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(); } }); });