gh-157443: Track the argument of sys.unraisablehook in the GC

PyStructSequence_New() does not track the created object, and the
argument of sys.unraisablehook holds the exception and its traceback,
so a reference cycle through it was never collected.

Co-authored-by: Claude Opus 5 (1M context) <noreply@anthropic.com>

Author: serhiy-storchaka

Lib/test/test_sys.py

@@ -1515,6 +1515,27 @@ def hook_func(args):
             expected = None
             hook_args = None
 
+    def test_unraisablehook_args_reference_cycle(self):
+        # gh-157443: The hook argument can be a part of a reference cycle
+        # via the traceback, which must be collected by the GC.
+        import weakref
+        class Holder:
+            pass
+        def f():
+            holder = Holder()
+            def hook(unraisable):
+                holder.unraisable = unraisable
+            with test.support.swap_attr(sys, 'unraisablehook', hook):
+                class C:
+                    def __del__(self):
+                        raise ValueError
+                C()
+            self.assertTrue(gc.is_tracked(holder.unraisable))
+            return weakref.ref(holder)
+        wr = f()
+        support.gc_collect()
+        self.assertIsNone(wr())
+
     def test_custom_unraisablehook_fail(self):
         _testcapi = import_helper.import_module('_testcapi')
         from _testcapi import err_writeunraisable

Misc/NEWS.d/next/Core_and_Builtins/2026-09-13-18-11-56.gh-issue-157443.3jfom1.rst

@@ -0,0 +1,3 @@
+The argument of :func:`sys.unraisablehook` is now tracked by the garbage
+collector, so that a reference cycle through it (for example via the
+traceback) can be collected.

Python/errors.c

@@ -1447,6 +1447,9 @@ make_unraisable_hook_args(PyThreadState *tstate, PyObject *exc_type,
         Py_DECREF(args);
         return NULL;
     }
+    // PyStructSequence_New() does not track the object, and it can be
+    // a part of a reference cycle via the traceback.
+    PyObject_GC_Track(args);
     return args;
 }