Dispatch in UndertowHttpHandlerAdapter

This ensures that the reactive handling of the request is dispatched
from the Undertow IO thread, marking the exchange as async rather than
ending it once the Undertow `handleRequest` method returns.

See gh-33885
Closes gh-33969

Author: simonbasle

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

@@ -66,25 +66,27 @@ public DataBufferFactory getDataBufferFactory() {
 
 	@Override
 	public void handleRequest(HttpServerExchange exchange) {
-		UndertowServerHttpRequest request = null;
-		try {
-			request = new UndertowServerHttpRequest(exchange, getDataBufferFactory());
-		}
-		catch (URISyntaxException ex) {
-			if (logger.isWarnEnabled()) {
-				logger.debug("Failed to get request URI: " + ex.getMessage());
+		exchange.dispatch(() -> {
+			UndertowServerHttpRequest request = null;
+			try {
+				request = new UndertowServerHttpRequest(exchange, getDataBufferFactory());
 			}
-			exchange.setStatusCode(400);
-			return;
-		}
-		ServerHttpResponse response = new UndertowServerHttpResponse(exchange, getDataBufferFactory(), request);
+			catch (URISyntaxException ex) {
+				if (logger.isWarnEnabled()) {
+					logger.debug("Failed to get request URI: " + ex.getMessage());
+				}
+				exchange.setStatusCode(400);
+				return;
+			}
+			ServerHttpResponse response = new UndertowServerHttpResponse(exchange, getDataBufferFactory(), request);
 
-		if (request.getMethod() == HttpMethod.HEAD) {
-			response = new HttpHeadResponseDecorator(response);
-		}
+			if (request.getMethod() == HttpMethod.HEAD) {
+				response = new HttpHeadResponseDecorator(response);
+			}
 
-		HandlerResultSubscriber resultSubscriber = new HandlerResultSubscriber(exchange, request);
-		this.httpHandler.handle(request, response).subscribe(resultSubscriber);
+			HandlerResultSubscriber resultSubscriber = new HandlerResultSubscriber(exchange, request);
+			this.httpHandler.handle(request, response).subscribe(resultSubscriber);
+		});
 	}
 
 

spring-webflux/src/test/java/org/springframework/web/reactive/result/method/annotation/RequestMappingMessageConversionIntegrationTests.java

@@ -17,6 +17,7 @@
 package org.springframework.web.reactive.result.method.annotation;
 
 import java.nio.ByteBuffer;
+import java.time.Duration;
 import java.util.ArrayList;
 import java.util.Arrays;
 import java.util.Collections;
@@ -331,6 +332,17 @@ void personTransformWithFlux(HttpServer httpServer) throws Exception {
 		assertThat(performPost("/person-transform/flux", JSON, req, JSON, PERSON_LIST).getBody()).isEqualTo(res);
 	}
 
+	@ParameterizedHttpServerTest // see gh-33885
+	void personTransformWithFluxDelayed(HttpServer httpServer) throws Exception {
+		startServer(httpServer);
+
+		List<?> req = asList(new Person("Robert"), new Person("Marie"));
+		List<?> res = asList(new Person("ROBERT"), new Person("MARIE"));
+		assertThat(performPost("/person-transform/flux-delayed", JSON, req, JSON, PERSON_LIST))
+				.satisfies(r -> assertThat(r.getBody()).isEqualTo(res))
+				.satisfies(r -> assertThat(r.getHeaders().getContentLength()).isNotZero());
+	}
+
 	@ParameterizedHttpServerTest
 	void personTransformWithObservable(HttpServer httpServer) throws Exception {
 		startServer(httpServer);
@@ -632,6 +644,11 @@ Flux<Person> transformFlux(@RequestBody Flux<Person> persons) {
 			return persons.map(person -> new Person(person.getName().toUpperCase()));
 		}
 
+		@PostMapping("/flux-delayed")
+		Flux<Person> transformDelayed(@RequestBody Flux<Person> persons) {
+			return transformFlux(persons).delayElements(Duration.ofMillis(10));
+		}
+
 		@PostMapping("/observable")
 		Observable<Person> transformObservable(@RequestBody Observable<Person> persons) {
 			return persons.map(person -> new Person(person.getName().toUpperCase()));