
+        return NULL;
+    }
+    Py_SETREF(obj, NULL);
+    assert(obj == NULL);
+
+    // Py_XSETREF() simple case with a variable
+    PyObject *obj2 = PyList_New(0);
+    if (obj2 == NULL) {
+        return NULL;
+    }
+    Py_XSETREF(obj2, NULL);
+    assert(obj2 == NULL);
+    // test Py_XSETREF() when the argument is NULL
+    Py_XSETREF(obj2, NULL);
+    assert(obj2 == NULL);
+
+    // gh-98724: complex case, Py_SETREF() argument has a side effect
+    PyObject* array[1];
+    array[0] = PyList_New(0);
+    if (array[0] == NULL) {
+        return NULL;
+    }
+
+    PyObject **p = array;
+    Py_SETREF(*p++, NULL);
+    assert(array[0] == NULL);
+    assert(p == array + 1);
+
+    // gh-98724: complex case, Py_XSETREF() argument has a side effect
+    PyObject* array2[1];
+    array2[0] = PyList_New(0);
+    if (array2[0] == NULL) {
+        return NULL;
+    }
+
+    PyObject **p2 = array2;
+    Py_XSETREF(*p2++, NULL);
+    assert(array2[0] == NULL);
+    assert(p2 == array2 + 1);
+
+    // test Py_XSETREF() when the argument is NULL
+    p2 = array2;
+    Py_XSETREF(*p2++, NULL);
+    assert(array2[0] == NULL);
+    assert(p2 == array2 + 1);
+
+    Py_RETURN_NONE;
+}
+
+
 #define TEST_REFCOUNT() \
     do { \
         PyObject *obj = PyList_New(0); \
@@ -3252,6 +3337,8 @@ static PyMethodDef TestMethods[] = {
     {"pynumber_tobase", pynumber_tobase, METH_VARARGS},
     {"without_gc", without_gc, METH_O},
     {"test_set_type_size", test_set_type_size, METH_NOARGS},
+    {"test_py_clear", test_py_clear, METH_NOARGS},
+    {"test_py_setref", test_py_setref, METH_NOARGS},
     {"test_refcount_macros", test_refcount_macros, METH_NOARGS},
     {"test_refcount_funcs", test_refcount_funcs, METH_NOARGS},
     {"test_py_is_macros", test_py_is_macros, METH_NOARGS},