[FIX] Proxy upload to correct instance

Proxy the file upload parts to the instance who created the file when running with multiple instances to prevent errors when sticky sessions does not works.

packages/rocketchat-lib/server/models/Avatars.js

+/* globals InstanceStatus */
+
 RocketChat.models.Avatars = new class extends RocketChat.models._Base {
 	constructor() {
 		super('avatars');
 
+		this.model.before.insert((userId, doc) => {
+			doc.instanceId = InstanceStatus.id();
+		});
+
 		this.tryEnsureIndex({ name: 1 });
 	}
 

packages/rocketchat-lib/server/models/Uploads.js

+/* globals InstanceStatus */
+
 RocketChat.models.Uploads = new class extends RocketChat.models._Base {
 	constructor() {
 		super('uploads');
 
+		this.model.before.insert((userId, doc) => {
+			doc.instanceId = InstanceStatus.id();
+		});
+
 		this.tryEnsureIndex({ 'rid': 1 });
 		this.tryEnsureIndex({ 'uploadedAt': 1 });
 	}

server/startup/avatar.js

-/* globals FileUpload */
+/* globals FileUpload, UploadFS, InstanceStatus */
+
+import http from 'http';
+import URL from 'url';
+
+const logger = new Logger('UploadProxy');
+
+WebApp.connectHandlers.stack.unshift({
+	route: '',
+	handle: Meteor.bindEnvironment(function(req, res, next) {
+		// Quick check to see if request should be catch
+		if (req.url.indexOf(UploadFS.config.storesPath) === -1) {
+			return next();
+		}
+
+		logger.debug('Upload URL:', req.url);
+
+		if (req.method !== 'POST') {
+			return next();
+		}
+
+		// Remove store path
+		const parsedUrl = URL.parse(req.url);
+		const path = parsedUrl.pathname.substr(UploadFS.config.storesPath.length + 1);
+
+		// Get store
+		const regExp = new RegExp('^\/([^\/\?]+)\/([^\/\?]+)$');
+		const match = regExp.exec(path);
+
+		// Request is not valid
+		if (match === null) {
+			res.writeHead(400);
+			res.end();
+			return;
+		}
+
+		// Get store
+		const store = UploadFS.getStore(match[1]);
+		if (!store) {
+			res.writeHead(404);
+			res.end();
+			return;
+		}
+
+		// Get file
+		const fileId = match[2];
+		const file = store.getCollection().findOne({_id: fileId});
+		if (file === undefined) {
+			res.writeHead(404);
+			res.end();
+			return;
+		}
+
+		if (file.instanceId === InstanceStatus.id()) {
+			logger.debug('Correct instance');
+			return next();
+		}
+
+		// Proxy to other instance
+		const instance = InstanceStatus.getCollection().findOne({_id: file.instanceId});
+
+		if (instance == null) {
+			res.writeHead(404);
+			res.end();
+			return;
+		}
+
+		if (instance.extraInformation.host === process.env.INSTANCE_IP && RocketChat.isDocker() === false) {
+			instance.extraInformation.host = 'localhost';
+		}
+
+		logger.debug('Wrong instance, proxing to:', `${ instance.extraInformation.host }:${ instance.extraInformation.port }`);
+
+		const options = {
+			hostname: instance.extraInformation.host,
+			port: instance.extraInformation.port,
+			path: req.url,
+			method: 'POST'
+		};
+
+		const proxy = http.request(options, function(proxy_res) {
+			proxy_res.pipe(res, {
+				end: true
+			});
+		});
+
+		req.pipe(proxy, {
+			end: true
+		});
+	})
+});
+
 Meteor.startup(function() {
 	WebApp.connectHandlers.use('/avatar/', Meteor.bindEnvironment(function(req, res/*, next*/) {
 		const params = {