gh-117649: Raise ImportError for unsupported modules in free-threaded build (#117651)

The free-threaded build does not currently support the combination of
single-phase init modules and non-isolated subinterpreters. Ensure that
`check_multi_interp_extensions` is always `True` for subinterpreters in
the free-threaded build so that importing these modules raises an
`ImportError`.

1 files changed, 12 insertions, 1 deletions

diff --git a/Python/pylifecycle.c b/Python/pylifecycle.c
index 4e83b16..efb2587 100644
--- a/Python/pylifecycle.c
+++ b/Python/pylifecycle.c
@@ -559,6 +559,15 @@ init_interp_settings(PyInterpreterState *interp,
         return _PyStatus_ERR("per-interpreter obmalloc does not support "
                              "single-phase init extension modules");
     }
+#ifdef Py_GIL_DISABLED
+    if (!_Py_IsMainInterpreter(interp) &&
+        !config->check_multi_interp_extensions)
+    {
+        return _PyStatus_ERR("The free-threaded build does not support "
+                             "single-phase init extension modules in "
+                             "subinterpreters");
+    }
+#endif
 
     if (config->allow_fork) {
         interp->feature_flags |= Py_RTFLAGS_FORK;
@@ -647,8 +656,10 @@ pycore_create_interpreter(_PyRuntimeState *runtime,
     }
 
     PyInterpreterConfig config = _PyInterpreterConfig_LEGACY_INIT;
-    // The main interpreter always has its own GIL.
+    // The main interpreter always has its own GIL and supports single-phase
+    // init extensions.
     config.gil = PyInterpreterConfig_OWN_GIL;
+    config.check_multi_interp_extensions = 0;
     status = init_interp_settings(interp, &config);
     if (_PyStatus_EXCEPTION(status)) {
         return status;