pythonGH-91636: Clear weakrefs created by finalizers. (pythonGH-136401)

Weakrefs to unreachable garbage that are created during running of
finalizers need to be cleared.  This avoids exposing objects that
have `tp_clear` called on them to Python-level code.

Author: nascheme

Lib/test/test_gc.py

@@ -262,9 +262,11 @@ class Cyclic(tuple):
             #    finalizer.
             def __del__(self):
 
-                # 5. Create a weakref to `func` now. If we had created
-                #    it earlier, it would have been cleared by the
-                #    garbage collector before calling the finalizers.
+                # 5. Create a weakref to `func` now. In previous
+                #    versions of Python, this would avoid having it
+                #    cleared by the garbage collector before calling
+                #    the finalizers.  Now, weakrefs get cleared after
+                #    calling finalizers.
                 self[1].ref = weakref.ref(self[0])
 
                 # 6. Drop the global reference to `latefin`. The only
@@ -293,14 +295,18 @@ def func():
         #    which will find `cyc` and `func` as garbage.
         gc.collect()
 
-        # 9. Previously, this would crash because `func_qualname`
-        #    had been NULL-ed out by func_clear().
+        # 9. Previously, this would crash because the weakref
+        #    created in the finalizer revealed the function after
+        #    `tp_clear` was called and `func_qualname`
+        #    had been NULL-ed out by func_clear().  Now, we clear
+        #    weakrefs to unreachable objects before calling `tp_clear`
+        #    but after calling finalizers.
         print(f"{func=}")
         """
-        # We're mostly just checking that this doesn't crash.
         rc, stdout, stderr = assert_python_ok("-c", code)
         self.assertEqual(rc, 0)
-        self.assertRegex(stdout, rb"""\A\s*func=<function  at \S+>\s*\z""")
+        # The `func` global is None because the weakref was cleared.
+        self.assertRegex(stdout, rb"""\A\s*func=None""")
         self.assertFalse(stderr)
 
     @refcount_test

Misc/NEWS.d/next/Core_and_Builtins/2025-07-07-17-26-06.gh-issue-91636.GyHU72.rst

@@ -0,0 +1,3 @@
+While performing garbage collection, clear weakrefs to unreachable objects
+that are created during running of finalizers.  If those weakrefs were are
+not cleared, they could reveal unreachable objects.

Python/gc.c

@@ -870,7 +870,7 @@ move_legacy_finalizer_reachable(PyGC_Head *finalizers)
  * no object in `unreachable` is weakly referenced anymore.
  */
 static int
-handle_weakrefs(PyGC_Head *unreachable, PyGC_Head *old)
+handle_weakrefs(PyGC_Head *unreachable, PyGC_Head *old, bool allow_callbacks)
 {
     PyGC_Head *gc;
     PyObject *op;               /* generally FROM_GC(gc) */
@@ -879,7 +879,9 @@ handle_weakrefs(PyGC_Head *unreachable, PyGC_Head *old)
     PyGC_Head *next;
     int num_freed = 0;
 
-    gc_list_init(&wrcb_to_call);
+    if (allow_callbacks) {
+        gc_list_init(&wrcb_to_call);
+    }
 
     /* Clear all weakrefs to the objects in unreachable.  If such a weakref
      * also has a callback, move it into `wrcb_to_call` if the callback
@@ -935,6 +937,11 @@ handle_weakrefs(PyGC_Head *unreachable, PyGC_Head *old)
             _PyObject_ASSERT((PyObject *)wr, wr->wr_object == op);
             _PyWeakref_ClearRef(wr);
             _PyObject_ASSERT((PyObject *)wr, wr->wr_object == Py_None);
+
+            if (!allow_callbacks) {
+                continue;
+            }
+
             if (wr->wr_callback == NULL) {
                 /* no callback */
                 continue;
@@ -987,6 +994,10 @@ handle_weakrefs(PyGC_Head *unreachable, PyGC_Head *old)
         }
     }
 
+    if (!allow_callbacks) {
+        return 0;
+    }
+
     /* Invoke the callbacks we decided to honor.  It's safe to invoke them
      * because they can't reference unreachable objects.
      */
@@ -1737,7 +1748,7 @@ gc_collect_region(PyThreadState *tstate,
     }
 
     /* Clear weakrefs and invoke callbacks as necessary. */
-    stats->collected += handle_weakrefs(&unreachable, to);
+    stats->collected += handle_weakrefs(&unreachable, to, true);
     gc_list_validate_space(to, gcstate->visited_space);
     validate_list(to, collecting_clear_unreachable_clear);
     validate_list(&unreachable, collecting_set_unreachable_clear);
@@ -1751,6 +1762,14 @@ gc_collect_region(PyThreadState *tstate,
     gc_list_init(&final_unreachable);
     handle_resurrected_objects(&unreachable, &final_unreachable, to);
 
+    /* Clear weakrefs to objects in the unreachable set.  No Python-level
+     * code must be allowed to access those unreachable objects.  During
+     * delete_garbage(), finalizers outside the unreachable set might run
+     * and create new weakrefs.  If those weakrefs were not cleared, they
+     * could reveal unreachable objects.  Callbacks are not executed.
+     */
+    handle_weakrefs(&final_unreachable, NULL, false);
+
     /* Call tp_clear on objects in the final_unreachable set.  This will cause
     * the reference cycles to be broken.  It may also cause some objects
     * in finalizers to be freed.

Python/gc_free_threading.c

@@ -1492,9 +1492,9 @@ move_legacy_finalizer_reachable(struct collection_state *state)
 }
 
 // Clear all weakrefs to unreachable objects. Weakrefs with callbacks are
-// enqueued in `wrcb_to_call`, but not invoked yet.
+// optionally enqueued in `wrcb_to_call`, but not invoked yet.
 static void
-clear_weakrefs(struct collection_state *state)
+clear_weakrefs(struct collection_state *state, bool enqueue_callbacks)
 {
     PyObject *op;
     WORKSTACK_FOR_EACH(&state->unreachable, op) {
@@ -1526,6 +1526,10 @@ clear_weakrefs(struct collection_state *state)
             _PyWeakref_ClearRef(wr);
             _PyObject_ASSERT((PyObject *)wr, wr->wr_object == Py_None);
 
+            if (!enqueue_callbacks) {
+                continue;
+            }
+
             // We do not invoke callbacks for weakrefs that are themselves
             // unreachable. This is partly for historical reasons: weakrefs
             // predate safe object finalization, and a weakref that is itself
@@ -2211,7 +2215,7 @@ gc_collect_internal(PyInterpreterState *interp, struct collection_state *state,
     interp->gc.long_lived_total = state->long_lived_total;
 
     // Clear weakrefs and enqueue callbacks (but do not call them).
-    clear_weakrefs(state);
+    clear_weakrefs(state, true);
     _PyEval_StartTheWorld(interp);
 
     // Deallocate any object from the refcount merge step
@@ -2222,11 +2226,19 @@ gc_collect_internal(PyInterpreterState *interp, struct collection_state *state,
     call_weakref_callbacks(state);
     finalize_garbage(state);
 
-    // Handle any objects that may have resurrected after the finalization.
     _PyEval_StopTheWorld(interp);
+    // Handle any objects that may have resurrected after the finalization.
     err = handle_resurrected_objects(state);
     // Clear free lists in all threads
     _PyGC_ClearAllFreeLists(interp);
+    if (err == 0) {
+        // Clear weakrefs to objects in the unreachable set.  No Python-level
+        // code must be allowed to access those unreachable objects.  During
+        // delete_garbage(), finalizers outside the unreachable set might
+        // run and create new weakrefs.  If those weakrefs were not cleared,
+        // they could reveal unreachable objects.
+        clear_weakrefs(state, false);
+    }
     _PyEval_StartTheWorld(interp);
 
     if (err < 0) {