pythongh-106168: PyTuple_SET_ITEM() now checks the index

PyTuple_SET_ITEM() and PyList_SET_ITEM() now check the index argument
with an assertion if Python is built in debug mode or is built with
assertions.

* PyStructSequence_GET_ITEM() and PyStructSequence_SET_ITEM() are now
  aliases to PyStructSequence_GetItem() and
  PyStructSequence_SetItem()
* PyStructSequence_GetItem() and PyStructSequence_SetItem() now check
  the index argument: must be lesser than REAL_SIZE(op).

Doc/whatsnew/3.13.rst

@@ -441,6 +441,12 @@ New Features
   ``NULL`` if the referent is no longer live.
   (Contributed by Victor Stinner in :gh:`105927`.)
 
+* If Python is built in :ref:`debug mode <debug-build>` or :option:`with
+  assertions <--with-assertions>`, :c:func:`PyTuple_SET_ITEM` and
+  :c:func:`PyList_SET_ITEM` now check the index argument with an assertion.
+  If the assertion fails, make sure that the size is set before.
+  (Contributed by Victor Stinner in :gh:`106168`.)
+
 Porting to Python 3.13
 ----------------------
 

Include/cpython/listobject.h

@@ -41,6 +41,8 @@ static inline Py_ssize_t PyList_GET_SIZE(PyObject *op) {
 static inline void
 PyList_SET_ITEM(PyObject *op, Py_ssize_t index, PyObject *value) {
     PyListObject *list = _PyList_CAST(op);
+    assert(0 <= index);
+    assert(index < Py_SIZE(list));
     list->ob_item[index] = value;
 }
 #define PyList_SET_ITEM(op, index, value) \

Include/cpython/tupleobject.h

@@ -31,6 +31,8 @@ static inline Py_ssize_t PyTuple_GET_SIZE(PyObject *op) {
 static inline void
 PyTuple_SET_ITEM(PyObject *op, Py_ssize_t index, PyObject *value) {
     PyTupleObject *tuple = _PyTuple_CAST(op);
+    assert(0 <= index);
+    assert(index < Py_SIZE(tuple));
     tuple->ob_item[index] = value;
 }
 #define PyTuple_SET_ITEM(op, index, value) \

Include/internal/pycore_list.h

@@ -49,8 +49,8 @@ _PyList_AppendTakeRef(PyListObject *self, PyObject *newitem)
     Py_ssize_t allocated = self->allocated;
     assert((size_t)len + 1 < PY_SSIZE_T_MAX);
     if (allocated > len) {
-        PyList_SET_ITEM(self, len, newitem);
         Py_SET_SIZE(self, len + 1);
+        PyList_SET_ITEM(self, len, newitem);
         return 0;
     }
     return _PyList_AppendTakeRefListResize(self, newitem);

Include/structseq.h

@@ -31,18 +31,15 @@ PyAPI_FUNC(PyTypeObject*) PyStructSequence_NewType(PyStructSequence_Desc *desc);
 
 PyAPI_FUNC(PyObject *) PyStructSequence_New(PyTypeObject* type);
 
+PyAPI_FUNC(void) PyStructSequence_SetItem(PyObject*, Py_ssize_t, PyObject*);
+PyAPI_FUNC(PyObject*) PyStructSequence_GetItem(PyObject*, Py_ssize_t);
+
 #ifndef Py_LIMITED_API
 typedef PyTupleObject PyStructSequence;
-
-/* Macro, *only* to be used to fill in brand new objects */
-#define PyStructSequence_SET_ITEM(op, i, v) PyTuple_SET_ITEM((op), (i), (v))
-
-#define PyStructSequence_GET_ITEM(op, i) PyTuple_GET_ITEM((op), (i))
+#define PyStructSequence_SET_ITEM PyStructSequence_SetItem
+#define PyStructSequence_GET_ITEM PyStructSequence_GetItem
 #endif
 
-PyAPI_FUNC(void) PyStructSequence_SetItem(PyObject*, Py_ssize_t, PyObject*);
-PyAPI_FUNC(PyObject*) PyStructSequence_GetItem(PyObject*, Py_ssize_t);
-
 #ifdef __cplusplus
 }
 #endif

Misc/NEWS.d/next/C API/2023-06-28-02-30-50.gh-issue-106168.NFOZPv.rst

@@ -0,0 +1,5 @@
+If Python is built in :ref:`debug mode <debug-build>` or :option:`with
+assertions <--with-assertions>`, :c:func:`PyTuple_SET_ITEM` and
+:c:func:`PyList_SET_ITEM` now check the index argument with an assertion. If
+the assertion fails, make sure that the size is set before. Patch by Victor
+Stinner.

Objects/listobject.c

@@ -953,8 +953,9 @@ list_extend(PyListObject *self, PyObject *iterable)
         }
         if (Py_SIZE(self) < self->allocated) {
             /* steals ref */
-            PyList_SET_ITEM(self, Py_SIZE(self), item);
-            Py_SET_SIZE(self, Py_SIZE(self) + 1);
+            Py_ssize_t len = Py_SIZE(self);
+            Py_SET_SIZE(self, len + 1);
+            PyList_SET_ITEM(self, len, item);
         }
         else {
             if (_PyList_AppendTakeRef(self, item) < 0)

Objects/structseq.c

@@ -74,15 +74,28 @@ PyStructSequence_New(PyTypeObject *type)
 }
 
 void
-PyStructSequence_SetItem(PyObject* op, Py_ssize_t i, PyObject* v)
+PyStructSequence_SetItem(PyObject *op, Py_ssize_t index, PyObject *value)
 {
-    PyStructSequence_SET_ITEM(op, i, v);
+    PyTupleObject *tuple = _PyTuple_CAST(op);
+    assert(0 <= index);
+#ifndef NDEBUG
+    Py_ssize_t n_fields = REAL_SIZE(op);
+    assert(n_fields >= 0);
+    assert(index < n_fields);
+#endif
+    tuple->ob_item[index] = value;
 }
 
 PyObject*
-PyStructSequence_GetItem(PyObject* op, Py_ssize_t i)
+PyStructSequence_GetItem(PyObject *op, Py_ssize_t index)
 {
-    return PyStructSequence_GET_ITEM(op, i);
+    assert(0 <= index);
+#ifndef NDEBUG
+    Py_ssize_t n_fields = REAL_SIZE(op);
+    assert(n_fields >= 0);
+    assert(index < n_fields);
+#endif
+    return PyTuple_GET_ITEM(op, index);
 }
 
 
@@ -287,7 +300,7 @@ structseq_repr(PyStructSequence *obj)
             goto error;
         }
 
-        PyObject *value = PyStructSequence_GET_ITEM(obj, i);
+        PyObject *value = PyStructSequence_GetItem((PyObject*)obj, i);
         assert(value != NULL);
         PyObject *repr = PyObject_Repr(value);
         if (repr == NULL) {