Read __spec__ from the module dict when checking for initialization

Author: brittanyrey

Lib/test/test_lazy_import/__init__.py

@@ -1372,6 +1372,25 @@ def test_initializing_module_is_still_tracked(self):
         """)
         assert_python_ok("-c", code)
 
+    def test_module_spec_descriptor_is_not_run(self):
+        """Checking whether a module is loaded must not run its descriptors."""
+        code = textwrap.dedent("""
+            import sys
+            import types
+
+            class RaisingSpec(types.ModuleType):
+                @property
+                def __spec__(self):
+                    raise RuntimeError("__spec__ descriptor was run")
+
+            sys.modules["raising_spec"] = RaisingSpec("raising_spec")
+            lazy import raising_spec
+            assert "raising_spec" not in sys.lazy_modules, (
+                f"expected 'raising_spec' untracked, got {sys.lazy_modules}"
+            )
+        """)
+        assert_python_ok("-c", code)
+
     def test_pending_submodule_is_still_tracked(self):
         """`lazy from` a submodule that is not loaded must stay tracked."""
         code = textwrap.dedent("""

Python/import.c

@@ -4390,10 +4390,12 @@ lazy_modules_add(PyThreadState *tstate, PyObject *name)
     }
     // A None entry blocks the import rather than satisfying it.
     int loaded = (existing != NULL && existing != Py_None);
-    if (loaded) {
-        // Check if the module is still initializing.
+    if (loaded && PyModule_Check(existing)) {
+        // Check if the module is still initializing.  Read __spec__ from the
+        // module dict so that a descriptor on a module subclass is not run.
         PyObject *spec;
-        int rc = PyObject_GetOptionalAttr(existing, &_Py_ID(__spec__), &spec);
+        int rc = PyDict_GetItemRef(_PyModule_GetDict(existing),
+                                   &_Py_ID(__spec__), &spec);
         if (rc > 0) {
             rc = _PyModuleSpec_IsInitializing(spec);
             Py_DECREF(spec);