GH-454 - Fix event externalization for RabbitMQ.

We now properly wrap the null value returned as a result from the RabbitMQ event externalization into a CompletableFuture. Tightened test case to prevent regressions going forward.

Author: odrotbohm

spring-modulith-events/spring-modulith-events-amqp/src/main/java/org/springframework/modulith/events/amqp/RabbitEventExternalizerConfiguration.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2023 the original author or authors.
+ * Copyright 2023-2024 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.
@@ -15,6 +15,8 @@
  */
 package org.springframework.modulith.events.amqp;
 
+import java.util.concurrent.CompletableFuture;
+
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
 import org.springframework.amqp.rabbit.core.RabbitMessageOperations;
@@ -63,7 +65,7 @@ DelegatingEventExternalizer rabbitEventExternalizer(EventExternalizationConfigur
 
 			operations.convertAndSend(routing.getTarget(), routing.getKey(payload), payload);
 
-			return null;
+			return CompletableFuture.completedFuture(null);
 		});
 	}
 }

spring-modulith-events/spring-modulith-events-amqp/src/test/java/org/springframework/modulith/events/amqp/RabbitEventPublicationIntegrationTests.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2023 the original author or authors.
+ * Copyright 2023-2024 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.
@@ -32,6 +32,7 @@
 import org.springframework.context.ApplicationEventPublisher;
 import org.springframework.context.annotation.Bean;
 import org.springframework.modulith.events.ApplicationModuleListener;
+import org.springframework.modulith.events.CompletedEventPublications;
 import org.springframework.modulith.events.Externalized;
 import org.springframework.transaction.annotation.Transactional;
 import org.testcontainers.containers.RabbitMQContainer;
@@ -47,6 +48,7 @@ class RabbitEventPublicationIntegrationTests {
 
 	@Autowired TestPublisher publisher;
 	@Autowired RabbitAdmin rabbit;
+	@Autowired CompletedEventPublications completed;
 
 	@SpringBootApplication
 	static class TestConfiguration {
@@ -87,6 +89,7 @@ void publishesEventToRabbitMq() throws Exception {
 		var info = rabbit.getQueueInfo("queue");
 
 		assertThat(info.getMessageCount()).isEqualTo(1);
+		assertThat(completed.findAll()).hasSize(1);
 	}
 
 	@Externalized("target")