gh-154561: Check module dict after trying to load submodule

Misc/NEWS.d/next/Core_and_Builtins/2026-07-26-18-34-45.gh-issue-154561.vuDLtN.rst

@@ -0,0 +1,3 @@
+Fix a race condition in the free-threaded build where concurrent access to
+the same lazy-imported submodule attribute could raise :exc:`AttributeError`.
+Patch by Bartosz Sławecki.

Objects/moduleobject.c

@@ -1398,6 +1398,16 @@ _Py_module_getattro_impl(PyModuleObject *m, PyObject *name, int suppress)
     if (PyErr_Occurred()) {
         return NULL;
     }
+    // Other threads could have raced to load the submodule.
+    // Re-check the module dict to avoid race condition.
+    attr = _PyObject_GenericGetAttrWithDict((PyObject *)m, name, NULL, 1);
+    if (attr != NULL) {
+        return attr;
+    }
+    if (PyErr_Occurred()) {
+        // pass up non-AttributeError exception
+        return NULL;
+    }
     if (PyDict_GetItemRef(m->md_dict, &_Py_ID(__getattr__), &getattr) < 0) {
         return NULL;
     }