gh-102013: Add PyUnstable_GC_VisitObjects (#102014)

Modules/gcmodule.c

@@ -2401,3 +2401,27 @@ PyObject_GC_IsFinalized(PyObject *obj)
     }
     return 0;
 }
+
+void
+PyUnstable_GC_VisitObjects(gcvisitobjects_t callback, void *arg)
+{
+    size_t i;
+    GCState *gcstate = get_gc_state();
+    int origenstate = gcstate->enabled;
+    gcstate->enabled = 0;
+    for (i = 0; i < NUM_GENERATIONS; i++) {
+        PyGC_Head *gc_list, *gc;
+        gc_list = GEN_HEAD(gcstate, i);
+        for (gc = GC_NEXT(gc_list); gc != gc_list; gc = GC_NEXT(gc)) {
+            PyObject *op = FROM_GC(gc);
+            Py_INCREF(op);
+            int res = callback(op, arg);
+            Py_DECREF(op);
+            if (!res) {
+                goto done;
+            }
+        }
+    }
+done:
+    gcstate->enabled = origenstate;
+}