public String toString() {
        return super.toString() + (completeFuture().isDone() ? " [completed]" : " [incomplete, " + incoming() + "]");
    }
Returns the holder of incoming data to this.
This may be used to add, consume, wait for and be notified about incoming data.
If this instance does not support late incoming data, the read methods of the return object behaves
as expected and is synchronization free. The write methods throws an exception.
container-core/src/main/java/com/yahoo/processing/response/AbstractDataList.java