Pass current request to filter in RouterFunction extensions

This commit ensures that the proper request is passed in the
Kotlin RouterFunction extension DSL.

Closes gh-27086

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

@@ -653,7 +653,7 @@ class RouterFunctionDsl internal constructor (private val init: RouterFunctionDs
 	fun filter(filterFunction: (ServerRequest, (ServerRequest) -> Mono<ServerResponse>) -> Mono<ServerResponse>) {
 		builder.filter { request, next ->
 			filterFunction(request) {
-				next.handle(request)
+				next.handle(it)
 			}
 		}
 	}

spring-webflux/src/test/kotlin/org/springframework/web/reactive/function/server/RouterFunctionDslTests.kt

@@ -24,11 +24,13 @@ import org.springframework.http.HttpHeaders.*
 import org.springframework.http.HttpMethod.*
 import org.springframework.http.HttpStatus
 import org.springframework.http.MediaType.*
+import org.springframework.web.reactive.function.server.support.ServerRequestWrapper
 import org.springframework.web.testfixture.http.server.reactive.MockServerHttpRequest.*
 import org.springframework.web.testfixture.server.MockServerWebExchange
 import org.springframework.web.reactive.function.server.AttributesTestVisitor
 import reactor.core.publisher.Mono
 import reactor.test.StepVerifier
+import java.security.Principal
 
 /**
  * Tests for [RouterFunctionDsl].
@@ -169,6 +171,53 @@ class RouterFunctionDslTests {
 		assertThat(visitor.visitCount()).isEqualTo(7);
 	}
 
+	@Test
+	fun acceptFilterAndPOST() {
+		val mockRequest = post("https://example.com/filter")
+			.header(ACCEPT, APPLICATION_JSON_VALUE)
+			.build()
+		val request = DefaultServerRequest(MockServerWebExchange.from(mockRequest), emptyList())
+		StepVerifier.create(filteredRouter.route(request).flatMap { it.handle(request) })
+			.expectNextCount(1)
+			.verifyComplete()
+	}
+
+	private val filteredRouter = router {
+		POST("/filter", ::handleRequestWrapper)
+
+		filter (TestFilterProvider.provide())
+		before {
+			it
+		}
+		after { _, response ->
+			response
+		}
+		onError({it is IllegalStateException}) { _, _ ->
+			ServerResponse.status(HttpStatus.INTERNAL_SERVER_ERROR).build()
+		}
+		onError<IllegalStateException> { _, _ ->
+			ServerResponse.status(HttpStatus.INTERNAL_SERVER_ERROR).build()
+		}
+	}
+
+	private class TestServerRequestWrapper(delegate: ServerRequest, private val principalOverride: String = "foo"): ServerRequestWrapper(delegate) {
+		override fun principal(): Mono<out Principal> = Mono.just(Principal { principalOverride })
+	}
+
+	private object TestFilterProvider {
+		fun provide(): (ServerRequest, (ServerRequest) -> Mono<ServerResponse>) -> Mono<ServerResponse> = { request, next ->
+			next(TestServerRequestWrapper(request))
+		}
+	}
+
+	private fun handleRequestWrapper(req: ServerRequest): Mono<ServerResponse> {
+		return req.principal()
+			.flatMap {
+				assertThat(it.name).isEqualTo("foo")
+				ServerResponse.ok().build()
+			}
+	}
+
 	private fun sampleRouter() = router {
 		(GET("/foo/") or GET("/foos/")) { req -> handle(req) }
 		"/api".nest {