调用一个 nodeapi 始终没有返回对象

来实现页面截屏功能，为了避免重复截屏就加入了锁。不知道是哪个环节有问题，求大牛看看。。

router.post('/shot', function(req, res) {
  var token = req.param('token'); 
  var url = req.param('url', false);

  debug('url : ' + url);
  if (!url) { res.send('must with url') }
  url = decodeURIComponent(url);
  var filename = '';
  var uri = new URI(url);
  uri.search(function(data){
    filename = data['preview_theme_id'] + '.png';
  });

  var mongo = req.mongo;
  var collection = mongo.collection('web_shots');
  collection.findOne({'url_md5':filename}, function(err, doc){
    if(!err && doc){
      debug(JSON.stringify(doc));
      return res.send(doc.content);
    } else {
      debug('filename: ', filename);
      mongo.collection('web_shots_lock').findOne({'url_md5':filename},function(err,doc){
        if(!doc)  { 
          mongo.collection('web_shots_lock').save({url_md5:filename,created_at:new Date()},{w:1},function(){
            shot.shot(url, function (err, data) {
              if(!err && data) {
                mongo.collection('web_shots').save({url_md5:filename,content:data,created_at:new Date()},{w:1},function(){
                  debug('update success!');
                  mongo.collection('web_shots_lock').findAndRemove({url_md5:filename},function(){});
                });
              } else {
                mongo.collection('web_shots_lock').findAndRemove({url_md5:filename},function(){});
              }
            });
          });
          return res.status(201).end();
        }
      });
      return res.status(202).end();
    }
  });
});