gh-106168: PyTuple_SET_ITEM() now checks the index (#106164)

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.

* list_extend() and _PyList_AppendTakeRef() now set the list size
  before calling PyList_SET_ITEM().
* PyStructSequence_GetItem() and PyStructSequence_SetItem() now check
  the index argument: must be lesser than REAL_SIZE(op).
* PyStructSequence_GET_ITEM() and PyStructSequence_SET_ITEM() are now
  aliases to PyStructSequence_GetItem() and
  PyStructSequence_SetItem().

2 files changed, 21 insertions, 7 deletions

diff --git a/Objects/listobject.c b/Objects/listobject.c
index f1edfb3..f1f324f 100644
--- a/Objects/listobject.c
+++ b/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)
diff --git a/Objects/structseq.c b/Objects/structseq.c
index 8b18959..4901113 100644
--- a/Objects/structseq.c
+++ b/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) {