Headers Already Sent When Page Reloads

[Caleb Beery]

I've got a separate front end application that is calling my Loopback 4 API endpoints. When the front end application's page is refreshed too quickly, my Loopback 4 API produces this error:

Error: Cannot set headers after they are sent to the client
    at new NodeError (node:internal/errors:371:5)
    at ServerResponse.setHeader (node:_http_outgoing:576:11)
    at ServerResponse.header (/Users/caleb.beery/sites/trimaster-lb4/node_modules/express/lib/response.js:794:10)
    at HttpContext.done (/Users/caleb.beery/sites/trimaster-lb4/node_modules/strong-remoting/lib/http-context.js:580:9)
    at /Users/caleb.beery/sites/trimaster-lb4/node_modules/strong-remoting/lib/rest-adapter.js:540:11
    at /Users/caleb.beery/sites/trimaster-lb4/node_modules/async/dist/async.js:3681:19
    at wrapper (/Users/caleb.beery/sites/trimaster-lb4/node_modules/async/dist/async.js:271:20)
    at replenish (/Users/caleb.beery/sites/trimaster-lb4/node_modules/async/dist/async.js:441:29)
    at iterateeCallback (/Users/caleb.beery/sites/trimaster-lb4/node_modules/async/dist/async.js:430:21)
    at /Users/caleb.beery/sites/trimaster-lb4/node_modules/async/dist/async.js:327:20
    at /Users/caleb.beery/sites/trimaster-lb4/node_modules/async/dist/async.js:3679:17
    at interceptInvocationErrors (/Users/caleb.beery/sites/trimaster-lb4/node_modules/strong-remoting/lib/remote-objects.js:731:22)
    at /Users/caleb.beery/sites/trimaster-lb4/node_modules/loopback-phase/node_modules/async/dist/async.js:473:16
    at replenish (/Users/caleb.beery/sites/trimaster-lb4/node_modules/loopback-phase/node_modules/async/dist/async.js:1009:25)
    at iterateeCallback (/Users/caleb.beery/sites/trimaster-lb4/node_modules/loopback-phase/node_modules/async/dist/async.js:998:17)
    at /Users/caleb.beery/sites/trimaster-lb4/node_modules/loopback-phase/node_modules/async/dist/async.js:972:16

Some information about the Loopback 4 app: 

This contains a partially migrated Loopback 3 application in the "lb3app" directory in the root of the project. Some of the front end application's API calls are hitting the Loopback 4 endpoints and some of them are hitting the Loopback 3 endpoints.

I'm not really even sure how to start debugging this. Any suggestions?

[Caleb Beery]

It looks like this is a similar issue with no resolution:

https://github.com/loopbackio/loopback-next/discussions/7196

[Caleb Beery]

Adding this to my application seems to keep the node process from crashing, allowing pages to refresh with no issues. Not sure this is a good idea, though.

process.on('uncaughtException', (reason, promise) => {
   console.log('uncaughtException at:', reason.stack ?? reason);
});

[Rifa Achrinza]

Hi Caleb Beery,

Based on the error stack trace, the frontend app is using the Strong
Remoting feature of LB3.

Would it be possible to provide some steps and/or a minimal PoC repo to consistently replicate the issue?

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[Caleb Beery]

Hi Rifa,

This post documents the issue I was having. They also provide an example repo and a change you can make to reproduce the issue.

https://github.com/loopbackio/loopback-next/discussions/7196#discussioncomment-1494410

These are the conditions needed to reproduce:

1. A Loopback 3 app is contained in a Loopback 4 app, in the "lb3app" directory.

2. One of the Loopback 3 models contains a remote method that uses a callback (not async/await)

3. The remote method takes some time to run, and before it returns, the request is cancelled (front end that calls it is reloaded, for example)

4. Error

[Rifa Achrinza]

Hi Caleb,

Thanks for the additional info! As I wasn't able to immediately find the
root cause, I've went ahead and opened it as a bug in our issue
tracker and added it to our icebox for the time being. If you are
able to find a patch, please do submit it as a PR on the `loopback-next`
Git repository; The maintainers will help with reviewing and landing the
PR.

Best regards,
Rifa

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