gh-156117: Fix crash on non-UTF-8 gil in interpreter config

Lib/test/test_interpreters/test_api.py

@@ -2071,6 +2071,10 @@ def test_new_config(self):
             with self.subTest(f'bad override (gil={value!r})'):
                 with self.assertRaises(ValueError):
                     _interpreters.new_config(gil=value)
+        with self.subTest('bad override (gil=lone surrogate)'):
+            # gh-156117: a lone surrogate cannot be encoded as UTF-8
+            with self.assertRaises(UnicodeEncodeError):
+                _interpreters.new_config(gil='\ud800')
 
     def test_get_main(self):
         interpid, whence = _interpreters.get_main()

Misc/NEWS.d/next/Core_and_Builtins/2026-08-21-01-10-18.gh-issue-156117.MEgefB.rst

@@ -0,0 +1,2 @@
+Fix a crash in :func:`!_interpreters.new_config`
+when the ``gil`` argument cannot be encoded as UTF-8.

Python/interpconfig.c

@@ -133,7 +133,12 @@ _config_dict_copy_str(PyObject *dict, const char *name,
         config_dict_invalid_type(name);
         return -1;
     }
-    strncpy(buf, PyUnicode_AsUTF8(item), bufsize-1);
+    const char *str = PyUnicode_AsUTF8(item);
+    if (str == NULL) {
+        Py_DECREF(item);
+        return -1;
+    }
+    strncpy(buf, str, bufsize-1);
     buf[bufsize-1] = '\0';
     Py_DECREF(item);
     return 0;