SpringBoot functional Web MVC, missing a way to return CompletableFuture<ResponseEntity<String>>

I have a working production SpringBoot application, and part of it is getting a do-over. It would be very beneficial for me to delete my old @RequestMapping from the ResponseEntity<String> foo()s of my world, keeping the old code as an as a duplicate while we try to roll out the new functionality behind a feature gate.. All production tenants go through my no-longer-declarative foo() function, while all my test and automation tenants can start to tinker with a brand new EntityResponse<String> bar().

The way to implement the change was so clear in my mind:

class Router{ 
  @Bean
  RouterFunction<ServerResponse> helloWorldRouterFunction(OldHelloWorldService oldHelloWorldService) {
    return RouterFunctions.route()
        .route(RequestPredicates.path("/helloWorld/{option}"), x ->
            {
              String option = x.pathVariable("option");
              if (FeatureManager.isActive()) {
                return ServerResponse.ok().body(String.format("New implementation of Hello World! your option is: %s", option));
              } else {
                // FutureServerResponse is my own bad implementation of the ServerResponse interface
                return FutureServerResponse.from(oldHelloWorldService.futureFoo(Integer.parseInt(option)));
              }
            }
        )
        .build();
  }
}

Here's the implementation for OldHelloWorldService::futureFoo

@RestController
static class OldHelloWorldService {
  @RequestMapping("/specialCase")
  ResponseEntity<String> specialCase() {
    // some business logic
    return ResponseEntity.ok().body("Special case for Hello World with option 2");
  }
  /**
   * Old declarative implementation, routed via functional {@link ServerRouteConfiguration}
   * to allow dynamic choice based on {@link FeatureManager#isActive()}
   * <p>
   * as you can see, before the change, this function was a {@link RequestMapping} and it handled the
   * completable future, we could return both concrete OK responses with a body, and FOUND responses with a location.
   */
  // @RequestMapping("/helloWorld/{option}")
  CompletableFuture<ResponseEntity<String>> futureFoo(
      // @PathVariable
      int option) {
    return CompletableFuture.supplyAsync(() -> {
      if (option == 2) {
        return ResponseEntity.status(HttpStatus.FOUND)
            .location(URI.create("/specialCase"))
            .build();
      } else {
        return ResponseEntity.ok().body(String.format("Old implementation of Hello World! your option is: %s", option));
      }
    });
  }
}

This feature lets my backend code decide what kind of ResponseEntity it will send, in the future. As you see, a smart function might for instance decide to either show a String message with an OK status, or give a Location header, and declare FOUND status, and not even give a String body at all. because the result type was a full fluid ResponseEntity, I had the power to do what I will.

Now with a EntityResponse you may still use a CompletionStage, but only as the actual entity. while building the EntityResponse I am required to give it a definitive final status. If it was OK, I can't decide it will be FOUND when my CompletionStage ran it's course.

The only problem with the above code, is that org.springframework.web.servlet.function does not contain the FutureServerResponse implementation I need. I created my own, and it works, but it feels hacky, And I wouldn't want it in my production code.

Now I feel like the functionality should still be there somewhere, Why isn't there a FutureServerResponse that can decide in the future what it is? Is there a workaround to this problem maybe somehow (ab)using views?

To state the maybe not-so-obvious.. I am contemplating a move to reactive and WebFlux, but changing the entire runtime will have more dramatic implications on current production tenants, and making that move with a feature gate would be impossible because the urls would have to be shared between MVC and Flux.

It's a niche problem, and Functional Web MVC has little resources so I will appreciate greatly any help I can get.

I have created a github companion for this question.

I've had to patch spring web to get what I needed. I pushed a pull-request to spring web with my patch, in the end something similar was created and pushed to the 5.3 release of spring.

if anybody else is looking for the async behavior described in the question, ServerResponse.async function in spring 5.3.0+ (spring boot 2.4.0+) solves the issue.