Mutated ServerHttpRequest returns native request correctly

Closes gh-26304

spring-web/src/main/java/org/springframework/http/server/reactive/DefaultServerHttpRequestBuilder.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2020 the original author or authors.
+ * Copyright 2002-2021 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.
@@ -236,7 +236,7 @@ class DefaultServerHttpRequestBuilder implements ServerHttpRequest.Builder {
 		@SuppressWarnings("unchecked")
 		@Override
 		public <T> T getNativeRequest() {
-			return (T) this.originalRequest;
+			return ServerHttpRequestDecorator.getNativeRequest(this.originalRequest);
 		}
 
 		@Override

spring-web/src/main/java/org/springframework/http/server/reactive/ServerHttpRequestDecorator.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2020 the original author or authors.
+ * Copyright 2002-2021 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.
@@ -120,6 +120,28 @@ public class ServerHttpRequestDecorator implements ServerHttpRequest {
 	}
 
 
+	/**
+	 * Return the native request of the underlying server API, if possible,
+	 * also unwrapping {@link ServerHttpRequestDecorator} if necessary.
+	 * @param request the request to check
+	 * @param <T> the expected native request type
+	 * @throws IllegalArgumentException if the native request can't be obtained
+	 * @since 5.3.3
+	 */
+	public static <T> T getNativeRequest(ServerHttpRequest request) {
+		if (request instanceof AbstractServerHttpRequest) {
+			return ((AbstractServerHttpRequest) request).getNativeRequest();
+		}
+		else if (request instanceof ServerHttpRequestDecorator) {
+			return getNativeRequest(((ServerHttpRequestDecorator) request).getDelegate());
+		}
+		else {
+			throw new IllegalArgumentException(
+					"Can't find native request in " + request.getClass().getName());
+		}
+	}
+
+
 	@Override
 	public String toString() {
 		return getClass().getSimpleName() + " [delegate=" + getDelegate() + "]";

spring-web/src/main/java/org/springframework/http/server/reactive/ServerHttpResponseDecorator.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2018 the original author or authors.
+ * Copyright 2002-2021 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.
@@ -110,6 +110,28 @@ public class ServerHttpResponseDecorator implements ServerHttpResponse {
 	}
 
 
+	/**
+	 * Return the native response of the underlying server API, if possible,
+	 * also unwrapping {@link ServerHttpResponseDecorator} if necessary.
+	 * @param response the response to check
+	 * @param <T> the expected native response type
+	 * @throws IllegalArgumentException if the native response can't be obtained
+	 * @since 5.3.3
+	 */
+	public static <T> T getNativeResponse(ServerHttpResponse response) {
+		if (response instanceof AbstractServerHttpResponse) {
+			return ((AbstractServerHttpResponse) response).getNativeResponse();
+		}
+		else if (response instanceof ServerHttpResponseDecorator) {
+			return getNativeResponse(((ServerHttpResponseDecorator) response).getDelegate());
+		}
+		else {
+			throw new IllegalArgumentException(
+					"Can't find native response in " + response.getClass().getName());
+		}
+	}
+
+
 	@Override
 	public String toString() {
 		return getClass().getSimpleName() + " [delegate=" + getDelegate() + "]";