presto-raptor-legacy/src/main/java/io/prestosql/plugin/raptor/legacy/RaptorSplitManager.java/RaptorSplitSource/getNextBatch
class RaptorSplitSource:        @Override
        public CompletableFuture<ConnectorSplitBatch> getNextBatch(ConnectorPartitionHandle partitionHandle, int maxSize)
        {
            checkState((future == null) || future.isDone(), "previous batch not completed");
            future = supplyAsync(batchSupplier(maxSize), executor);
            return future;
        }