Adds FilterFunctions.setRequestHostHeader()

See gh-2949

spring-cloud-gateway-server-mvc/src/main/java/org/springframework/cloud/gateway/server/mvc/filter/FilterFunctions.java

@@ -177,6 +177,21 @@ public interface FilterFunctions {
 		};
 	}
 
+	@Shortcut
+	static HandlerFilterFunction<ServerResponse, ServerResponse> setRequestHostHeader(String host) {
+		return (request, next) -> {
+			String expandedValue = MvcUtils.expand(request, host);
+			ServerRequest modified = ServerRequest.from(request).headers(httpHeaders -> {
+				httpHeaders.remove(HttpHeaders.HOST);
+				httpHeaders.set(HttpHeaders.HOST, expandedValue);
+			}).build();
+
+			// Make sure the header we just set is preserved
+			modified.attributes().put(MvcUtils.PRESERVE_HOST_HEADER_ATTRIBUTE, true);
+			return next.handle(modified);
+		};
+	}
+
 	static HandlerFilterFunction<ServerResponse, ServerResponse> stripPrefix() {
 		return stripPrefix(1);
 	}

spring-cloud-gateway-server-mvc/src/test/java/org/springframework/cloud/gateway/server/mvc/ServerMvcIntegrationTests.java

@@ -81,6 +81,7 @@ import static org.springframework.cloud.gateway.server.mvc.filter.FilterFunction
 import static org.springframework.cloud.gateway.server.mvc.filter.FilterFunctions.routeId;
 import static org.springframework.cloud.gateway.server.mvc.filter.FilterFunctions.setPath;
 import static org.springframework.cloud.gateway.server.mvc.filter.FilterFunctions.setRequestHeader;
+import static org.springframework.cloud.gateway.server.mvc.filter.FilterFunctions.setRequestHostHeader;
 import static org.springframework.cloud.gateway.server.mvc.filter.FilterFunctions.setStatus;
 import static org.springframework.cloud.gateway.server.mvc.filter.FilterFunctions.stripPrefix;
 import static org.springframework.cloud.gateway.server.mvc.filter.LoadBalancerFilterFunctions.lb;
@@ -373,13 +374,21 @@ public class ServerMvcIntegrationTests {
 	public void setRequestHeaderWorks() {
 		restClient.get().uri("/headers").header("test", "setrequestheader").exchange().expectStatus().isOk()
 				.expectBody(Map.class).consumeWith(res -> {
-					Map<String, Object> map = res.getResponseBody();
-					Map<String, Object> headers = getMap(map, "headers");
+					Map<String, Object> headers = getMap(res.getResponseBody(), "headers");
 					assertThat(headers).doesNotContainEntry("X-Test", "value1");
 					assertThat(headers).containsEntry("X-Test", "value2");
 				});
 	}
 
+	@Test
+	public void setRequestHeaderHostWorks() {
+		restClient.get().uri("/headers").header("Host", "www.setrequesthostheader.org").exchange().expectStatus().isOk()
+				.expectBody(Map.class).consumeWith(res -> {
+					Map<String, Object> headers = getMap(res.getResponseBody(), "headers");
+					assertThat(headers).containsEntry("Host", "otherhost.io");
+				});
+	}
+
 	@SpringBootConfiguration
 	@EnableAutoConfiguration
 	@LoadBalancerClient(name = "httpbin", configuration = TestLoadBalancerConfig.Httpbin.class)
@@ -635,6 +644,17 @@ public class ServerMvcIntegrationTests {
 			// @formatter:on
 		}
 
+		@Bean
+		public RouterFunction<ServerResponse> gatewayRouterFunctionsSetRequestHostHeader() {
+			// @formatter:off
+			return route("testsetrequestheader")
+					.route(host("**.setrequesthostheader.org"), http())
+					.filter(new HttpbinUriResolver())
+					.filter(setRequestHostHeader("otherhost.io"))
+					.build();
+			// @formatter:on
+		}
+
 	}
 
 	@RestController