presto-blackhole/src/main/java/io/prestosql/plugin/blackhole/BlackHolePageSource.java/BlackHolePageSource/isBlocked
class BlackHolePageSource:    @Override
    public CompletableFuture<?> isBlocked()
    {
        if (currentPage != null) {
            return currentPage;
        }
        return NOT_BLOCKED;
    }