gh-142029: Raise ModuleNotFoundError instead of crashing on nonexsistent module name given to create_builtin()

Lib/test/test_import/__init__.py

@@ -1253,6 +1253,21 @@ class Spec2:
                 origin = "a\x00b"
             _imp.create_dynamic(Spec2())
 
+    def test_create_builtin(self):
+        class Spec:
+            name = "sys"
+
+        spec = Spec()
+        self.assertIs(_imp.create_builtin(spec), sys)
+
+        spec.name = "builtins"
+        self.assertIs(_imp.create_builtin(spec), builtins)
+
+        # gh-142029
+        spec.name = "nonexistent_lib"
+        with self.assertRaises(ModuleNotFoundError):
+            _imp.create_builtin(spec)
+
     def test_filter_syntax_warnings_by_module(self):
         module_re = r'test\.test_import\.data\.syntax_warnings\z'
         unload('test.test_import.data.syntax_warnings')

Misc/NEWS.d/next/Core_and_Builtins/2025-11-29-08-51-56.gh-issue-142029.rUpcmt.rst

@@ -0,0 +1,2 @@
+Raise :exc:`ModuleNotFoundError` instead of crashing when a nonexistent module
+is used as a name in ``_imp.create_builtin()``.

Python/import.c

@@ -2383,12 +2383,12 @@ is_builtin(PyObject *name)
     return 0;
 }
 
-static PyModInitFunction
-lookup_inittab_initfunc(const struct _Py_ext_module_loader_info* info)
+static struct _inittab*
+lookup_inittab_entry(const struct _Py_ext_module_loader_info* info)
 {
     for (struct _inittab *p = INITTAB; p->name != NULL; p++) {
         if (_PyUnicode_EqualToASCIIString(info->name, p->name)) {
-            return (PyModInitFunction)p->initfunc;
+            return p;
         }
     }
     // not found
@@ -2430,16 +2430,28 @@ create_builtin(
         _extensions_cache_delete(info.path, info.name);
     }
 
-    PyModInitFunction p0 = initfunc;
-    if (p0 == NULL) {
-        p0 = lookup_inittab_initfunc(&info);
-        if (p0 == NULL) {
-            /* Cannot re-init internal module ("sys" or "builtins") */
-            assert(is_core_module(tstate->interp, info.name, info.path));
-            mod = import_add_module(tstate, info.name);
+    PyModInitFunction p0 = NULL;
+    if (initfunc == NULL) {
+        struct _inittab *entry = lookup_inittab_entry(&info);
+        if (entry == NULL) {
+            mod = NULL;
+            _PyErr_SetModuleNotFoundError(name);
             goto finally;
         }
+
+        p0 = (PyModInitFunction)entry->initfunc;
     }
+    else {
+        p0 = initfunc;
+    }
+
+    if (p0 == NULL) {
+        /* Cannot re-init internal module ("sys" or "builtins") */
+        assert(is_core_module(tstate->interp, info.name, info.path));
+        mod = import_add_module(tstate, info.name);
+        goto finally;
+    }
+
 
 #ifdef Py_GIL_DISABLED
     // This call (and the corresponding call to _PyImport_CheckGILForModule())