Introduce ForwardedHeaderFilter for WebFlux

This commit introduces a ForwardedHeaderFilter for WebFlux, similar to
the existing Servlet version. As part of this the
DefaultServerHttpRequestBuilder had to be changed to no longer use
delegation, but instead use a deep copy at the point of mutate().
Otherwise, headers could not be removed.

Issue: SPR-15954

spring-web/src/test/java/org/springframework/web/filter/reactive/ForwardedHeaderFilterTests.java

@@ -0,0 +1,146 @@
+/*
+ * Copyright 2002-2017 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.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.web.filter.reactive;
+
+import java.net.URI;
+import java.time.Duration;
+
+import org.junit.Test;
+import reactor.core.publisher.Mono;
+
+import org.springframework.http.HttpHeaders;
+import org.springframework.lang.Nullable;
+import org.springframework.mock.http.server.reactive.test.MockServerHttpRequest;
+import org.springframework.mock.http.server.reactive.test.MockServerWebExchange;
+import org.springframework.web.server.ServerWebExchange;
+import org.springframework.web.server.WebFilterChain;
+
+import static org.junit.Assert.*;
+
+/**
+ * @author Arjen Poutsma
+ */
+public class ForwardedHeaderFilterTests {
+
+	private final ForwardedHeaderFilter filter = new ForwardedHeaderFilter();
+
+	private final TestWebFilterChain filterChain = new TestWebFilterChain();
+
+
+	@Test
+	public void removeOnly() {
+		MockServerWebExchange exchange = MockServerHttpRequest.get("/")
+				.header("Forwarded", "for=192.0.2.60;proto=http;by=203.0.113.43")
+				.header("X-Forwarded-Host", "example.com")
+				.header("X-Forwarded-Port", "8080")
+				.header("X-Forwarded-Proto", "http")
+				.header("X-Forwarded-Prefix", "prefix")
+				.toExchange();
+
+		this.filter.setRemoveOnly(true);
+		this.filter.filter(exchange, this.filterChain).block(Duration.ZERO);
+
+		HttpHeaders result = this.filterChain.getHeaders();
+		assertNotNull(result);
+		assertFalse(result.containsKey("Forwarded"));
+		assertFalse(result.containsKey("X-Forwarded-Host"));
+		assertFalse(result.containsKey("X-Forwarded-Port"));
+		assertFalse(result.containsKey("X-Forwarded-Proto"));
+		assertFalse(result.containsKey("X-Forwarded-Prefix"));
+	}
+
+	@Test
+	public void xForwardedRequest() throws Exception {
+		MockServerWebExchange exchange = MockServerHttpRequest.get("http://example.com/path")
+				.header("X-Forwarded-Host", "84.198.58.199")
+				.header("X-Forwarded-Port", "443")
+				.header("X-Forwarded-Proto", "https")
+				.toExchange();
+
+		this.filter.filter(exchange, this.filterChain).block(Duration.ZERO);
+
+		URI uri = this.filterChain.uri;
+		assertEquals(new URI("https://84.198.58.199/path"), uri);
+	}
+
+	@Test
+	public void forwardedRequest() throws Exception {
+		MockServerWebExchange exchange = MockServerHttpRequest.get("http://example.com/path")
+				.header("Forwarded", "host=84.198.58.199;proto=https")
+
+				.toExchange();
+
+		this.filter.filter(exchange, this.filterChain).block(Duration.ZERO);
+
+		URI uri = this.filterChain.uri;
+		assertEquals(new URI("https://84.198.58.199/path"), uri);
+	}
+
+	@Test
+	public void requestUriWithForwardedPrefix() throws Exception {
+		MockServerWebExchange exchange = MockServerHttpRequest.get("http://example.com/path")
+				.header("X-Forwarded-Prefix", "/prefix")
+				.toExchange();
+
+		this.filter.filter(exchange, this.filterChain).block(Duration.ZERO);
+
+		URI uri = this.filterChain.uri;
+		assertEquals(new URI("http://example.com/prefix/path"), uri);
+	}
+
+	@Test
+	public void requestUriWithForwardedPrefixTrailingSlash() throws Exception {
+		MockServerWebExchange exchange = MockServerHttpRequest.get("http://example.com/path")
+				.header("X-Forwarded-Prefix", "/prefix/")
+				.toExchange();
+
+		this.filter.filter(exchange, this.filterChain).block(Duration.ZERO);
+
+		URI uri = this.filterChain.uri;
+		assertEquals(new URI("http://example.com/prefix/path"), uri);
+	}
+
+
+	private static class TestWebFilterChain implements WebFilterChain {
+
+		@Nullable
+		private HttpHeaders httpHeaders;
+
+		@Nullable
+		private URI uri;
+
+		@Nullable
+		public HttpHeaders getHeaders() {
+			return this.httpHeaders;
+		}
+
+		@Nullable
+		public URI getUri() {
+			return this.uri;
+		}
+
+		@Override
+		public Mono<Void> filter(ServerWebExchange exchange) {
+			this.httpHeaders = exchange.getRequest().getHeaders();
+			this.uri = exchange.getRequest().getURI();
+			return Mono.empty();
+		}
+	}
+
+
+
+}