presto-main/src/main/java/io/prestosql/operator/index/PageBufferOperator.java/PageBufferOperator/addInput
class PageBufferOperator:    @Override
    public void addInput(Page page)
    {
        requireNonNull(page, "page is null");
        checkState(blocked == NOT_BLOCKED, "output is already blocked");
        ListenableFuture<?> future = pageBuffer.add(page);
        if (!future.isDone()) {
            this.blocked = future;
        }
        operatorContext.recordOutput(page.getSizeInBytes(), page.getPositionCount());
    }