presto-blackhole/src/main/java/io/prestosql/plugin/blackhole/BlackHolePageSource.java/BlackHolePageSource/getNextPage
class BlackHolePageSource:    @Override
    public Page getNextPage()
    {
        if (isFinished()) {
            return null;
        }

        if (currentPage != null) {
            Page page = getFutureValue(currentPage);
            currentPage = null;
            return page;
        }

        pagesLeft--;
        completedBytes += page.getSizeInBytes();

        if (pageProcessingDelayInMillis == 0) {
            return page;
        }
        else {
            currentPage = toCompletableFuture(executorService.schedule(() -> page, pageProcessingDelayInMillis, MILLISECONDS));
            return null;
        }
    }