Issue #20440: Massive replacing unsafe attribute setting code with special

macro Py_SETREF.

Include/object.h

@@ -846,6 +846,32 @@ PyAPI_FUNC(void) _Py_Dealloc(PyObject *);
             Py_DECREF(_py_xdecref_tmp);               \
     } while (0)
 
+#ifndef Py_LIMITED_API
+/* Safely decref `op` and set `op` to `op2`.
+ *
+ * As in case of Py_CLEAR "the obvious" code can be deadly:
+ *
+ *     Py_XDECREF(op);
+ *     op = op2;
+ *
+ * The safe way is:
+ *
+ *      Py_SETREF(op, op2);
+ *
+ * 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.
+ */
+
+#define Py_SETREF(op, op2)                      \
+    do {                                        \
+        PyObject *_py_tmp = (PyObject *)(op);   \
+        (op) = (op2);                           \
+        Py_XDECREF(_py_tmp);                    \
+    } while (0)
+
+#endif /* ifndef Py_LIMITED_API */
+
 /*
 These are provided as conveniences to Python runtime embedders, so that
 they can have object code that is not dependent on Python compilation flags.

Misc/NEWS

@@ -10,6 +10,9 @@ Release date: tba
 Core and Builtins
 -----------------
 
+- Issue #20440: Massive replacing unsafe attribute setting code with special
+  macro Py_SETREF.
+
 - Issue #25766: Special method __bytes__() now works in str subclasses.
 
 - Issue #25421: __sizeof__ methods of builtin types now use dynamic basic size.

Modules/_csv.c

@@ -276,9 +276,8 @@ _set_str(const char *name, PyObject **target, PyObject *src, const char *dflt)
         else {
             if (PyUnicode_READY(src) == -1)
                 return -1;
-            Py_XDECREF(*target);
             Py_INCREF(src);
-            *target = src;
+            Py_SETREF(*target, src);
         }
     }
     return 0;
@@ -784,8 +783,7 @@ parse_process_char(ReaderObj *self, Py_UCS4 c)
 static int
 parse_reset(ReaderObj *self)
 {
-    Py_XDECREF(self->fields);
+    Py_SETREF(self->fields, PyList_New(0));
-    self->fields = PyList_New(0);
     if (self->fields == NULL)
         return -1;
     self->field_len = 0;

Modules/_ctypes/_ctypes.c

@@ -391,8 +391,7 @@ StructUnionType_new(PyTypeObject *type, PyObject *args, PyObject *kwds, int isSt
         Py_DECREF((PyObject *)dict);
         return NULL;
     }
-    Py_DECREF(result->tp_dict);
+    Py_SETREF(result->tp_dict, (PyObject *)dict);
-    result->tp_dict = (PyObject *)dict;
     dict->format = _ctypes_alloc_format_string(NULL, "B");
     if (dict->format == NULL) {
         Py_DECREF(result);
@@ -871,8 +870,7 @@ PyCPointerType_SetProto(StgDictObject *stgdict, PyObject *proto)
         return -1;
     }
     Py_INCREF(proto);
-    Py_XDECREF(stgdict->proto);
+    Py_SETREF(stgdict->proto, proto);
-    stgdict->proto = proto;
     return 0;
 }
 
@@ -962,8 +960,7 @@ PyCPointerType_new(PyTypeObject *type, PyObject *args, PyObject *kwds)
         Py_DECREF((PyObject *)stgdict);
         return NULL;
     }
-    Py_DECREF(result->tp_dict);
+    Py_SETREF(result->tp_dict, (PyObject *)stgdict);
-    result->tp_dict = (PyObject *)stgdict;
 
     return (PyObject *)result;
 }
@@ -1406,8 +1403,7 @@ PyCArrayType_new(PyTypeObject *type, PyObject *args, PyObject *kwds)
     /* replace the class dict by our updated spam dict */
     if (-1 == PyDict_Update((PyObject *)stgdict, result->tp_dict))
         goto error;
-    Py_DECREF(result->tp_dict);
+    Py_SETREF(result->tp_dict, (PyObject *)stgdict);  /* steal the reference */
-    result->tp_dict = (PyObject *)stgdict;  /* steal the reference */
     stgdict = NULL;
 
     /* Special case for character arrays.
@@ -1820,8 +1816,7 @@ static PyObject *CreateSwappedType(PyTypeObject *type, PyObject *args, PyObject
         Py_DECREF((PyObject *)stgdict);
         return NULL;
     }
-    Py_DECREF(result->tp_dict);
+    Py_SETREF(result->tp_dict, (PyObject *)stgdict);
-    result->tp_dict = (PyObject *)stgdict;
 
     return (PyObject *)result;
 }
@@ -1949,8 +1944,7 @@ PyCSimpleType_new(PyTypeObject *type, PyObject *args, PyObject *kwds)
         Py_DECREF((PyObject *)stgdict);
         return NULL;
     }
-    Py_DECREF(result->tp_dict);
+    Py_SETREF(result->tp_dict, (PyObject *)stgdict);
-    result->tp_dict = (PyObject *)stgdict;
 
     /* Install from_param class methods in ctypes base classes.
        Overrides the PyCSimpleType_from_param generic method.
@@ -2313,8 +2307,7 @@ PyCFuncPtrType_new(PyTypeObject *type, PyObject *args, PyObject *kwds)
         Py_DECREF((PyObject *)stgdict);
         return NULL;
     }
-    Py_DECREF(result->tp_dict);
+    Py_SETREF(result->tp_dict, (PyObject *)stgdict);
-    result->tp_dict = (PyObject *)stgdict;
 
     if (-1 == make_funcptrtype_dict(stgdict)) {
         Py_DECREF(result);
@@ -2458,8 +2451,7 @@ KeepRef(CDataObject *target, Py_ssize_t index, PyObject *keep)
         return -1;
     }
     if (ob->b_objects == NULL || !PyDict_CheckExact(ob->b_objects)) {
-        Py_XDECREF(ob->b_objects);
+        Py_SETREF(ob->b_objects, keep); /* refcount consumed */
-        ob->b_objects = keep; /* refcount consumed */
         return 0;
     }
     key = unique_key(target, index);
@@ -2962,9 +2954,8 @@ PyCFuncPtr_set_errcheck(PyCFuncPtrObject *self, PyObject *ob)
                         "the errcheck attribute must be callable");
         return -1;
     }
-    Py_XDECREF(self->errcheck);
     Py_XINCREF(ob);
-    self->errcheck = ob;
+    Py_SETREF(self->errcheck, ob);
     return 0;
 }
 
@@ -2993,9 +2984,8 @@ PyCFuncPtr_set_restype(PyCFuncPtrObject *self, PyObject *ob)
         return -1;
     }
     Py_XDECREF(self->checker);
-    Py_XDECREF(self->restype);
     Py_INCREF(ob);
-    self->restype = ob;
+    Py_SETREF(self->restype, ob);
     self->checker = PyObject_GetAttrString(ob, "_check_retval_");
     if (self->checker == NULL)
         PyErr_Clear();
@@ -3033,11 +3023,9 @@ PyCFuncPtr_set_argtypes(PyCFuncPtrObject *self, PyObject *ob)
         converters = converters_from_argtypes(ob);
         if (!converters)
             return -1;
-        Py_XDECREF(self->converters);
+        Py_SETREF(self->converters, converters);
-        self->converters = converters;
-        Py_XDECREF(self->argtypes);
         Py_INCREF(ob);
-        self->argtypes = ob;
+        Py_SETREF(self->argtypes, ob);
     }
     return 0;
 }
@@ -5164,9 +5152,8 @@ comerror_init(PyObject *self, PyObject *args, PyObject *kwds)
         return -1;
 
     bself = (PyBaseExceptionObject *)self;
-    Py_DECREF(bself->args);
+    Py_INCREF(args);
-    bself->args = args;
+    Py_SETREF(bself->args, args);
-    Py_INCREF(bself->args);
 
     return 0;
 }

Modules/_curses_panel.c

@@ -312,9 +312,8 @@ PyCursesPanel_replace_panel(PyCursesPanelObject *self, PyObject *args)
         PyErr_SetString(_curses_panelstate_global->PyCursesError, "replace_panel() returned ERR");
         return NULL;
     }
-    Py_DECREF(po->wo);
+    Py_INCREF(temp);
-    po->wo = temp;
+    Py_SETREF(po->wo, temp);
-    Py_INCREF(po->wo);
     Py_INCREF(Py_None);
     return Py_None;
 }

Modules/_io/bytesio.c

@@ -969,8 +969,7 @@ _io_BytesIO___init___impl(bytesio *self, PyObject *initvalue)
     if (initvalue && initvalue != Py_None) {
         if (PyBytes_CheckExact(initvalue)) {
             Py_INCREF(initvalue);
-            Py_XDECREF(self->buf);
+            Py_SETREF(self->buf, initvalue);
-            self->buf = initvalue;
             self->string_size = PyBytes_GET_SIZE(initvalue);
         }
         else {

Modules/_sqlite/connection.c

@@ -204,8 +204,8 @@ void pysqlite_flush_statement_cache(pysqlite_Connection* self)
         node = node->next;
     }
 
-    Py_DECREF(self->statement_cache);
+    Py_SETREF(self->statement_cache,
-    self->statement_cache = (pysqlite_Cache*)PyObject_CallFunction((PyObject*)&pysqlite_CacheType, "O", self);
+              (pysqlite_Cache *)PyObject_CallFunction((PyObject *)&pysqlite_CacheType, "O", self));
     Py_DECREF(self);
     self->statement_cache->decref_factory = 0;
 }
@@ -318,9 +318,8 @@ PyObject* pysqlite_connection_cursor(pysqlite_Connection* self, PyObject* args,
     _pysqlite_drop_unused_cursor_references(self);
 
     if (cursor && self->row_factory != Py_None) {
-        Py_XDECREF(((pysqlite_Cursor*)cursor)->row_factory);
         Py_INCREF(self->row_factory);
-        ((pysqlite_Cursor*)cursor)->row_factory = self->row_factory;
+        Py_SETREF(((pysqlite_Cursor *)cursor)->row_factory, self->row_factory);
     }
 
     return cursor;
@@ -795,8 +794,7 @@ static void _pysqlite_drop_unused_statement_references(pysqlite_Connection* self
         }
     }
 
-    Py_DECREF(self->statements);
+    Py_SETREF(self->statements, new_list);
-    self->statements = new_list;
 }
 
 static void _pysqlite_drop_unused_cursor_references(pysqlite_Connection* self)
@@ -827,8 +825,7 @@ static void _pysqlite_drop_unused_cursor_references(pysqlite_Connection* self)
         }
     }
 
-    Py_DECREF(self->cursors);
+    Py_SETREF(self->cursors, new_list);
-    self->cursors = new_list;
 }
 
 PyObject* pysqlite_connection_create_function(pysqlite_Connection* self, PyObject* args, PyObject* kwargs)

Modules/_sqlite/cursor.c

@@ -170,8 +170,7 @@ int pysqlite_build_row_cast_map(pysqlite_Cursor* self)
         return 0;
     }
 
-    Py_XDECREF(self->row_cast_map);
+    Py_SETREF(self->row_cast_map, PyList_New(0));
-    self->row_cast_map = PyList_New(0);
 
     for (i = 0; i < sqlite3_column_count(self->statement->st); i++) {
         converter = NULL;
@@ -510,9 +509,8 @@ PyObject* _pysqlite_query_execute(pysqlite_Cursor* self, int multiple, PyObject*
         goto error;
 
     /* reset description and rowcount */
-    Py_DECREF(self->description);
     Py_INCREF(Py_None);
-    self->description = Py_None;
+    Py_SETREF(self->description, Py_None);
     self->rowcount = -1L;
 
     func_args = PyTuple_New(1);
@@ -537,8 +535,8 @@ PyObject* _pysqlite_query_execute(pysqlite_Cursor* self, int multiple, PyObject*
     }
 
     if (self->statement->in_use) {
-        Py_DECREF(self->statement);
+        Py_SETREF(self->statement,
-        self->statement = PyObject_New(pysqlite_Statement, &pysqlite_StatementType);
+                  PyObject_New(pysqlite_Statement, &pysqlite_StatementType));
         if (!self->statement) {
             goto error;
         }
@@ -654,8 +652,7 @@ PyObject* _pysqlite_query_execute(pysqlite_Cursor* self, int multiple, PyObject*
                 numcols = sqlite3_column_count(self->statement->st);
                 Py_END_ALLOW_THREADS
 
-                Py_DECREF(self->description);
+                Py_SETREF(self->description, PyTuple_New(numcols));
-                self->description = PyTuple_New(numcols);
                 if (!self->description) {
                     goto error;
                 }

Modules/_sre.c

@@ -753,8 +753,7 @@ deepcopy(PyObject** object, PyObject* memo)
     if (!copy)
         return 0;
 
-    Py_DECREF(*object);
+    Py_SETREF(*object, copy);
-    *object = copy;
 
     return 1; /* success */
 }

Modules/_ssl.c

@@ -1589,8 +1589,7 @@ static int PySSL_set_context(PySSLSocket *self, PyObject *value,
         return -1;
 #else
         Py_INCREF(value);
-        Py_DECREF(self->ctx);
+        Py_SETREF(self->ctx, (PySSLContext *)value);
-        self->ctx = (PySSLContext *) value;
         SSL_set_SSL_CTX(self->ssl, self->ctx->ctx);
 #endif
     } else {
@@ -1647,8 +1646,7 @@ PySSL_get_owner(PySSLSocket *self, void *c)
 static int
 PySSL_set_owner(PySSLSocket *self, PyObject *value, void *c)
 {
-    Py_XDECREF(self->owner);
+    Py_SETREF(self->owner, PyWeakref_NewRef(value, NULL));
-    self->owner = PyWeakref_NewRef(value, NULL);
     if (self->owner == NULL)
         return -1;
     return 0;

Modules/faulthandler.c

@@ -380,9 +380,8 @@ faulthandler_enable(PyObject *self, PyObject *args, PyObject *kwargs)
     if (tstate == NULL)
         return NULL;
 
-    Py_XDECREF(fatal_error.file);
     Py_XINCREF(file);
-    fatal_error.file = file;
+    Py_SETREF(fatal_error.file, file);
     fatal_error.fd = fd;
     fatal_error.all_threads = all_threads;
     fatal_error.interp = tstate->interp;
@@ -599,9 +598,8 @@ faulthandler_dump_traceback_later(PyObject *self,
     /* Cancel previous thread, if running */
     cancel_dump_traceback_later();
 
-    Py_XDECREF(thread.file);
     Py_XINCREF(file);
-    thread.file = file;
+    Py_SETREF(thread.file, file);
     thread.fd = fd;
     thread.timeout_us = timeout_us;
     thread.repeat = repeat;
@@ -778,9 +776,8 @@ faulthandler_register_py(PyObject *self,
         user->previous = previous;
     }
 
-    Py_XDECREF(user->file);
     Py_XINCREF(file);
-    user->file = file;
+    Py_SETREF(user->file, file);
     user->fd = fd;
     user->all_threads = all_threads;
     user->chain = chain;

Modules/itertoolsmodule.c

@@ -634,8 +634,7 @@ tee_next(teeobject *to)
         link = teedataobject_jumplink(to->dataobj);
         if (link == NULL)
             return NULL;
-        Py_DECREF(to->dataobj);
+        Py_SETREF(to->dataobj, (teedataobject *)link);
-        to->dataobj = (teedataobject *)link;
         to->index = 0;
     }
     value = teedataobject_getitem(to->dataobj, to->index);

Modules/signalmodule.c

@@ -1266,8 +1266,7 @@ PyInit__signal(void)
     if (Handlers[SIGINT].func == DefaultHandler) {
         /* Install default int handler */
         Py_INCREF(IntHandler);
-        Py_DECREF(Handlers[SIGINT].func);
+        Py_SETREF(Handlers[SIGINT].func, IntHandler);
-        Handlers[SIGINT].func = IntHandler;
         old_siginthandler = PyOS_setsig(SIGINT, signal_handler);
     }
 

Modules/zipimport.c

@@ -155,8 +155,7 @@ zipimporter_init(ZipImporter *self, PyObject *args, PyObject *kwds)
             tmp = PyUnicode_FromFormat("%U%c", self->prefix, SEP);
             if (tmp == NULL)
                 goto error;
-            Py_DECREF(self->prefix);
+            Py_SETREF(self->prefix, tmp);
-            self->prefix = tmp;
         }
     }
     else

Modules/zlibmodule.c

@@ -667,8 +667,7 @@ save_unconsumed_input(compobject *self, int err)
                       PyBytes_AS_STRING(self->unused_data), old_size);
             Py_MEMCPY(PyBytes_AS_STRING(new_data) + old_size,
                       self->zst.next_in, self->zst.avail_in);
-            Py_DECREF(self->unused_data);
+            Py_SETREF(self->unused_data, new_data);
-            self->unused_data = new_data;
             self->zst.avail_in = 0;
         }
     }
@@ -680,8 +679,7 @@ save_unconsumed_input(compobject *self, int err)
                 (char *)self->zst.next_in, self->zst.avail_in);
         if (new_data == NULL)
             return -1;
-        Py_DECREF(self->unconsumed_tail);
+        Py_SETREF(self->unconsumed_tail, new_data);
-        self->unconsumed_tail = new_data;
     }
     return 0;
 }

Objects/bytesobject.c

@@ -3688,8 +3688,7 @@ PyBytes_Concat(PyObject **pv, PyObject *w)
         /* Multiple references, need to create new object */
         PyObject *v;
         v = bytes_concat(*pv, w);
-        Py_DECREF(*pv);
+        Py_SETREF(*pv, v);
-        *pv = v;
     }
 }
 

Objects/descrobject.c

@@ -1509,8 +1509,7 @@ property_init(PyObject *self, PyObject *args, PyObject *kwds)
         PyObject *get_doc = _PyObject_GetAttrId(get, &PyId___doc__);
         if (get_doc) {
             if (Py_TYPE(self) == &PyProperty_Type) {
-                Py_XDECREF(prop->prop_doc);
+                Py_SETREF(prop->prop_doc, get_doc);
-                prop->prop_doc = get_doc;
             }
             else {
                 /* If this is a property subclass, put __doc__

Objects/exceptions.c

@@ -236,8 +236,7 @@ BaseException_set_tb(PyBaseExceptionObject *self, PyObject *tb)
     }
 
     Py_XINCREF(tb);
-    Py_XDECREF(self->traceback);
+    Py_SETREF(self->traceback, tb);
-    self->traceback = tb;
     return 0;
 }
 

Objects/frameobject.c

@@ -857,8 +857,7 @@ dict_to_map(PyObject *map, Py_ssize_t nmap, PyObject *dict, PyObject **values,
             }
         } else if (values[j] != value) {
             Py_XINCREF(value);
-            Py_XDECREF(values[j]);
+            Py_SETREF(values[j], value);
-            values[j] = value;
         }
         Py_XDECREF(value);
     }

Objects/funcobject.c

@@ -127,8 +127,7 @@ PyFunction_SetDefaults(PyObject *op, PyObject *defaults)
         PyErr_SetString(PyExc_SystemError, "non-tuple default args");
         return -1;
     }
-    Py_XDECREF(((PyFunctionObject *) op) -> func_defaults);
+    Py_SETREF(((PyFunctionObject *)op)->func_defaults, defaults);
-    ((PyFunctionObject *) op) -> func_defaults = defaults;
     return 0;
 }
 
@@ -159,8 +158,7 @@ PyFunction_SetKwDefaults(PyObject *op, PyObject *defaults)
                         "non-dict keyword only default args");
         return -1;
     }
-    Py_XDECREF(((PyFunctionObject *)op) -> func_kwdefaults);
+    Py_SETREF(((PyFunctionObject *)op)->func_kwdefaults, defaults);
-    ((PyFunctionObject *) op) -> func_kwdefaults = defaults;
     return 0;
 }
 
@@ -192,8 +190,7 @@ PyFunction_SetClosure(PyObject *op, PyObject *closure)
                      closure->ob_type->tp_name);
         return -1;
     }
-    Py_XDECREF(((PyFunctionObject *) op) -> func_closure);
+    Py_SETREF(((PyFunctionObject *)op)->func_closure, closure);
-    ((PyFunctionObject *) op) -> func_closure = closure;
     return 0;
 }
 
@@ -224,8 +221,7 @@ PyFunction_SetAnnotations(PyObject *op, PyObject *annotations)
                         "non-dict annotations");
         return -1;
     }
-    Py_XDECREF(((PyFunctionObject *)op) -> func_annotations);
+    Py_SETREF(((PyFunctionObject *)op)->func_annotations, annotations);
-    ((PyFunctionObject *) op) -> func_annotations = annotations;
     return 0;
 }
 
@@ -531,8 +527,7 @@ func_new(PyTypeObject* type, PyObject* args, PyObject* kw)
 
     if (name != Py_None) {
         Py_INCREF(name);
-        Py_DECREF(newfunc->func_name);
+        Py_SETREF(newfunc->func_name, name);
-        newfunc->func_name = name;
     }
     if (defaults != Py_None) {
         Py_INCREF(defaults);

Objects/moduleobject.c

@@ -69,8 +69,7 @@ module_init_dict(PyModuleObject *mod, PyObject *md_dict,
         return -1;
     if (PyUnicode_CheckExact(name)) {
         Py_INCREF(name);
-        Py_XDECREF(mod->md_name);
+        Py_SETREF(mod->md_name, name);
-        mod->md_name = name;
     }
 
     return 0;

Objects/rangeobject.c

@@ -1066,8 +1066,7 @@ longrangeiter_next(longrangeiterobject *r)
     result = PyNumber_Add(r->start, product);
     Py_DECREF(product);
     if (result) {
-        Py_DECREF(r->index);
+        Py_SETREF(r->index, new_index);
-        r->index = new_index;
     }
     else {
         Py_DECREF(new_index);

Objects/typeobject.c

@@ -315,9 +315,8 @@ assign_version_tag(PyTypeObject *type)
            are borrowed reference */
         for (i = 0; i < (1 << MCACHE_SIZE_EXP); i++) {
             method_cache[i].value = NULL;
-            Py_XDECREF(method_cache[i].name);
-            method_cache[i].name = Py_None;
             Py_INCREF(Py_None);
+            Py_SETREF(method_cache[i].name, Py_None);
         }
         /* mark all version tags as invalid */
         PyType_Modified(&PyBaseObject_Type);
@@ -462,8 +461,7 @@ type_set_qualname(PyTypeObject *type, PyObject *value, void *context)
 
     et = (PyHeapTypeObject*)type;
     Py_INCREF(value);
-    Py_DECREF(et->ht_qualname);
+    Py_SETREF(et->ht_qualname, value);
-    et->ht_qualname = value;
     return 0;
 }
 
@@ -2918,8 +2916,7 @@ _PyType_Lookup(PyTypeObject *type, PyObject *name)
         else
             method_cache_misses++;
 #endif
-        Py_DECREF(method_cache[h].name);
+        Py_SETREF(method_cache[h].name, name);
-        method_cache[h].name = name;
     }
     return res;
 }

Objects/unicodeobject.c

@@ -1828,8 +1828,7 @@ unicode_resize(PyObject **p_unicode, Py_ssize_t length)
         _Py_INCREF_UNICODE_EMPTY();
         if (!unicode_empty)
             return -1;
-        Py_DECREF(*p_unicode);
+        Py_SETREF(*p_unicode, unicode_empty);
-        *p_unicode = unicode_empty;
         return 0;
     }
 
@@ -1837,8 +1836,7 @@ unicode_resize(PyObject **p_unicode, Py_ssize_t length)
         PyObject *copy = resize_copy(unicode, length);
         if (copy == NULL)
             return -1;
-        Py_DECREF(*p_unicode);
+        Py_SETREF(*p_unicode, copy);
-        *p_unicode = copy;
         return 0;
     }
 
@@ -13543,8 +13541,7 @@ _PyUnicodeWriter_PrepareInternal(_PyUnicodeWriter *writer,
             return -1;
         _PyUnicode_FastCopyCharacters(newbuffer, 0,
                                       writer->buffer, 0, writer->pos);
-        Py_DECREF(writer->buffer);
+        Py_SETREF(writer->buffer, newbuffer);
-        writer->buffer = newbuffer;
     }
     _PyUnicodeWriter_Update(writer);
     return 0;
@@ -15261,8 +15258,7 @@ PyUnicode_InternInPlace(PyObject **p)
 
     if (t) {
         Py_INCREF(t);
-        Py_DECREF(*p);
+        Py_SETREF(*p, t);
-        *p = t;
         return;
     }
 

Parser/tokenizer.c

@@ -524,9 +524,8 @@ fp_setreadl(struct tok_state *tok, const char* enc)
     if (stream == NULL)
         goto cleanup;
 
-    Py_XDECREF(tok->decoding_readline);
     readline = _PyObject_GetAttrId(stream, &PyId_readline);
-    tok->decoding_readline = readline;
+    Py_SETREF(tok->decoding_readline, readline);
     if (pos > 0) {
         if (PyObject_CallObject(readline, NULL) == NULL) {
             readline = NULL;

Python/_warnings.c

@@ -680,8 +680,7 @@ setup_context(Py_ssize_t stack_level, PyObject **filename, int *lineno,
                     goto handle_error;
                 }
                 else if (!is_true) {
-                    Py_DECREF(*filename);
+                    Py_SETREF(*filename, PyUnicode_FromString("__main__"));
-                    *filename = PyUnicode_FromString("__main__");
                     if (*filename == NULL)
                         goto handle_error;
                 }

Python/ceval.c

@@ -3221,8 +3221,7 @@ PyEval_EvalFrameEx(PyFrameObject *f, int throwflag)
                 Py_INCREF(self);
                 func = PyMethod_GET_FUNCTION(func);
                 Py_INCREF(func);
-                Py_DECREF(*pfunc);
+                Py_SETREF(*pfunc, self);
-                *pfunc = self;
                 na++;
                 /* n++; */
             } else
@@ -4734,8 +4733,7 @@ call_function(PyObject ***pp_stack, int oparg
             Py_INCREF(self);
             func = PyMethod_GET_FUNCTION(func);
             Py_INCREF(func);
-            Py_DECREF(*pfunc);
+            Py_SETREF(*pfunc, self);
-            *pfunc = self;
             na++;
             n++;
         } else

Python/compile.c

@@ -1800,8 +1800,7 @@ compiler_class(struct compiler *c, stmt_ty s)
     {
         /* use the class name for name mangling */
         Py_INCREF(s->v.ClassDef.name);
-        Py_XDECREF(c->u->u_private);
+        Py_SETREF(c->u->u_private, s->v.ClassDef.name);
-        c->u->u_private = s->v.ClassDef.name;
         /* load (global) __name__ ... */
         str = PyUnicode_InternFromString("__name__");
         if (!str || !compiler_nameop(c, str, Load)) {