fix: return the correct serializer function when no content-type is defined

[DouglasdeMoura]

Description

While I was creating am API and validating the response, I ran into Error: serializerFn is not a function. By investigating the issue, I've seen that, in my specific case, the serializerFn returned an object , as follows:

{
  'application/json': Function
}

Therefore, with this fix, I set the default content type to pick the correct serializer.

In this repo, you can find a minimal reproducible error. Start the project and send the following request:

curl --json '{"foo":"bar"}' http://localhost:3000/example/1

Checklist

• run npm run test and npm run benchmark
• tests and/or benchmarks are included
• documentation is changed or added
• commit message and code follows the Developer's Certification of Origin
  and the Code of conduct

[jsumners]

The "reproduction" is a lot of code. Please highlight what the actual reproduction is or narrow it down to a single snippet.

I suspect the error is due to improper usage of the API. If not, the suggested fix is rather fragile and at the very least needs a unit test to cover it.

[mcollina]

Thanks for opening a PR! Can you please add a unit test?

[DouglasdeMoura]

@jsumners the "reproduction" is the template generated by Fastify's CLI (I got this error while bootstrapping a new API with the CLI).

@mcollina will do!

[mcollina]

Can you add a test?

[DouglasdeMoura]

Guys, I made a better fix for this issue. I realized that the serializers from context['response-schema'] for responses > 299 are returning inside an object (e.g. { 'application/json': Function }) when the reply[kReplyHeaders]['content-type'] returned undefined. Therefore I setted up a default content type from where the getSchemaSerializer can pick up the correct serializer function. Sorry for the delay, I didn't had time to work on this until today.

[DouglasdeMoura]

@jsumners you'll be able to reproduce the error using the test I added. Here's the result running it on the main branch:

npx tap test/serializeResponse.test.js
​ FAIL ​ test/serializeResponse.test.js
 ✖ should be equal

  test/serializeResponse.test.js                 
  125 |     }, (err, response, body) => {        
  126 |       t.error(err)                       
> 127 |       t.equal(response.statusCode, 400)  
      | --------^                                
  128 |       t.same(body, {                     
  129 |         statusCode: 400,                 
  130 |         error: 'Bad Request',            

  --- expected    
  +++ actual      
  @@ -1,1 +1,1 @@ 
  -400            
  +500            

  test: test/serializeResponse.test.js serialize the response for a Bad
Request
    error, as defined on the schema
  stack: |
    test/serializeResponse.test.js:127:9
    node_modules/simple-get/index.js:98:7
    IncomingMessage.<anonymous>
(node_modules/simple-concat/index.js:8:13)

​ FAIL ​ test/serializeResponse.test.js
 ✖ should be equivalent

  test/serializeResponse.test.js                 
  126 |       t.error(err)                       
  127 |       t.equal(response.statusCode, 400)  
> 128 |       t.same(body, {                     
      | --------^                                
  129 |         statusCode: 400,                 
  130 |         error: 'Bad Request',            
  131 |         message: 'body must be object'   

  --- expected                                                         
  +++ actual                                                           
  @@ -1,5 +1,5 @@                                                      
   Object {                                                            
  -  "statusCode": 400,                                                
  -  "message": "body must be object",                                 
  -  "error": "Bad Request",                                           
  +  "statusCode": 500,                                                
  +  "code": "FST_ERR_FAILED_ERROR_SERIALIZATION",                     
  +  "message": "Failed to serialize an error. Error: serializerFn is not
 a function. Original error: body must be object",
   }                                                                   

  test: test/serializeResponse.test.js serialize the response for a Bad
Request
    error, as defined on the schema
  stack: |
    test/serializeResponse.test.js:128:9
    node_modules/simple-get/index.js:98:7
    IncomingMessage.<anonymous>
(node_modules/simple-concat/index.js:8:13)

​ FAIL ​ test/serializeResponse.test.js 2 failed of 16 114.562ms
 ✖ should be equal
 ✖ should be equivalent



                         
  🌈 SUMMARY RESULTS 🌈  
                         
​ FAIL ​ test/serializeResponse.test.js 2 failed of 16 114.562ms
 ✖ should be equal
 ✖ should be equivalent


Suites:   ​1 failed​, ​1 of 1 completed
Asserts:  ​​​2 failed​, ​14 passed​, ​of 16
​Time:​   ​394.496ms

[DouglasdeMoura]

@mcollina and @jsumners, I think this PR needs your approval to be merged.

[mcollina]

lgtm

[mcollina]

Could you update the PR title? It's not clear what is fixing.

[jsumners]

I'm still not clear how the reported error happens (clarification was never provided), but all of my feedback has been addressed.

[Uzlopak]

RSLGTM