presto-main/src/main/java/io/prestosql/operator/ExchangeClient.java/ExchangeClient/pollPage
class ExchangeClient:    @Nullable
    public SerializedPage pollPage()
    {
        checkState(!Thread.holdsLock(this), "Can not get next page while holding a lock on this");

        throwIfFailed();

        if (closed.get()) {
            return null;
        }

        SerializedPage page = pageBuffer.poll();
        return postProcessPage(page);
    }