presto-main/src/main/java/io/prestosql/operator/PartitionedOutputOperator.java/PartitionedOutputOperator/isBlocked
class PartitionedOutputOperator:    @Override
    public ListenableFuture<?> isBlocked()
    {
        ListenableFuture<?> blocked = partitionFunction.isFull();
        return blocked.isDone() ? NOT_BLOCKED : blocked;
    }