Support suspending functions returning Unit in WebFlux

Closes gh-27629

Author: sdeleuze

spring-webflux/src/main/java/org/springframework/web/reactive/result/method/InvocableHandlerMethod.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2021 the original author or authors.
+ * Copyright 2002-2022 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.
@@ -129,15 +129,16 @@ public void setReactiveAdapterRegistry(ReactiveAdapterRegistry registry) {
 	 * @param providedArgs optional list of argument values to match by type
 	 * @return a Mono with a {@link HandlerResult}
 	 */
-	@SuppressWarnings("KotlinInternalInJava")
+	@SuppressWarnings({"KotlinInternalInJava", "unchecked"})
 	public Mono<HandlerResult> invoke(
 			ServerWebExchange exchange, BindingContext bindingContext, Object... providedArgs) {
 
 		return getMethodArgumentValues(exchange, bindingContext, providedArgs).flatMap(args -> {
 			Object value;
+			Method method = getBridgedMethod();
+			boolean isSuspendingFunction = KotlinDetector.isSuspendingFunction(method);
 			try {
-				Method method = getBridgedMethod();
-				if (KotlinDetector.isSuspendingFunction(method)) {
+				if (isSuspendingFunction) {
 					value = CoroutinesUtils.invokeSuspendingFunction(method, getBean(), args);
 				}
 				else {
@@ -163,10 +164,16 @@ public Mono<HandlerResult> invoke(
 			}
 
 			MethodParameter returnType = getReturnType();
-			ReactiveAdapter adapter = this.reactiveAdapterRegistry.getAdapter(returnType.getParameterType());
-			boolean asyncVoid = isAsyncVoidReturnType(returnType, adapter);
-			if ((value == null || asyncVoid) && isResponseHandled(args, exchange)) {
-				return (asyncVoid ? Mono.from(adapter.toPublisher(value)) : Mono.empty());
+			if (isResponseHandled(args, exchange)) {
+				Class<?> parameterType = returnType.getParameterType();
+				ReactiveAdapter adapter = this.reactiveAdapterRegistry.getAdapter(parameterType);
+				boolean asyncVoid = isAsyncVoidReturnType(returnType, adapter);
+				if (value == null || asyncVoid) {
+					return (asyncVoid ? Mono.from(adapter.toPublisher(value)) : Mono.empty());
+				}
+				if (isSuspendingFunction && parameterType == void.class) {
+					return (Mono<HandlerResult>) value;
+				}
 			}
 
 			HandlerResult result = new HandlerResult(this, value, returnType, bindingContext);

spring-webflux/src/test/kotlin/org/springframework/web/reactive/result/KotlinInvocableHandlerMethodTests.kt

@@ -34,6 +34,7 @@ import org.springframework.web.reactive.result.method.annotation.ContinuationHan
 import reactor.core.publisher.Mono
 import reactor.test.StepVerifier
 import java.lang.reflect.Method
+import java.time.Duration
 import kotlin.reflect.jvm.javaMethod
 
 class KotlinInvocableHandlerMethodTests {
@@ -89,11 +90,9 @@ class KotlinInvocableHandlerMethodTests {
 		val response = this.exchange.response
 		this.resolvers.add(stubResolver(response))
 		val method = CoroutinesController::response.javaMethod!!
-		val result = invoke(CoroutinesController(), method)
+		val result = invokeForResult(CoroutinesController(), method, response)
 
-		StepVerifier.create(result)
-				.consumeNextWith { StepVerifier.create(it.returnValue as Mono<*>).verifyComplete() }
-				.verifyComplete()
+		assertThat(result).`as`("Expected no result (i.e. fully handled)").isNull()
 		assertThat(this.exchange.response.headers.getFirst("foo")).isEqualTo("bar")
 	}
 
@@ -105,6 +104,10 @@ class KotlinInvocableHandlerMethodTests {
 		assertHandlerResultValue(result, "success:foo")
 	}
 
+	private fun invokeForResult(handler: Any, method: Method, vararg providedArgs: Any): HandlerResult? {
+		return invoke(handler, method, *providedArgs).block(Duration.ofSeconds(5))
+	}
+
 	private fun invoke(handler: Any, method: Method, vararg providedArgs: Any?): Mono<HandlerResult> {
 		val invocable = InvocableHandlerMethod(handler, method)
 		invocable.setArgumentResolvers(this.resolvers)