presto-main/src/main/java/io/prestosql/operator/WorkProcessorSourceOperatorAdapter.java/WorkProcessorSourceOperatorAdapter/process
class WorkProcessorSourceOperatorAdapter:        @Override
        public WorkProcessor.ProcessState<Split> process()
        {
            if (pendingSplits.isEmpty()) {
                if (noMoreSplits) {
                    return finished();
                }

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

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