koa integration overwrites content-type of response

[mrl5]

This applies only to koa (express and hapi properly return content-type) and is related to this known koa bug koajs/koa#1120 - I've discovered it while working on PR #1129 (#1129 (review))

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Expected Behavior

koa returns custom content-type just like express and hapi currently does

Current Behavior

koa ignores this.setHeader('content-type', 'application/vnd.mycompany.myapp.v2+json'); and returns application/json

Possible Solution

as described in koajs/koa#1120 (comment) by @brunoabreu

I can make it work if I write the header after setting the body. It only affects json responses.

so the solution is as simple as this:

diff --git a/packages/cli/src/routeGeneration/templates/koa.hbs b/packages/cli/src/routeGeneration/templates/koa.hbs
index 6716837..b2d2ee9 100644
--- a/packages/cli/src/routeGeneration/templates/koa.hbs
+++ b/packages/cli/src/routeGeneration/templates/koa.hbs
@@ -204,8 +204,6 @@ export function RegisterRoutes(router: KoaRouter) {
 
     function returnHandler(context: any, next: () => any, statusCode?: number, data?: any, headers: any={}) {
         if (!context.headerSent && !context.response.__tsoaResponded) {
-            context.set(headers);
-
             if (data !== null && data !== undefined) {
                 context.body = data;
                 context.status = 200;
@@ -217,6 +215,7 @@ export function RegisterRoutes(router: KoaRouter) {
                 context.status = statusCode;
             }
 
+            context.set(headers);
             context.response.__tsoaResponded = true;
             return next();
         }

Steps to Reproduce

1. use koa as a server
2. use this controller:

@Route('MediaTypeTest')
export class MediaTypeTestController extends Controller {
  @SuccessResponse('202', 'Accepted')
  @Post('Custom')
  async postCustomProduces(@Body() model: UserRequestModel): Promise<UserResponseModel> {
    const body = { id: model.name.length, name: model.name };
    this.setStatus(202);
    this.setHeader('content-type', 'application/vnd.mycompany.myapp.v2+json'); // express and hapi returns it, koa ignores
    return body;
  }
}

3. generate routes
4. do request (e.g. with curl)

$ curl -i \
    -H 'content-type: application/json' \
    -X POST -d '{"name": "foo"}' \
    http://localhost:3000/MediaTypeTest/Custom

Context (Environment)

Version of the library: 3.14.0
Version of NodeJS: v14.17.6

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Detailed Description

I think everything was described already

Breaking change?

hard to tell, I guess not

[mrl5]

I can provide a fix if maintainers agree that it's a bug that can be fixed as proposed

[WoH]

Seems like we should obviously fix this, can you open a PR?

[mrl5]

Seems like we should obviously fix this, can you open a PR?

how about pushing additional commit that fixes it to the PR #1129 ? this is kind of related and additional benefit is that I can introduce integration tests :)