presto-raptor-legacy/src/main/java/io/prestosql/plugin/raptor/legacy/RaptorSplitManager.java/RaptorSplitSource/close
class RaptorSplitSource:        @Override
        public synchronized void close()
        {
            if (future != null) {
                future.cancel(true);
                future = null;
            }
            executor.execute(iterator::close);
        }