NestJS 9 with Fastify ignores @Res() in controller

[coryroloff]

Did you read the migration guide?

• I have read the whole migration guide

Is there an existing issue that is already proposing this?

• I have searched the existing issues

Potential Commit/PR that introduced the regression

No response

NestJS version

8.4.7 -> 9.1.2

Describe the regression

NestJS appears to be ignoring the use of @res() to override Nest's standard code for responding to a request, specifically when using Fastify.

In version 8, Nest doesn't attempt to respond to requests at all if it sees @res() in the controller's parameters and allows you to respond when needed (for example, any asynchronous code not through a Promise). In version 9, the same code will respond immediately, ignoring your res.send if it doesn't happen synchronously.

Note: NestJS 9 using Express behaves the same way as NestJS 8 using Fastify.

Minimum reproduction code

https://github.com/coryroloff/nestjs-res-bug

Input code

No response

Expected behavior

To test NestJS 8, run the following:

cd nestjs-8
npm install
npm run start:dev

curl localhost:3000/test-res-bug -v

You will get the response "hello world" back. This is expected behavior.

To test NestJS 9 with Fastify, run the following:

cd nestjs-9
npm install
npm run start:dev

curl localhost:3000/test-res-bug -v

You will get an empty response body back, instead of the expected "hello world".

To test NestJS 9 with Express, run the following:

cd nestjs-9-express
npm install
npm run start:dev

curl localhost:3000/test-res-bug -v

You will get the response "hello world" back. This is expected behavior.

Other

No response

[jmcdo29]

This actually might be related to something I'm looking into with @fastify/view integration and not being able to res.view() from an exception filter. I think there's somewhere that an await is possibly missing that's causing fastify to send a response before we expect it to. I'll try to update this as I can

[kamilmysliwiec]

Did you have a chance to find the root cause @jmcdo29? Let me know and I can take over if you're busy 🙌

[SirReiva]

This may be the problem

[jmcdo29]

Did you have a chance to find the root cause @jmcdo29? Let me know and I can take over if you're busy raised_hands

@kamilmysliwiec that might be best. What SirReiva added might be relevant here too

[coryroloff]

@SirReiva's answer explains and solves the issue I was running into. Thanks, everyone.

[SirReiva]

making this change apparently works

[kamilmysliwiec]

Let's track this here #10459