presto-main/src/main/java/io/prestosql/execution/buffer/LazyOutputBuffer.java/LazyOutputBuffer/acknowledge
class LazyOutputBuffer:    @Override
    public void acknowledge(OutputBufferId bufferId, long token)
    {
        OutputBuffer outputBuffer;
        synchronized (this) {
            checkState(delegate != null, "delegate is null");
            outputBuffer = delegate;
        }
        outputBuffer.acknowledge(bufferId, token);
    }