presto-main/src/main/java/io/prestosql/operator/PageSourceOperator.java/PageSourceOperator/isBlocked
class PageSourceOperator:    @Override
    public ListenableFuture<?> isBlocked()
    {
        CompletableFuture<?> pageSourceBlocked = pageSource.isBlocked();
        return pageSourceBlocked.isDone() ? NOT_BLOCKED : toListenableFuture(pageSourceBlocked);
    }