Pass preflight request to upstream. Fixes gh-2472

Author: tommas1988

docs/src/main/asciidoc/spring-cloud-gateway.adoc

@@ -2396,6 +2396,31 @@ In the preceding example, CORS requests are allowed from requests that originate
 To provide the same CORS configuration to requests that are not handled by some gateway route predicate, set the `spring.cloud.gateway.globalcors.add-to-simple-url-handler-mapping` property  to `true`.
 This is useful when you try to support CORS preflight requests and your route predicate does not evalute to `true` because the HTTP method is `options`.
 
+== Handle CORS Preflight Request By Upstream
+
+You can configure the gateway to pass CORS preflight request to upstream per route configuration.
+The following example shows how to do this:
+
+.application.yml
+====
+[source,yaml]
+----
+spring:
+  cloud:
+    gateway:
+      routes:
+      - id: handle_pre_flight_request_by_upstream
+        uri: ${upstream-uri}
+        handle-preflight-request-by-upstream: true
+        predicates:
+          - Path=/some-path/**
+----
+====
+
+In the preceding example, CORS preflight request will be handled by upstream for this route configuration, no matter what `globalcors` is configured.
+
+Route configuration without `handle-preflight-request-by-upstream` or explicitly setting to `false` will obey gateway CORS configuration.
+
 == Actuator API
 
 The `/gateway` actuator endpoint lets you monitor and interact with a Spring Cloud Gateway application.

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

@@ -52,4 +52,9 @@ public int getOrder() {
 		return 0;
 	}
 
+	@Override
+	public boolean handlePreFlightRequest() {
+		return true;
+	}
+
 }

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

@@ -31,7 +31,7 @@
  * @author Rossen Stoyanchev
  * @since 5.0
  */
-public interface GatewayFilter extends ShortcutConfigurable {
+public interface GatewayFilter extends PreFlightRequestFilter, ShortcutConfigurable {
 
 	/**
 	 * Name key.
@@ -52,4 +52,9 @@ public interface GatewayFilter extends ShortcutConfigurable {
 	 */
 	Mono<Void> filter(ServerWebExchange exchange, GatewayFilterChain chain);
 
+	@Override
+	default boolean handlePreFlightRequest() {
+		return true;
+	}
+
 }

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

@@ -28,7 +28,7 @@
  * @author Rossen Stoyanchev
  * @since 5.0
  */
-public interface GlobalFilter {
+public interface GlobalFilter extends PreFlightRequestFilter {
 
 	/**
 	 * Process the Web request and (optionally) delegate to the next {@code WebFilter}
@@ -39,4 +39,9 @@ public interface GlobalFilter {
 	 */
 	Mono<Void> filter(ServerWebExchange exchange, GatewayFilterChain chain);
 
+	@Override
+	default boolean handlePreFlightRequest() {
+		return false;
+	}
+
 }

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

@@ -69,7 +69,7 @@
  * @author Spencer Gibb
  * @author Biju Kunjummen
  */
-public class NettyRoutingFilter implements GlobalFilter, Ordered {
+public class NettyRoutingFilter implements GlobalFilter, PreFlightRequestFilter, Ordered {
 
 	/**
 	 * The order of the NettyRoutingFilter. See {@link Ordered#LOWEST_PRECEDENCE}.
@@ -198,6 +198,11 @@ public Mono<Void> filter(ServerWebExchange exchange, GatewayFilterChain chain) {
 		return responseFlux.then(chain.filter(exchange));
 	}
 
+	@Override
+	public boolean handlePreFlightRequest() {
+		return true;
+	}
+
 	protected ByteBuf getByteBuf(DataBuffer dataBuffer) {
 		if (dataBuffer instanceof NettyDataBuffer) {
 			NettyDataBuffer buffer = (NettyDataBuffer) dataBuffer;

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

@@ -44,6 +44,11 @@ public Mono<Void> filter(ServerWebExchange exchange, GatewayFilterChain chain) {
 		return this.delegate.filter(exchange, chain);
 	}
 
+	@Override
+	public boolean handlePreFlightRequest() {
+		return delegate.handlePreFlightRequest();
+	}
+
 	@Override
 	public int getOrder() {
 		return this.order;

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

@@ -0,0 +1,32 @@
+/*
+ * Copyright 2013-2022 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
+ *
+ *      https://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.cloud.gateway.filter;
+
+/**
+ * Indicate a filter should handle pre-flight request and pass the request to upstream.
+ * Only filters that affect route process should return true with handlePreFlightRequest method.
+ *
+ * @author Tommas Yuan
+ */
+public interface PreFlightRequestFilter {
+
+	/**
+	 * @return {@code true} to indicate this filter will handle pre-flight request
+	 */
+	boolean handlePreFlightRequest();
+
+}

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

@@ -180,4 +180,9 @@ private String getHint(String serviceId) {
 		return hintPropertyValue != null ? hintPropertyValue : defaultHint;
 	}
 
+	@Override
+	public boolean handlePreFlightRequest() {
+		return true;
+	}
+
 }

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

@@ -92,4 +92,9 @@ public Mono<Void> filter(ServerWebExchange exchange, GatewayFilterChain chain) {
 		return chain.filter(exchange);
 	}
 
+	@Override
+	public boolean handlePreFlightRequest() {
+		return true;
+	}
+
 }

spring-cloud-gateway-server/src/main/java/org/springframework/cloud/gateway/handler/FilteringWebHandler.java

@@ -51,8 +51,12 @@ public class FilteringWebHandler implements WebHandler {
 
 	private final List<GatewayFilter> globalFilters;
 
+	private final List<GatewayFilter> preflightRequestFilters;
+
 	public FilteringWebHandler(List<GlobalFilter> globalFilters) {
 		this.globalFilters = loadFilters(globalFilters);
+		this.preflightRequestFilters = this.globalFilters.stream().filter(filter -> filter.handlePreFlightRequest())
+				.toList();
 	}
 
 	private static List<GatewayFilter> loadFilters(List<GlobalFilter> filters) {
@@ -88,6 +92,23 @@ public Mono<Void> handle(ServerWebExchange exchange) {
 		return new DefaultGatewayFilterChain(combined).filter(exchange);
 	}
 
+	public Mono<Void> handlePreFlight(ServerWebExchange exchange) {
+		Route route = exchange.getRequiredAttribute(GATEWAY_ROUTE_ATTR);
+		List<GatewayFilter> preflightRequestFilters = route.getFilters().stream()
+				.filter(filter -> filter.handlePreFlightRequest()).toList();
+
+		List<GatewayFilter> combined = new ArrayList<>(this.preflightRequestFilters);
+		combined.addAll(preflightRequestFilters);
+
+		AnnotationAwareOrderComparator.sort(combined);
+
+		if (logger.isDebugEnabled()) {
+			logger.debug("Sorted preflight request filters: " + combined);
+		}
+
+		return new DefaultGatewayFilterChain(combined).filter(exchange);
+	}
+
 	private static class DefaultGatewayFilterChain implements GatewayFilterChain {
 
 		private final int index;