INT-4350: Handle ResponseEntity in HTTP Inbounds

JIRA: https://jira.spring.io/browse/INT-4350

Spring MVC and Spring WebFlux handles `ResponseEntity` via appropriate
`ReturnValue` handlers.
This way all the headers and status code are fully up to end-user.
The body is handled by the appropriate converter/writer as before

* Add `ResponseEntity` handling to the `HttpRequestHandlingMessagingGateway`
and `WebFluxInboundEndpoint`.
The logic mostly compy/pasted from the `ResponseEntityResultHandler`

spring-integration-webflux/src/main/java/org/springframework/integration/webflux/inbound/WebFluxInboundEndpoint.java

@@ -16,7 +16,9 @@
 
 package org.springframework.integration.webflux.inbound;
 
+import java.time.Instant;
 import java.util.ArrayList;
+import java.util.Arrays;
 import java.util.Collections;
 import java.util.Comparator;
 import java.util.LinkedHashSet;
@@ -35,8 +37,10 @@ import org.springframework.expression.Expression;
 import org.springframework.expression.spel.support.StandardEvaluationContext;
 import org.springframework.http.HttpEntity;
 import org.springframework.http.HttpHeaders;
+import org.springframework.http.HttpMethod;
 import org.springframework.http.HttpStatus;
 import org.springframework.http.MediaType;
+import org.springframework.http.ResponseEntity;
 import org.springframework.http.codec.HttpMessageReader;
 import org.springframework.http.codec.HttpMessageWriter;
 import org.springframework.http.codec.ServerCodecConfigurer;
@@ -75,6 +79,8 @@ public class WebFluxInboundEndpoint extends BaseHttpInboundEndpoint implements W
 
 	private static final MediaType MEDIA_TYPE_APPLICATION_ALL = new MediaType("application");
 
+	private static final List<HttpMethod> SAFE_METHODS = Arrays.asList(HttpMethod.GET, HttpMethod.HEAD);
+
 	private ServerCodecConfigurer codecConfigurer = ServerCodecConfigurer.create();
 
 	private RequestedContentTypeResolver requestedContentTypeResolver = new HeaderContentTypeResolver();
@@ -318,12 +324,44 @@ public class WebFluxInboundEndpoint extends BaseHttpInboundEndpoint implements W
 			return response.setComplete();
 		}
 		else {
-			HttpStatus httpStatus = resolveHttpStatusFromHeaders(replyMessage.getHeaders());
+			final HttpStatus httpStatus = resolveHttpStatusFromHeaders(replyMessage.getHeaders());
 			if (httpStatus != null) {
 				response.setStatusCode(httpStatus);
 			}
 
-			return writeResponseBody(exchange, responseContent);
+			if (responseContent instanceof ResponseEntity) {
+				return Mono.just((ResponseEntity<?>) responseContent)
+						.flatMap(e -> {
+							if (httpStatus == null) {
+								exchange.getResponse().setStatusCode(e.getStatusCode());
+							}
+
+							HttpHeaders entityHeaders = e.getHeaders();
+							HttpHeaders responseHeaders = exchange.getResponse().getHeaders();
+
+							if (!entityHeaders.isEmpty()) {
+								entityHeaders.entrySet().stream()
+										.filter(entry -> !responseHeaders.containsKey(entry.getKey()))
+										.forEach(entry -> responseHeaders.put(entry.getKey(), entry.getValue()));
+							}
+
+							if (e.getBody() == null) {
+								return exchange.getResponse().setComplete();
+							}
+
+							String etag = entityHeaders.getETag();
+							Instant lastModified = Instant.ofEpochMilli(entityHeaders.getLastModified());
+							HttpMethod httpMethod = exchange.getRequest().getMethod();
+							if (SAFE_METHODS.contains(httpMethod) && exchange.checkNotModified(etag, lastModified)) {
+								return exchange.getResponse().setComplete();
+							}
+
+							return writeResponseBody(exchange, e.getBody());
+						});
+			}
+			else {
+				return writeResponseBody(exchange, responseContent);
+			}
 		}
 	}
 

spring-integration-webflux/src/test/java/org/springframework/integration/webflux/inbound/WebFluxInboundEndpointTests.java

@@ -27,6 +27,7 @@ import org.springframework.context.annotation.Bean;
 import org.springframework.context.annotation.Configuration;
 import org.springframework.http.HttpStatus;
 import org.springframework.http.MediaType;
+import org.springframework.http.ResponseEntity;
 import org.springframework.integration.annotation.ServiceActivator;
 import org.springframework.integration.channel.FluxMessageChannel;
 import org.springframework.integration.config.EnableIntegration;
@@ -83,6 +84,17 @@ public class WebFluxInboundEndpointTests {
 				.jsonPath("$[2].name").isEqualTo("John");
 	}
 
+	@Test
+	public void testServerInternalErrorRequest() {
+		this.webTestClient
+				.get()
+				.uri("/error")
+				.accept(MediaType.TEXT_PLAIN)
+				.exchange()
+				.expectStatus()
+				.is5xxServerError();
+	}
+
 	@Configuration
 	@EnableWebFlux
 	@EnableIntegration
@@ -128,6 +140,22 @@ public class WebFluxInboundEndpointTests {
 			return Flux.just(new Person("Jane"), new Person("Jason"), new Person("John"));
 		}
 
+
+		@Bean
+		public WebFluxInboundEndpoint errorInboundEndpoint() {
+			WebFluxInboundEndpoint endpoint = new WebFluxInboundEndpoint();
+			RequestMapping requestMapping = new RequestMapping();
+			requestMapping.setPathPatterns("/error");
+			endpoint.setRequestMapping(requestMapping);
+			endpoint.setRequestChannelName("errorServiceChannel");
+			return endpoint;
+		}
+
+		@ServiceActivator(inputChannel = "errorServiceChannel")
+		public ResponseEntity<String> processHttpRequest() {
+			return new ResponseEntity<>("<500 Internal Server Error,{}>", HttpStatus.INTERNAL_SERVER_ERROR);
+		}
+
 	}