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

macro Py_SETREF.

Author: serhiy-storchaka

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);
-    self->fields = PyList_New(0);
+    Py_SETREF(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);
-    result->tp_dict = (PyObject *)dict;
+    Py_SETREF(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);
-    stgdict->proto = proto;
+    Py_SETREF(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);
-    result->tp_dict = (PyObject *)stgdict;
+    Py_SETREF(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);
-    result->tp_dict = (PyObject *)stgdict;  /* steal the reference */
+    Py_SETREF(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);
-    result->tp_dict = (PyObject *)stgdict;
+    Py_SETREF(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);
-    result->tp_dict = (PyObject *)stgdict;
+    Py_SETREF(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);
-    result->tp_dict = (PyObject *)stgdict;
+    Py_SETREF(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);
-        ob->b_objects = keep; /* refcount consumed */
+        Py_SETREF(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);
-        self->converters = converters;
-        Py_XDECREF(self->argtypes);
+        Py_SETREF(self->converters, converters);
         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);
-    bself->args = args;
-    Py_INCREF(bself->args);
+    Py_INCREF(args);
+    Py_SETREF(bself->args, 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);
-    po->wo = temp;
-    Py_INCREF(po->wo);
+    Py_INCREF(temp);
+    Py_SETREF(po->wo, temp);
     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);
-            self->buf = initvalue;
+            Py_SETREF(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);
-    self->statement_cache = (pysqlite_Cache*)PyObject_CallFunction((PyObject*)&pysqlite_CacheType, "O", self);
+    Py_SETREF(self->statement_cache,
+              (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);
-    self->statements = new_list;
+    Py_SETREF(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);
-    self->cursors = new_list;
+    Py_SETREF(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);
-    self->row_cast_map = PyList_New(0);
+    Py_SETREF(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);
-        self->statement = PyObject_New(pysqlite_Statement, &pysqlite_StatementType);
+        Py_SETREF(self->statement,
+                  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);
-                self->description = PyTuple_New(numcols);
+                Py_SETREF(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);
-    *object = copy;
+    Py_SETREF(*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);
-        self->ctx = (PySSLContext *) value;
+        Py_SETREF(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);
-    self->owner = PyWeakref_NewRef(value, NULL);
+    Py_SETREF(self->owner, PyWeakref_NewRef(value, NULL));
     if (self->owner == NULL)
         return -1;
     return 0;