presto-main/src/main/java/io/prestosql/operator/PageBuffer.java/PageBuffer/pages
class PageBuffer:    public WorkProcessor<Page> pages()
    {
        return WorkProcessor.create(() -> {
            if (isFinished() && isEmpty()) {
                return finished();
            }

            if (!initialFuture.isDone()) {
                return blocked(initialFuture);
            }

            if (!isEmpty()) {
                Page result = page;
                page = null;
                return ofResult(result);
            }

            return yield();
        });
    }