gh-160682: Track running function objects in free-threaded GC

Lib/test/test_free_threading/test_gc.py

@@ -97,6 +97,19 @@ def evil():
         thread.start()
         thread.join()
 
+    def test_running_method_not_collected(self):
+        class C:
+            def m(self):
+                wr = weakref.ref(C.__dict__["m"])
+                del C.m
+                gc.collect()
+                return wr() is not None
+
+            def run(self):
+                return self.m()
+
+        self.assertTrue(C().run())
+
     def test_merge_brc_queue_of_detached_thread(self):
         # GH-157838: objects queued for merging by a thread that is detached
         # (blocked in a lock acquire, sleep, etc.) are merged and freed on its

Misc/NEWS.d/next/Core_and_Builtins/2026-10-26-12-00-00.gh-issue-160682.Q7mK2x.rst

@@ -0,0 +1,2 @@
+Fix a bug in the :term:`free-threaded build` where garbage collection could
+prematurely collect the function object of a running method.

Python/gc_free_threading.c

@@ -470,6 +470,7 @@ gc_visit_thread_stacks(PyInterpreterState *interp, struct collection_state *stat
                 continue;
             }
 
+            gc_visit_stackref(f->f_funcobj);
             gc_visit_stackref(f->f_executable);
             while (top != f->localsplus) {
                 --top;
@@ -866,6 +867,10 @@ gc_visit_thread_stacks_mark_alive(PyInterpreterState *interp, gc_mark_args_t *ar
             }
 
             _PyStackRef *top = f->stackpointer;
+            if (gc_visit_stackref_mark_alive(args, f->f_funcobj) < 0) {
+                err = -1;
+                goto exit;
+            }
             if (gc_visit_stackref_mark_alive(args, f->f_executable) < 0) {
                 err = -1;
                 goto exit;