Ensure old attributes are not removed by accident

This commit ensures that a copy is made of old attributes before
replacing it with new attributes. Because new attributes can be
composed of old, clearing the old would also remove entries from the
 new.

See gh-32245

Author: poutsma

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

@@ -1265,9 +1265,13 @@ public Mono<HandlerFunction<T>> route(ServerRequest serverRequest) {
 								return this.routerFunction.route(nestedRequest)
 										.doOnNext(match -> {
 											if (nestedRequest != serverRequest) {
-												serverRequest.attributes().clear();
-												serverRequest.attributes()
-														.putAll(nestedRequest.attributes());
+												// new attributes map from nestedRequest.attributes() can be composed of the old attributes,
+												// which means that clearing the old attributes will remove those values from new attributes as well
+												// so let's make a copy
+												Map<String, Object> newAttributes = new LinkedHashMap<>(nestedRequest.attributes());
+												Map<String, Object> oldAttributes = serverRequest.attributes();
+												oldAttributes.clear();
+												oldAttributes.putAll(newAttributes);
 											}
 										});
 							}

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

@@ -1181,8 +1181,13 @@ public Optional<HandlerFunction<T>> route(ServerRequest serverRequest) {
 								Optional<HandlerFunction<T>> result =
 										this.routerFunction.route(nestedRequest);
 								if (result.isPresent() && nestedRequest != serverRequest) {
-									serverRequest.attributes().clear();
-									serverRequest.attributes().putAll(nestedRequest.attributes());
+									// new attributes map from nestedRequest.attributes() can be composed of the old attributes,
+									// which means that clearing the old attributes will remove those values from new attributes as well
+									// so let's make a copy
+									Map<String, Object> newAttributes = new LinkedHashMap<>(nestedRequest.attributes());
+									Map<String, Object> oldAttributes = serverRequest.attributes();
+									oldAttributes.clear();
+									oldAttributes.putAll(newAttributes);
 								}
 								return result;
 							}