Incorrect serializer used when a coroutine controller method returns a ResponseEntity

Hello Spring Hateoas!

I have a Spring reactive application written in Kotlin which uses coroutines in controller methods. There is one use case where the correct serializer (from spring-hateoas-jsonapi) doesn't kick in: when my response is wrapped in a ResponseEntity.

I have made an example repository with controller tests to illustrate the issue. There I have four controller methods one of which doesn't return correctly formatted data to the client:

✅    fun monoEntityModel(): Mono<EntityModel<Payload>>
✅    suspend fun coroutineEntityModel(): EntityModel<Payload>
✅    fun monoResponseEntityModel(): Mono<ResponseEntity<EntityModel<Payload>>>
❌    suspend fun coroutineResponseEntityModel(): ResponseEntity<EntityModel<Payload>>

It does work fine if instead of a suspend function a Mono is returned from the controller method. It also works well if the coroutine returns data without wrapping in ResponseEntity. Just the mix of suspend fun returning ResponseEntity is problematic.

I initially thought this was a serializer issue but this has been proven wrong.

Start with the linked example repository and its controller tests, comparing the four return signatures and their serialized responses. Trace the handling of a suspend method returning ResponseEntity against the working Mono and unwrapped coroutine cases. Done means the ResponseEntity coroutine case uses the same correctly formatted representation as the other working cases.