From 4fbba931a58b57301a59f65ec3d11697d1c30b47 Mon Sep 17 00:00:00 2001
From: Arjen Poutsma <apoutsma@pivotal.io>
Date: Fri, 13 Mar 2020 16:26:15 +0100
Subject: [PATCH] Improve RouterFunction composition

This commit changes the way two RouterFunctions are composed in
WebFlux.fn. Prior to this commit, two were composed with
`switchIfEmpty()`, switching from the first to the second route if the
first did not provide an element.
After this commit, two router functions are compose using `concat`,
which results in a smaller stack trace.

See gh-24652
---
 .../reactive/function/server/RouterFunctions.java | 15 ++++++++-------
 1 file changed, 8 insertions(+), 7 deletions(-)

diff --git a/spring-webflux/src/main/java/org/springframework/web/reactive/function/server/RouterFunctions.java b/spring-webflux/src/main/java/org/springframework/web/reactive/function/server/RouterFunctions.java
index 9068ff7b32..9b869c5ba8 100644
--- a/spring-webflux/src/main/java/org/springframework/web/reactive/function/server/RouterFunctions.java
+++ b/spring-webflux/src/main/java/org/springframework/web/reactive/function/server/RouterFunctions.java
@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2018 the original author or authors.
+ * Copyright 2002-2020 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -26,6 +26,7 @@ import java.util.function.Supplier;
 
 import org.apache.commons.logging.Log;
 import org.apache.commons.logging.LogFactory;
+import reactor.core.publisher.Flux;
 import reactor.core.publisher.Mono;
 
 import org.springframework.core.io.Resource;
@@ -486,7 +487,7 @@ public abstract class RouterFunctions {
 		 * {@code OrderController.routerFunction()}.
 		 * to the {@code changeUser} method in {@code userController}:
 		 * <pre class="code">
-		 * RouterFunctionlt;ServerResponsegt; route =
+		 * RouterFunction&lt;ServerResponse&gt; route =
 		 *   RouterFunctions.route()
 		 *     .GET("/users", userController::listUsers)
 		 *     .add(orderController.routerFunction());
@@ -803,8 +804,8 @@ public abstract class RouterFunctions {
 
 		@Override
 		public Mono<HandlerFunction<T>> route(ServerRequest request) {
-			return this.first.route(request)
-					.switchIfEmpty(Mono.defer(() -> this.second.route(request)));
+			return Flux.concat(this.first.route(request), Mono.defer(() -> this.second.route(request)))
+					.next();
 		}
 
 		@Override
@@ -833,9 +834,9 @@ public abstract class RouterFunctions {
 
 		@Override
 		public Mono<HandlerFunction<ServerResponse>> route(ServerRequest request) {
-			return this.first.route(request)
-					.map(RouterFunctions::cast)
-					.switchIfEmpty(Mono.defer(() -> this.second.route(request).map(RouterFunctions::cast)));
+			return Flux.concat(this.first.route(request), Mono.defer(() -> this.second.route(request)))
+					.next()
+					.map(RouterFunctions::cast);
 		}
 
 		@Override