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

        return pages.getBlockedFuture();
    }