presto-main/src/main/java/io/prestosql/operator/index/PageBuffer.java/PageBuffer/add
class PageBuffer:    public synchronized ListenableFuture<?> add(Page page)
    {
        checkState(!isFull(), "PageBuffer is full!");
        pages.offer(page);
        if (isFull()) {
            if (settableFuture == null) {
                settableFuture = SettableFuture.create();
            }
            return settableFuture;
        }
        return NOT_FULL;
    }