presto-main/src/main/java/io/prestosql/operator/exchange/LocalMergeSourceOperator.java/LocalMergeSourceOperator/isBlocked
class LocalMergeSourceOperator:    @Override
    public ListenableFuture<?> isBlocked()
    {
        if (mergedPages.isBlocked()) {
            return mergedPages.getBlockedFuture();
        }

        return NOT_BLOCKED;
    }