GH-155728: Untrack tuples while unmarshalling

Lib/test/test_marshal.py

@@ -1,3 +1,5 @@
+import gc
+
 from test import support
 from test.support import is_apple_mobile, os_helper, requires_debug_ranges, is_emscripten
 from test.support.script_helper import assert_python_ok
@@ -872,6 +874,55 @@ def test_read_object_from_file(self):
                 _testcapi.pymarshal_read_object_from_file(os_helper.TESTFN)
             os_helper.unlink(os_helper.TESTFN)
 
+@support.cpython_only
+class GCTrackingTestCase(unittest.TestCase):
+
+    def _not_tracked_instantly(self, t):
+        new = marshal.loads(marshal.dumps(t))
+
+        self.assertFalse(gc.is_tracked(t), t)
+        self.assertFalse(gc.is_tracked(new), new)
+
+    def _not_tracked(self, t):
+        # Nested tuples can take several collections to untrack
+        gc.collect()
+        gc.collect()
+
+        new = marshal.loads(marshal.dumps(t))
+
+        self.assertFalse(gc.is_tracked(t), t)
+        self.assertFalse(gc.is_tracked(new), new)
+
+    def _tracked(self, t):
+        new = marshal.loads(marshal.dumps(t))
+
+        self.assertTrue(gc.is_tracked(t), t)
+        self.assertTrue(gc.is_tracked(new), new)
+
+    def testTuple(self):
+        x, y, z = 1.5, "a", []
+
+        self._not_tracked_instantly(())
+        self._not_tracked_instantly((1,))
+        self._not_tracked_instantly((1, 2))
+        self._not_tracked_instantly((1, 2, "a"))
+        self._not_tracked_instantly((12, 10**10, 'a_' * 100))
+
+        # Test for _PyTuple_Concat
+        self._not_tracked_instantly((1, 2) + (2, 3))
+
+        # Test for _PyTuple_Repeat
+        self._not_tracked_instantly((1, 2) * 5)
+
+        self._not_tracked(((1, x), y, (2, 3)))
+        self._not_tracked((1, 2, (True, False, ())))
+
+        self._tracked(([],))
+        self._tracked(([1],))
+        self._tracked(({},))
+        self._tracked((set(),))
+        self._tracked((x, y, z))
+
 
 if __name__ == "__main__":
     unittest.main()

Misc/NEWS.d/next/Core_and_Builtins/2026-08-26-19-40-02.gh-issue-155728.OvrUCl.rst

@@ -0,0 +1,2 @@
+Untrack tuples from the GC when possible during unmarshalling. Patch by
+Sergey Miryanov.

Python/marshal.c

@@ -1180,6 +1180,7 @@ r_object(RFILE *p)
     int type, code = r_byte(p);
     int flag, is_interned = 0;
     PyObject *retval = NULL;
+    bool track_tuple = false;
 
     if (code == EOF) {
         if (PyErr_ExceptionMatches(PyExc_EOFError)) {
@@ -1422,16 +1423,28 @@ r_object(RFILE *p)
         if (v == NULL)
             break;
 
+        // empty tuples are untracked, and we can check if n > 0,
+        // but using PyObject_GC_UnTrack is clearer
+        PyObject_GC_UnTrack(v);
+        track_tuple = false;
+
         for (i = 0; i < n; i++) {
             v2 = r_object(p);
             if ( v2 == NULL ) {
                 if (!PyErr_Occurred())
                     PyErr_SetString(PyExc_TypeError,
                         "NULL object in marshal data for tuple");
                 Py_SETREF(v, NULL);
+                track_tuple = false;
                 break;
             }
             PyTuple_SET_ITEM(v, i, v2);
+            if (!track_tuple && PyObject_GC_IsTracked(v2)) {
+                track_tuple = true;
+            }
+        }
+        if (track_tuple) {
+            _PyObject_GC_TRACK(v);
         }
         retval = v;
         break;