Clear weakrefs created by finalizers.

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

@@ -1037,6 +1037,62 @@ handle_weakrefs(PyGC_Head *unreachable, PyGC_Head *old)
     return num_freed;
 }
 
+/* Clear all weakrefs to unreachable objects.  When this returns, no object in
+ * `unreachable` is weakly referenced anymore.
+ */
+static void
+clear_weakrefs(PyGC_Head *unreachable)
+{
+    PyGC_Head *gc;
+    PyGC_Head *next;
+
+    for (gc = GC_NEXT(unreachable); gc != unreachable; gc = next) {
+        PyWeakReference **wrlist;
+
+        PyObject *op = FROM_GC(gc);
+        next = GC_NEXT(gc);
+
+        if (PyWeakref_Check(op)) {
+            /* A weakref inside the unreachable set must be cleared.  If we
+             * allow its callback to execute inside delete_garbage(), it
+             * could expose objects that have tp_clear already called on
+             * them.  Or, it could resurrect unreachable objects.  One way
+             * this can happen is if some container objects do not implement
+             * tp_traverse.  Then, wr_object can be outside the unreachable
+             * set but can be deallocated as a result of breaking the
+             * reference cycle.  If we don't clear the weakref, the callback
+             * will run and potentially cause a crash.  See bpo-38006 for
+             * one example.
+             */
+            _PyWeakref_ClearRef((PyWeakReference *)op);
+        }
+
+        if (! _PyType_SUPPORTS_WEAKREFS(Py_TYPE(op))) {
+            continue;
+        }
+
+        /* It supports weakrefs.  Does it have any?
+         *
+         * This is never triggered for static types so we can avoid the
+         * (slightly) more costly _PyObject_GET_WEAKREFS_LISTPTR().
+         */
+        wrlist = _PyObject_GET_WEAKREFS_LISTPTR_FROM_OFFSET(op);
+
+        /* `op` may have some weakrefs.  March over the list, clear
+         * all the weakrefs.
+         */
+        for (PyWeakReference *wr = *wrlist; wr != NULL; wr = *wrlist) {
+            /* _PyWeakref_ClearRef clears the weakref but leaves
+             * the callback pointer intact.  Obscure:  it also
+             * changes *wrlist.
+             */
+            _PyObject_ASSERT((PyObject *)wr, wr->wr_object == op);
+            _PyWeakref_ClearRef(wr);
+            _PyObject_ASSERT((PyObject *)wr, wr->wr_object == Py_None);
+        }
+    }
+}
+
 static void
 debug_cycle(const char *msg, PyObject *op)
 {
@@ -1751,6 +1807,14 @@ 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.
+     */
+    clear_weakrefs(&final_unreachable);
+
     /* 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

@@ -1494,7 +1494,7 @@ 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.
 static void
-clear_weakrefs(struct collection_state *state)
+handle_weakrefs(struct collection_state *state)
 {
     PyObject *op;
     WORKSTACK_FOR_EACH(&state->unreachable, op) {
@@ -1545,6 +1545,42 @@ clear_weakrefs(struct collection_state *state)
     }
 }
 
+// Clear all weakrefs to unreachable objects.
+static void
+clear_weakrefs(struct collection_state *state)
+{
+    PyObject *op;
+    WORKSTACK_FOR_EACH(&state->unreachable, op) {
+        if (PyWeakref_Check(op)) {
+            // Clear weakrefs that are themselves unreachable to ensure their
+            // callbacks will not be executed later from a `tp_clear()`
+            // inside delete_garbage(). That would be unsafe: it could
+            // resurrect a dead object or access a an already cleared object.
+            // See bpo-38006 for one example.
+            _PyWeakref_ClearRef((PyWeakReference *)op);
+        }
+
+        if (!_PyType_SUPPORTS_WEAKREFS(Py_TYPE(op))) {
+            continue;
+        }
+
+        // NOTE: This is never triggered for static types so we can avoid the
+        // (slightly) more costly _PyObject_GET_WEAKREFS_LISTPTR().
+        PyWeakReference **wrlist = _PyObject_GET_WEAKREFS_LISTPTR_FROM_OFFSET(op);
+
+        // `op` may have some weakrefs.  March over the list, clear
+        // all the weakrefs.
+        for (PyWeakReference *wr = *wrlist; wr != NULL; wr = *wrlist) {
+            // _PyWeakref_ClearRef clears the weakref but leaves
+            // the callback pointer intact.  Obscure: it also
+            // changes *wrlist.
+            _PyObject_ASSERT((PyObject *)wr, wr->wr_object == op);
+            _PyWeakref_ClearRef(wr);
+            _PyObject_ASSERT((PyObject *)wr, wr->wr_object == Py_None);
+        }
+    }
+}
+
 static void
 call_weakref_callbacks(struct collection_state *state)
 {
@@ -2211,7 +2247,7 @@ 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);
+    handle_weakrefs(state);
     _PyEval_StartTheWorld(interp);
 
     // Deallocate any object from the refcount merge step
@@ -2222,11 +2258,19 @@ 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);
+    }
     _PyEval_StartTheWorld(interp);
 
     if (err < 0) {