protected void flushIfLikelyToSuspend(DataList list) throws IOException {
            // If the listener is not complete, we will (likely) suspend rendering
            if ( ! list.incoming().isComplete()) stream.flush();
        }
The listener to completion of the data list this listens to new data in.
container-core/src/main/java/com/yahoo/processing/rendering/AsynchronousSectionedRenderer.java