Fix parameter bug of handler that Kotlin DSL defines as a parameter for the filterFunction.

[ShindongLee]

Resolve #26838
This PR fixes following bug.

spring-framework/spring-webmvc/src/main/kotlin/org/springframework/web/servlet/function/RouterFunctionDsl.kt

Lines 650 to 655 in 5b1ab31

	fun filter(filterFunction: (ServerRequest, (ServerRequest) -> ServerResponse) -> ServerResponse) {
	builder.filter { request, next ->
	filterFunction(request) {
	next.handle(request)
	}
	}

spring-framework/spring-webflux/src/main/kotlin/org/springframework/web/reactive/function/server/CoRouterFunctionDsl.kt

Lines 531 to 539 in 5b1ab31

	fun filter(filterFunction: suspend (ServerRequest, suspend (ServerRequest) -> ServerResponse) -> ServerResponse) {
	builder.filter { serverRequest, handlerFunction ->
	mono(Dispatchers.Unconfined) {
	filterFunction(serverRequest) {
	handlerFunction.handle(serverRequest).awaitSingle()
	}
	}
	}
	}

1. Function filterFunction takes serverRequest and function handler as parameters. (I just named them.)
2. Function handler also takes serverRequest as parameter.
3. These DSLs define handler.
4. Function handlers that these DSLs defined are now using mistakenly filterFunction's serverRequest, not their own serverRequest parameters, which makes their own parameters meaningless.

This bug only exists in Kotlin DSL.

You can see java codes have no problem as follows.

spring-framework/spring-webmvc/src/main/java/org/springframework/web/servlet/function/HandlerFilterFunction.java

Line 46 in 01e50fb

R filter(ServerRequest request, HandlerFunction<T> next) throws Exception;

spring-framework/spring-webflux/src/main/java/org/springframework/web/reactive/function/server/HandlerFilterFunction.java

Line 48 in 01e50fb

Mono<R> filter(ServerRequest request, HandlerFunction<T> next);

In java you can override filter something like

ServerRequest newRequest = someOperation(request)
return next.handle(newRequest)

but in Kotlin DSL, even if you write

filter { serverRequest, handlerFunction ->
    val newServerRequest = someOperation(serverRequest)
    handlerFunction(newServerRequest)
}

handlerFunction will use original serverRequest, not the newServerRequest

[sdeleuze]

Thanks for this PR, could you please add related tests for both WebMvc and WebFlux that show a failure with current implementation and would be green with your fix in the 2 RouterFunctionDslTests and CoRouterFunctionDslTests?

[ShindongLee]

@sdeleuze I added tests for Spring MVC (9d0fbbe) and Spring Webflux (b7e6d23). Thank you !

[ShindongLee]

@sdeleuze Do you have any updates or plans for this ...?

[sdeleuze]

Yes it is planned for inclusion in Spring Framework 5.3.7 as mentioned in #26838.

[sdeleuze]

Merged via 07ba957 (main) and 3ab270e (5.2.x), thanks!