gh-125859: Fix crash when gc.get_objects is called during GC

Lib/test/test_gc.py

@@ -1065,6 +1065,29 @@ def test_get_referents_on_capsule(self):
         self.assertEqual(len(gc.get_referents(untracked_capsule)), 0)
         gc.get_referents(tracked_capsule)
 
+    @cpython_only
+    def test_get_objects_during_gc(self):
+        # gh-125859: Calling gc.get_objects() or gc.get_referrers() during a
+        # collection should not crash.
+        test = self
+        collected = False
+
+        class GetObjectsOnDel:
+            def __del__(self):
+                nonlocal collected
+                collected = True
+                objs = gc.get_objects()
+                # NB: can't use "in" here because some objects override __eq__
+                for obj in objs:
+                    test.assertTrue(obj is not self)
+                test.assertEqual(gc.get_referrers(self), [])
+
+        obj = GetObjectsOnDel()
+        obj.cycle = obj
+        del obj
+
+        gc.collect()
+        self.assertTrue(collected)
 
 
 class IncrementalGCTests(unittest.TestCase):

Misc/NEWS.d/next/Core_and_Builtins/2024-10-23-14-42-27.gh-issue-125859.m3EF9E.rst

@@ -0,0 +1,2 @@
+Fix a crash in the free threading build when :func:`gc.get_objects` or
+:func:`gc.get_referrers` is called during an in-progress garbage collection.

Python/gc_free_threading.c

@@ -1404,7 +1404,7 @@ gc_collect_main(PyThreadState *tstate, int generation, _PyGC_Reason reason)
 struct get_referrers_args {
     struct visitor_args base;
     PyObject *objs;
-    struct worklist results;
+    _PyObjectStack results;
 };
 
 static int
@@ -1428,11 +1428,17 @@ visit_get_referrers(const mi_heap_t *heap, const mi_heap_area_t *area,
     if (op == NULL) {
         return true;
     }
+    if (op->ob_gc_bits & (_PyGC_BITS_UNREACHABLE | _PyGC_BITS_FROZEN)) {
+        // Exclude unreachable objects (in-progress GC) and frozen
+        // objects from gc.get_objects() to match the default build.
+        return true;
+    }
 
     struct get_referrers_args *arg = (struct get_referrers_args *)args;
     if (Py_TYPE(op)->tp_traverse(op, referrersvisit, arg->objs)) {
-        op->ob_tid = 0;  // we will restore the refcount later
-        worklist_push(&arg->results, op);
+        if (_PyObjectStack_Push(&arg->results, op) < 0) {
+            return false;
+        }
     }
 
     return true;
@@ -1446,43 +1452,36 @@ _PyGC_GetReferrers(PyInterpreterState *interp, PyObject *objs)
         return NULL;
     }
 
+    // NOTE: We can't append to the PyListObject during gc_visit_heaps()
+    // because PyList_Append() may reclaim an abandoned mimalloc segments
+    // while we are traversing them.
+    struct get_referrers_args args = { .objs = objs };
     _PyEval_StopTheWorld(interp);
+    int err = gc_visit_heaps(interp, &visit_get_referrers, &args.base);
+    _PyEval_StartTheWorld(interp);
 
-    // Append all objects to a worklist. This abuses ob_tid. We will restore
-    // it later. NOTE: We can't append to the PyListObject during
-    // gc_visit_heaps() because PyList_Append() may reclaim an abandoned
-    // mimalloc segments while we are traversing them.
-    struct get_referrers_args args = { .objs = objs };
-    gc_visit_heaps(interp, &visit_get_referrers, &args.base);
+    if (err < 0) {
+        PyErr_NoMemory();
+        goto error;
+    }
 
-    bool error = false;
     PyObject *op;
-    while ((op = worklist_pop(&args.results)) != NULL) {
-        gc_restore_tid(op);
+    while ((op = _PyObjectStack_Pop(&args.results)) != NULL) {
         if (op != objs && PyList_Append(result, op) < 0) {
-            error = true;
-            break;
+            goto error;
         }
     }
-
-    // In case of error, clear the remaining worklist
-    while ((op = worklist_pop(&args.results)) != NULL) {
-        gc_restore_tid(op);
-    }
-
-    _PyEval_StartTheWorld(interp);
-
-    if (error) {
-        Py_DECREF(result);
-        return NULL;
-    }
-
     return result;
+
+error:
+    Py_DECREF(result);
+    _PyObjectStack_Clear(&args.results);
+    return NULL;
 }
 
 struct get_objects_args {
     struct visitor_args base;
-    struct worklist objects;
+    _PyObjectStack objects;
 };
 
 static bool
@@ -1493,11 +1492,16 @@ visit_get_objects(const mi_heap_t *heap, const mi_heap_area_t *area,
     if (op == NULL) {
         return true;
     }
+    if (op->ob_gc_bits & (_PyGC_BITS_UNREACHABLE | _PyGC_BITS_FROZEN)) {
+        // Exclude unreachable objects (in-progress GC) and frozen
+        // objects from gc.get_objects() to match the default build.
+        return true;
+    }
 
     struct get_objects_args *arg = (struct get_objects_args *)args;
-    op->ob_tid = 0;  // we will restore the refcount later
-    worklist_push(&arg->objects, op);
-
+    if (_PyObjectStack_Push(&arg->objects, op) < 0) {
+        return false;
+    }
     return true;
 }
 
@@ -1509,38 +1513,31 @@ _PyGC_GetObjects(PyInterpreterState *interp, int generation)
         return NULL;
     }
 
+    // NOTE: We can't append to the PyListObject during gc_visit_heaps()
+    // because PyList_Append() may reclaim an abandoned mimalloc segments
+    // while we are traversing them.
+    struct get_objects_args args = { 0 };
     _PyEval_StopTheWorld(interp);
+    int err = gc_visit_heaps(interp, &visit_get_objects, &args.base);
+    _PyEval_StartTheWorld(interp);
 
-    // Append all objects to a worklist. This abuses ob_tid. We will restore
-    // it later. NOTE: We can't append to the list during gc_visit_heaps()
-    // because PyList_Append() may reclaim an abandoned mimalloc segment
-    // while we are traversing it.
-    struct get_objects_args args = { 0 };
-    gc_visit_heaps(interp, &visit_get_objects, &args.base);
+    if (err < 0) {
+        PyErr_NoMemory();
+        goto error;
+    }
 
-    bool error = false;
     PyObject *op;
-    while ((op = worklist_pop(&args.objects)) != NULL) {
-        gc_restore_tid(op);
+    while ((op = _PyObjectStack_Pop(&args.objects)) != NULL) {
         if (op != result && PyList_Append(result, op) < 0) {
-            error = true;
-            break;
+            goto error;
         }
     }
-
-    // In case of error, clear the remaining worklist
-    while ((op = worklist_pop(&args.objects)) != NULL) {
-        gc_restore_tid(op);
-    }
-
-    _PyEval_StartTheWorld(interp);
-
-    if (error) {
-        Py_DECREF(result);
-        return NULL;
-    }
-
     return result;
+
+error:
+    Py_DECREF(result);
+    _PyObjectStack_Clear(&args.objects);
+    return NULL;
 }
 
 static bool