bpo-32374: m_traverse may be called with m_state=NULL (GH-5140)

Multi-phase initialized modules allow m_traverse to be called while the
module is still being initialized, so module authors may need to account
for that.

Objects/moduleobject.c

@@ -21,6 +21,17 @@ static PyMemberDef module_members[] = {
     {0}
 };
 
+
+/* Helper for sanity check for traverse not handling m_state == NULL
+ * Issue #32374 */
+#ifdef Py_DEBUG
+static int
+bad_traverse_test(PyObject *self, void *arg) {
+    assert(self != NULL);
+    return 0;
+}
+#endif
+
 PyTypeObject PyModuleDef_Type = {
     PyVarObject_HEAD_INIT(&PyType_Type, 0)
     "moduledef",                                /* tp_name */
@@ -345,6 +356,16 @@ PyModule_FromDefAndSpec2(struct PyModuleDef* def, PyObject *spec, int module_api
         }
     }
 
+    /* Sanity check for traverse not handling m_state == NULL
+     * This doesn't catch all possible cases, but in many cases it should
+     * make many cases of invalid code crash or raise Valgrind issues
+     * sooner than they would otherwise.
+     * Issue #32374 */
+#ifdef Py_DEBUG
+    if (def->m_traverse != NULL) {
+        def->m_traverse(m, bad_traverse_test, NULL);
+    }
+#endif
     Py_DECREF(nameobj);
     return m;