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