Can't set headers after they are sent.] Error: Can't set headers after they are sent. 2.2.18

[gateway]

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Issue Description

While looking into a cloud code issue which I fixed, I saw this issue in my Heroku logs and Parse Dashboard logs.. I have no idea what this could be but doesnt sound to good.

Not sure how to debug this, but needed to provide this to the community.

I should note rolling back to 2.2.17 this does not happen.. :/

2016-08-24T18:17:46.973Z - Uncaught internal server error. [Error: Can't set headers after they are sent.] Error: Can't set headers after they are sent.
    at ServerResponse.OutgoingMessage.setHeader (_http_outgoing.js:346:11)
    at ServerResponse.header (/app/node_modules/express/lib/response.js:695:10)
    at allowCrossDomain (/app/node_modules/parse-server/lib/middlewares.js:229:7)
    at Layer.handle [as handle_request] (/app/node_modules/parse-server/node_modules/express/lib/router/layer.js:95:5)
    at next (/app/node_modules/parse-server/node_modules/express/lib/router/route.js:131:13)
    at Route.dispatch (/app/node_modules/parse-server/node_modules/express/lib/router/route.js:112:3)
    at Layer.handle [as handle_request] (/app/node_modules/parse-server/node_modules/express/lib/router/layer.js:95:5)
    at /app/node_modules/parse-server/node_modules/express/lib/router/index.js:277:22
    at param (/app/node_modules/parse-server/node_modules/express/lib/router/index.js:349:14)
    at param (/app/node_modules/parse-server/node_modules/express/lib/router/index.js:365:14)

Environment Setup

• Server
  • parse-server version (Be specific! Don't say 'latest'.) : 2.2.18
  • Operating System: Heroku Dyno
  • Hardware: Heroku Dyno
  • Localhost or remote server? (AWS, Heroku, Azure, Digital Ocean, etc): Heroku Dyno
• Database
  • MongoDB version: 3.2
  • Storage engine: WiredTiger
  • Hardware: M1 Cluster
  • Localhost or remote server? (AWS, mLab, ObjectRocket, Digital Ocean, etc): Heroku

[GuichiZhao]

My workaround is never putting any route after /parse route even the 404 route .Put all the other routes before /parse instead, for 404 just exclude /parse

[flovilmart]

@gateway what HTTP call trigger that? In 2.2.18 this should have probably been fixed for most of the calls, as well as by #2559

[gateway]

I think this happens when our client saves a userTag, stuff they tag in our app..

This happens on a few of our queries for POST from our app..

2016-08-25T17:34:32.327145+00:00 heroku[router]: at=info method=POST path="/parse/classes/Activity" host=xxx.herokuapp.com request_id=b6a0ec3d-876f-47a8-899e-8e0ff424714c fwd="12.215.39.106" dyno=web.1 connect=1ms service=1297ms status=200 bytes=58972
2016-08-25T17:35:40.593366+00:00 app[web.1]: error: Uncaught internal server error. [Error: Can't set headers after they are sent.] Error: Can't set headers after they are sent.

[jcampalo]

Did you try to update express to latest version?

[anhldbk]

@GuichiZhao Would you pls show me your workaround code?

[GuichiZhao]

@anhldbk
index.js

//Parse use an ancient version of express,that is how we mount path
require('cloud/routes')(app)
//mount parse-dashboard application to /dashboard
app.use('/dashboard', dashboard);
//mount parse api to /parse
app.use('/parse',api);

cloud/routes/index.js

module.exports=function (app) {
    //other routes
    app.use()
    //404 catcher
    app.all(/^\/(?!parse|dashboard).*$/, function (req, res, next) {
    });
}

[anhldbk]

@GuichiZhao Thanks.

[anhldbk]

@flovilmart When will your patches available in npm?

[kronos]

Same here. express 4.14.0

[flovilmart]

The fix (that will probably land in 2.2.19) is intended to fix the internal problems with 'Cannot set headers' errors. For you, that use a custom express installation, you should stick with the workaround.

[mcclaskiem]

@kronos so updating to 4.14.0 fixed this for you?

[kronos]

@mcclaskiem No. I meant that the problem is reproducible with the latest express

[sebastiankade]

Hey guys,

I wanted to upgrade to 2.2.18 for the mongo connection improvements and ran into this issue as well.

After digging into it I found that it was because of #2338 (this commit). The change was added to allow "developers to add express handlers for successful completion of parse-server requests" however this causes problems for anyone who uses Parse Server in an existing Express app like this:
app.use('/parse', new ParseServer({...}));

Why it is breaking is that by adding the next() calls to the PromiseRouter the request gets passed down onto the next app.use(), which might try to handle it again causing the "Can't set headers after they are sent" issue to arise.

The correct workaround for doing this atm is to add a callback that stops the propagation:

app.use('/parse', new ParseServer({...}), function(req, res, next) {
    // This will get called after every parse request 
    // and stops the request propagation by doing nothing
});

I don't think next() should be called inside the PromiseRouter as the whole point of an Express middleware is that if it handles a request it stops the propagation of the request. I think to solve the needs put forward in #2338 there could be a callback in the ParseServer options for this (a request callback).

👍

[flovilmart]

I ageee with your workaround, and it should be added onto the Parse-server-example repository (PR are welcome).

When using though the CLI / standalone, we should not yield any headers already sent, internally, as we manage our routes.

On another note, calling next after the response is sent is also an opportunity to cleanup stuff (like caches) after the response is sent.

It's also an opportunity to do request tracing, logging etc.. (as it's been intended by the commit you point out).

There is nothing in the express design that prevents having 'post' middlewares, and they can be quite useful.

For now, it's been decided that next call was here to stay, there are numerous was to work around that.

On another note, adding pre middlewares can have even worse side effects, like a json body parser would conflict with the files endpoint.

[sebastiankade]

This is not a middleware though (by definition something that sits inbetween the start of the request and the handler). This IS the handler. This makes routing messy.

In node if you wanted to do things like request tracing & logging, wouldn't you generally do the following:

app.use('/parse', function(req, res, next) {
    // Do PRE work
    next();
    // Do POST work
});

In our case it would be:

var parse = new ParseServer({...});
app.use('/parse', function(req, res, next) {
    req.on('response', function() {
        // Track anything on response, such as resp time
    });

    // Manually invoke the parse server router
    parse(req, res, next);

    // Immediate POST tracing etc
});

I disagree but fair enough 👍 I shall keep my hack in place