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