webmvc.fn onError doesn't work with CompletableFuture

[dudkiewicz-codes]

Affects: 5.3.6

webmvc.fn onError doesn't work with CompletableFuture.
Error handler predicates are never called.
Take a look at the code below.
error1 scenario shows the problem: application should return an error response with "error" body, returns Whitelabel Error Page instead.
error2 scenario works fine.

package com.example.demo;

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.context.annotation.Bean;
import org.springframework.http.HttpStatus;
import org.springframework.web.servlet.function.RouterFunction;
import org.springframework.web.servlet.function.ServerResponse;

import java.util.concurrent.CompletableFuture;

import static org.springframework.web.servlet.function.RouterFunctions.route;

@SpringBootApplication
public class DemoApplication {

    public static void main(String[] args) {
        SpringApplication.run(DemoApplication.class, args);
    }

    @Bean
    public RouterFunction<ServerResponse> routes() {
        return route()
            .GET("error1", request -> {
                CompletableFuture<String> result = CompletableFuture.supplyAsync(
                    () -> {
                        throwRuntimeException();
                        return "test";
                    }
                );
                return ServerResponse.ok().body(result);
            })
            .GET("error2", request -> {
                throwRuntimeException();
                CompletableFuture<String> result = CompletableFuture.supplyAsync(
                    () -> "test"
                );
                return ServerResponse.ok().body(result);
            })
            .onError(RuntimeException.class, (throwable, request) ->
                ServerResponse
                    .status(HttpStatus.INTERNAL_SERVER_ERROR)
                    .body("error")
            )
            .build();
    }

    private void throwRuntimeException() {
        throw new RuntimeException();
    }


}

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