gh-104621: Check for Incompatible Extensions in import_find_extension() (gh-107184)

This fixes a bug where incompatible modules could still be imported if attempted multiple times.

1 files changed, 11 insertions, 10 deletions

diff --git a/Python/import.c b/Python/import.c
index f8cae641a64..711ed5d32ca 100644
--- a/Python/import.c
+++ b/Python/import.c
@@ -1215,6 +1215,15 @@ import_find_extension(PyThreadState *tstate, PyObject *name,
         return NULL;
     }
 
+    /* It may have been successfully imported previously
+       in an interpreter that allows legacy modules
+       but is not allowed in the current interpreter. */
+    const char *name_buf = PyUnicode_AsUTF8(name);
+    assert(name_buf != NULL);
+    if (_PyImport_CheckSubinterpIncompatibleExtensionAllowed(name_buf) < 0) {
+        return NULL;
+    }
+
     PyObject *mod, *mdict;
     PyObject *modules = MODULES(tstate->interp);
 
@@ -3704,16 +3713,8 @@ _imp_create_dynamic_impl(PyObject *module, PyObject *spec, PyObject *file)
 
     PyThreadState *tstate = _PyThreadState_GET();
     mod = import_find_extension(tstate, name, path);
-    if (mod != NULL) {
-        const char *name_buf = PyUnicode_AsUTF8(name);
-        assert(name_buf != NULL);
-        if (_PyImport_CheckSubinterpIncompatibleExtensionAllowed(name_buf) < 0) {
-            Py_DECREF(mod);
-            mod = NULL;
-        }
-        goto finally;
-    }
-    else if (PyErr_Occurred()) {
+    if (mod != NULL || _PyErr_Occurred(tstate)) {
+        assert(mod == NULL || !_PyErr_Occurred(tstate));
         goto finally;
     }