can't seem to get progress events from node-formidable to send to the correct client over socket.io

Question

can't seem to get progress events from node-formidable to send to the correct client over socket.io

So I'm building a multipart form uploader over ajax on node.js, and sending progress events back to the client over socket.io to show the status of their upload. Everything works just fine until I have multiple clients trying to upload at the same time. Originally what would happen is while one upload is going, when a second one starts up it begins receiving progress events from both of the forms being parsed. The original form does not get affected and it only receives progress updates for itself. I tried creating a new formidable form object and storing it in an array along with the socket's session id to try to fix this, but now the first form stops receiving events while the second form gets processed. Here is my server code:

var http        = require('http'),
formidable  = require('formidable'),
fs          = require('fs'), 
io          = require('socket.io'),
mime        = require('mime'),
forms       = {};

var server = http.createServer(function (req, res) {

if (req.url.split("?")[0] == "/upload") {
    console.log("hit upload");
    if (req.method.toLowerCase() === 'post') {
        socket_id = req.url.split("sid=")[1];
        forms[socket_id] = new formidable.IncomingForm();
        form = forms[socket_id];

        form.addListener('progress', function (bytesReceived, bytesExpected) {
            progress = (bytesReceived / bytesExpected * 100).toFixed(0);

            socket.sockets.socket(socket_id).send(progress);
        });

        form.parse(req, function (err, fields, files) {
            file_name = escape(files.upload.name);

            fs.writeFile(file_name, files.upload, 'utf8', function (err) {
                if (err) throw err;
                console.log(file_name);
            })
        });
    }
}
});

var socket = io.listen(server);
server.listen(8000);

If anyone could be any help on this I would greatly appreciate it. I've been banging my head against my desk for a few days trying to figure this one out, and would really just like to get this solved so that I can move on. Thank you so much in advance!

ajax
node.js
socket.io
multipart-form
formidable

Answer 1

Can you try putting console.log(socket_id);

after form = forms[socket_id]; and
after progress = (bytesReceived / bytesExpected * 100).toFixed(0);, please?

I get the feeling that you might have to wrap that socket_id in a closure, like this:

form.addListener(
  'progress', 
  (function(socket_id) {
    return function (bytesReceived, bytesExpected) {
        progress = (bytesReceived / bytesExpected * 100).toFixed(0);
        socket.sockets.socket(socket_id).send(progress);
    };
  })(socket_id)
);

Answer 2

The problem is that you aren't declaring socket_id and form with var, so they're actually global.socket_id and global.form rather than local variables of your request handler. Consequently, separate requests step over each other since the callbacks are referring to the globals rather than being proper closures.

rdrey's solution works because it bypasses that problem (though only for socket_id; if you were to change the code in such a way that one of the callbacks referenced form you'd get in trouble). Normally you only need to use his technique if the variable in question is something that changes in the course of executing the outer function (e.g. if you're creating closures within a loop).