presto-main/src/main/java/io/prestosql/operator/MergeOperator.java/MergeOperator/isBlocked
class MergeOperator:    @Override
    public ListenableFuture<?> isBlocked()
    {
        if (!blockedOnSplits.isDone()) {
            return blockedOnSplits;
        }

        if (mergedPages.isBlocked()) {
            return mergedPages.getBlockedFuture();
        }

        return NOT_BLOCKED;
    }