Webflux: GATEWAY_ORIGINAL_REQUEST_URL_ATTR not set in ForwardRoutingFilter

[FrontierPsychiatrist]

This is a re-report of #3510.

Describe the bug
The documentation for Forward Routing Filter states:

The unmodified original URL is appended to the list in the ServerWebExchangeUtils.GATEWAY_ORIGINAL_REQUEST_URL_ATTR attribute

This is not the case, or I am misunderstsanding. The controller eventually handling the request (i.e. the one being forwarded to) does not see this attribute on the ServerWebExchange.

Sample
I have attached a minimal application that recreates the behavior.

It consists roughly of:

# application.yaml
spring:
  cloud:
    gateway:
      server:
        webflux:
          routes:
            - id: test
              uri: forward:/internal/test
              predicates:
                - Path=/test/**
                - Method=GET

@GetMapping("/internal/test")
public TestResponse test(ServerWebExchange exchange) {
  Set<URI> originalUris = exchange.getAttribute(ServerWebExchangeUtils.GATEWAY_ORIGINAL_REQUEST_URL_ATTR);
  return new TestResponse(true, originalUris);
}

and a test expecting the response to contain a non-null originalUris value.

forward-bug.zip

[FrontierPsychiatrist]

I tried to overcome this by overwriting the ForwardRoutingFilter, but it turns out the original request URL is not really available there anymore. I could accomplish setting the attribute with a custom ForwardPathFilter though:

package REDACTED.gateway.filter;

import org.springframework.cloud.gateway.filter.ForwardPathFilter;
import org.springframework.cloud.gateway.filter.GatewayFilterChain;
import org.springframework.cloud.gateway.route.Route;
import org.springframework.stereotype.Component;
import org.springframework.web.server.ServerWebExchange;
import reactor.core.publisher.Mono;

import java.net.URI;

import static org.springframework.cloud.gateway.support.ServerWebExchangeUtils.*;

@Component
public class MyForwardPathFilter extends ForwardPathFilter {

  @Override
  public Mono<Void> filter(ServerWebExchange exchange, GatewayFilterChain chain) {
    Route route = exchange.getAttribute(GATEWAY_ROUTE_ATTR);
    if (route != null) {
      URI routeUri = route.getUri();
      String scheme = routeUri.getScheme();
      if (isAlreadyRouted(exchange) || !"forward".equals(scheme)) {
        return chain.filter(exchange);
      }
      // This line is added
      addOriginalRequestUrl(exchange, exchange.getRequest().getURI());
      exchange = exchange.mutate()
        .request(exchange.getRequest().mutate().path(routeUri.getPath()).build())
        .build();

    }
    return chain.filter(exchange);
  }
}