fix: allow lone surrogates in module filenames for crossinterp (gh-156122)

Author: AdityaM06

Lib/test/test_interpreters/test_api.py

@@ -1683,6 +1683,25 @@ def get_count():
         self.assertEqual(after, 0)
         self.assertEqual(counts, [0, 1, 4])
 
+    def test_surrogate_filename_in___main__(self):
+        interp = interpreters.create()
+        import __main__
+        orig_file = getattr(__main__, '__file__', None)
+        try:
+            for surrogate in ('\ud800', '\udcff'):
+                with self.subTest(surrogate=ascii(surrogate)):
+                    __main__.__file__ = f'my_script_{surrogate}.py'
+                    res = interp.call(lambda x: x, [1])
+                    self.assertEqual(res, [1])
+        finally:
+            if orig_file is None:
+                try:
+                    del __main__.__file__
+                except AttributeError:
+                    pass
+            else:
+                __main__.__file__ = orig_file
+
     def test_raises(self):
         interp = interpreters.create()
         with self.assertRaises(ExecutionFailed):

Misc/NEWS.d/next/Core_and_Builtins/2026-08-22-03-54-22.gh-issue-156122.lYvmDy.rst

@@ -0,0 +1,2 @@
+Fix crash when module filenames containing lone surrogates are used during
+cross-interpreter unpickling.

Objects/moduleobject.c

@@ -991,14 +991,17 @@ _PyModule_GetFilenameUTF8(PyObject *mod, char *buffer, Py_ssize_t maxlen)
         size = 0;
     }
     else {
-        const char *filename = PyUnicode_AsUTF8AndSize(filenameobj, &size);
-        assert(size >= 0);
-        if (size > maxlen) {
-            size = -1;
-            PyErr_SetString(PyExc_ValueError, "__file__ too long");
-        }
-        else {
-            (void)strcpy(buffer, filename);
+        PyObject *bytes = PyUnicode_EncodeFSDefault(filenameobj);
+        if (bytes != NULL) {
+            size = PyBytes_GET_SIZE(bytes);
+            if (size > maxlen) {
+                size = -1;
+                PyErr_SetString(PyExc_ValueError, "__file__ too long");
+            }
+            else {
+                memcpy(buffer, PyBytes_AS_STRING(bytes), size + 1);
+            }
+            Py_DECREF(bytes);
         }
     }
     Py_DECREF(filenameobj);

Python/crossinterp.c

@@ -41,7 +41,13 @@ runpy_run_path(const char *filename, const char *modname)
     if (run_path == NULL) {
         return NULL;
     }
-    PyObject *args = Py_BuildValue("(sOs)", filename, Py_None, modname);
+    PyObject *path = PyUnicode_DecodeFSDefault(filename);
+    if (path == NULL) {
+        Py_DECREF(run_path);
+        return NULL;
+    }
+    PyObject *args = Py_BuildValue("(OOs)", path, Py_None, modname);
+    Py_DECREF(path);
     if (args == NULL) {
         Py_DECREF(run_path);
         return NULL;