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GH-91636: Clear weakrefs created by finalizers. (GH-136401)
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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.
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4 changed files with 54 additions and 14 deletions
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@ -262,9 +262,11 @@ class GCTests(unittest.TestCase):
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# finalizer.
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def __del__(self):
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# 5. Create a weakref to `func` now. If we had created
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# it earlier, it would have been cleared by the
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# garbage collector before calling the finalizers.
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# 5. Create a weakref to `func` now. In previous
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# versions of Python, this would avoid having it
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# cleared by the garbage collector before calling
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# the finalizers. Now, weakrefs get cleared after
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# calling finalizers.
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self[1].ref = weakref.ref(self[0])
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# 6. Drop the global reference to `latefin`. The only
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@ -293,14 +295,18 @@ class GCTests(unittest.TestCase):
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# which will find `cyc` and `func` as garbage.
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gc.collect()
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# 9. Previously, this would crash because `func_qualname`
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# had been NULL-ed out by func_clear().
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# 9. Previously, this would crash because the weakref
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# created in the finalizer revealed the function after
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# `tp_clear` was called and `func_qualname`
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# had been NULL-ed out by func_clear(). Now, we clear
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# weakrefs to unreachable objects before calling `tp_clear`
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# but after calling finalizers.
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print(f"{func=}")
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"""
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# We're mostly just checking that this doesn't crash.
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rc, stdout, stderr = assert_python_ok("-c", code)
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self.assertEqual(rc, 0)
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self.assertRegex(stdout, rb"""\A\s*func=<function at \S+>\s*\z""")
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# The `func` global is None because the weakref was cleared.
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self.assertRegex(stdout, rb"""\A\s*func=None""")
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self.assertFalse(stderr)
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@refcount_test
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@ -0,0 +1,3 @@
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While performing garbage collection, clear weakrefs to unreachable objects
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that are created during running of finalizers. If those weakrefs were are
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not cleared, they could reveal unreachable objects.
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23
Python/gc.c
23
Python/gc.c
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@ -870,7 +870,7 @@ move_legacy_finalizer_reachable(PyGC_Head *finalizers)
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* no object in `unreachable` is weakly referenced anymore.
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*/
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static int
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handle_weakrefs(PyGC_Head *unreachable, PyGC_Head *old)
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handle_weakrefs(PyGC_Head *unreachable, PyGC_Head *old, bool allow_callbacks)
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{
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PyGC_Head *gc;
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PyObject *op; /* generally FROM_GC(gc) */
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@ -879,7 +879,9 @@ handle_weakrefs(PyGC_Head *unreachable, PyGC_Head *old)
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PyGC_Head *next;
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int num_freed = 0;
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if (allow_callbacks) {
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gc_list_init(&wrcb_to_call);
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}
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/* Clear all weakrefs to the objects in unreachable. If such a weakref
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* also has a callback, move it into `wrcb_to_call` if the callback
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@ -935,6 +937,11 @@ handle_weakrefs(PyGC_Head *unreachable, PyGC_Head *old)
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_PyObject_ASSERT((PyObject *)wr, wr->wr_object == op);
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_PyWeakref_ClearRef(wr);
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_PyObject_ASSERT((PyObject *)wr, wr->wr_object == Py_None);
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if (!allow_callbacks) {
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continue;
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}
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if (wr->wr_callback == NULL) {
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/* no callback */
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continue;
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@ -987,6 +994,10 @@ handle_weakrefs(PyGC_Head *unreachable, PyGC_Head *old)
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}
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}
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if (!allow_callbacks) {
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return 0;
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}
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/* Invoke the callbacks we decided to honor. It's safe to invoke them
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* because they can't reference unreachable objects.
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*/
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@ -1737,7 +1748,7 @@ gc_collect_region(PyThreadState *tstate,
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}
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/* Clear weakrefs and invoke callbacks as necessary. */
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stats->collected += handle_weakrefs(&unreachable, to);
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stats->collected += handle_weakrefs(&unreachable, to, true);
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gc_list_validate_space(to, gcstate->visited_space);
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validate_list(to, collecting_clear_unreachable_clear);
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validate_list(&unreachable, collecting_set_unreachable_clear);
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@ -1751,6 +1762,14 @@ gc_collect_region(PyThreadState *tstate,
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gc_list_init(&final_unreachable);
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handle_resurrected_objects(&unreachable, &final_unreachable, to);
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/* Clear weakrefs to objects in the unreachable set. No Python-level
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* code must be allowed to access those unreachable objects. During
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* delete_garbage(), finalizers outside the unreachable set might run
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* and create new weakrefs. If those weakrefs were not cleared, they
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* could reveal unreachable objects. Callbacks are not executed.
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*/
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handle_weakrefs(&final_unreachable, NULL, false);
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/* Call tp_clear on objects in the final_unreachable set. This will cause
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* the reference cycles to be broken. It may also cause some objects
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* in finalizers to be freed.
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@ -1492,9 +1492,9 @@ move_legacy_finalizer_reachable(struct collection_state *state)
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}
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// Clear all weakrefs to unreachable objects. Weakrefs with callbacks are
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// enqueued in `wrcb_to_call`, but not invoked yet.
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// optionally enqueued in `wrcb_to_call`, but not invoked yet.
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static void
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clear_weakrefs(struct collection_state *state)
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clear_weakrefs(struct collection_state *state, bool enqueue_callbacks)
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{
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PyObject *op;
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WORKSTACK_FOR_EACH(&state->unreachable, op) {
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@ -1526,6 +1526,10 @@ clear_weakrefs(struct collection_state *state)
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_PyWeakref_ClearRef(wr);
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_PyObject_ASSERT((PyObject *)wr, wr->wr_object == Py_None);
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if (!enqueue_callbacks) {
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continue;
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}
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// We do not invoke callbacks for weakrefs that are themselves
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// unreachable. This is partly for historical reasons: weakrefs
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// predate safe object finalization, and a weakref that is itself
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@ -2211,7 +2215,7 @@ gc_collect_internal(PyInterpreterState *interp, struct collection_state *state,
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interp->gc.long_lived_total = state->long_lived_total;
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// Clear weakrefs and enqueue callbacks (but do not call them).
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clear_weakrefs(state);
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clear_weakrefs(state, true);
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_PyEval_StartTheWorld(interp);
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// Deallocate any object from the refcount merge step
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@ -2222,11 +2226,19 @@ gc_collect_internal(PyInterpreterState *interp, struct collection_state *state,
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call_weakref_callbacks(state);
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finalize_garbage(state);
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// Handle any objects that may have resurrected after the finalization.
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_PyEval_StopTheWorld(interp);
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// Handle any objects that may have resurrected after the finalization.
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err = handle_resurrected_objects(state);
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// Clear free lists in all threads
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_PyGC_ClearAllFreeLists(interp);
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if (err == 0) {
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// Clear weakrefs to objects in the unreachable set. No Python-level
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// code must be allowed to access those unreachable objects. During
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// delete_garbage(), finalizers outside the unreachable set might
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// run and create new weakrefs. If those weakrefs were not cleared,
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// they could reveal unreachable objects.
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clear_weakrefs(state, false);
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}
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_PyEval_StartTheWorld(interp);
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if (err < 0) {
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