presto-main/src/main/java/io/prestosql/operator/WorkProcessorPipelineSourceOperator.java/WorkProcessorPipelineSourceOperator/process
class WorkProcessorPipelineSourceOperator:        @Override
        public ProcessState<Split> process()
        {
            boolean noMoreSplits = sourceOperator == null;

            if (pendingSplits.isEmpty()) {
                if (noMoreSplits) {
                    return ProcessState.finished();
                }

                blockedOnSplits = SettableFuture.create();
                blockedFuture = blockedOnSplits;
                return ProcessState.blocked(blockedOnSplits);
            }

            return ProcessState.ofResult(pendingSplits.remove(0));
        }