presto-main/src/main/java/io/prestosql/operator/ExchangeClient.java/ExchangeClient/pages
class ExchangeClient:    public WorkProcessor<SerializedPage> pages()
    {
        return WorkProcessor.create(() -> {
            SerializedPage page = pollPage();
            if (page == null) {
                if (isFinished()) {
                    return ProcessState.finished();
                }

                ListenableFuture<?> blocked = isBlocked();
                if (!blocked.isDone()) {
                    return ProcessState.blocked(blocked);
                }

                return ProcessState.yield();
            }

            return ProcessState.ofResult(page);
        });
    }