bpo-44263: Py_TPFLAGS_HAVE_GC requires tp_traverse (GH-26463)

The PyType_Ready() function now raises an error if a type is defined
with the Py_TPFLAGS_HAVE_GC flag set but has no traverse function
(PyTypeObject.tp_traverse).

Doc/whatsnew/3.11.rst

@@ -144,6 +144,11 @@ New Features
 Porting to Python 3.11
 ----------------------
 
+* The :c:func:`PyType_Ready` function now raises an error if a type is defined
+  with the :const:`Py_TPFLAGS_HAVE_GC` flag set but has no traverse function
+  (:c:member:`PyTypeObject.tp_traverse`).
+  (Contributed by Victor Stinner in :issue:`44263`.)
+
 Deprecated
 ----------
 

Misc/NEWS.d/next/C API/2021-05-31-11-31-13.bpo-44263.8mIOfV.rst

@@ -0,0 +1,4 @@
+The :c:func:`PyType_Ready` function now raises an error if a type is defined
+with the :const:`Py_TPFLAGS_HAVE_GC` flag set but has no traverse function
+(:c:member:`PyTypeObject.tp_traverse`).
+Patch by Victor Stinner.

Objects/typeobject.c

@@ -158,6 +158,12 @@ _PyType_CheckConsistency(PyTypeObject *type)
     CHECK(!(type->tp_flags & Py_TPFLAGS_READYING));
     CHECK(type->tp_dict != NULL);
 
+    if (type->tp_flags & Py_TPFLAGS_HAVE_GC) {
+        // bpo-44263: tp_traverse is required if Py_TPFLAGS_HAVE_GC is set.
+        // Note: tp_clear is optional.
+        CHECK(type->tp_traverse != NULL);
+    }
+
     if (type->tp_flags & Py_TPFLAGS_DISALLOW_INSTANTIATION) {
         CHECK(type->tp_new == NULL);
         CHECK(_PyDict_ContainsId(type->tp_dict, &PyId___new__) == 0);
@@ -3607,6 +3613,7 @@ PyType_FromModuleAndSpec(PyObject *module, PyType_Spec *spec, PyObject *bases)
         }
     }
 
+    assert(_PyType_CheckConsistency(type));
     return (PyObject*)res;
 
  fail:
@@ -5944,7 +5951,7 @@ static int add_tp_new_wrapper(PyTypeObject *type);
 #define COLLECTION_FLAGS (Py_TPFLAGS_SEQUENCE | Py_TPFLAGS_MAPPING)
 
 static int
-type_ready_checks(PyTypeObject *type)
+type_ready_pre_checks(PyTypeObject *type)
 {
     /* Consistency checks for PEP 590:
      * - Py_TPFLAGS_METHOD_DESCRIPTOR requires tp_descr_get
@@ -6305,10 +6312,28 @@ type_ready_set_new(PyTypeObject *type)
 }
 
 
+static int
+type_ready_post_checks(PyTypeObject *type)
+{
+    // bpo-44263: tp_traverse is required if Py_TPFLAGS_HAVE_GC is set.
+    // Note: tp_clear is optional.
+    if (type->tp_flags & Py_TPFLAGS_HAVE_GC
+        && type->tp_traverse == NULL)
+    {
+        PyErr_Format(PyExc_SystemError,
+                     "type %s has the Py_TPFLAGS_HAVE_GC flag "
+                     "but has no traverse function",
+                     type->tp_name);
+        return -1;
+    }
+    return 0;
+}
+
+
 static int
 type_ready(PyTypeObject *type)
 {
-    if (type_ready_checks(type) < 0) {
+    if (type_ready_pre_checks(type) < 0) {
         return -1;
     }
 
@@ -6346,6 +6371,9 @@ type_ready(PyTypeObject *type)
     if (type_ready_add_subclasses(type) < 0) {
         return -1;
     }
+    if (type_ready_post_checks(type) < 0) {
+        return -1;
+    }
     return 0;
 }