GH-746 Fix support for Cloud Event properly enriching Function<Mono, Mono>

Resolves #746

spring-cloud-function-context/src/main/java/org/springframework/cloud/function/context/catalog/SimpleFunctionRegistry.java

@@ -841,13 +841,8 @@ public class SimpleFunctionRegistry implements FunctionRegistry, FunctionInspect
 			}
 			Object result = ((Function) this.target).apply(inputValue);
 
-			if (result instanceof Flux && functionInvocationHelper != null) {
-				result = ((Flux) result).map(v -> {
-					if (firstInputMessage.get() != null && CloudEventMessageUtils.isCloudEvent(firstInputMessage.get())) {
-						return functionInvocationHelper.postProcessResult(v, firstInputMessage.get());
-					}
-					return v;
-				});
+			if (result instanceof Publisher && functionInvocationHelper != null) {
+				result = this.postProcessFunction((Publisher) result, firstInputMessage);
 			}
 
 			return value instanceof OriginalMessageHolder
@@ -855,6 +850,40 @@ public class SimpleFunctionRegistry implements FunctionRegistry, FunctionInspect
 					: result;
 		}
 
+		@SuppressWarnings("unchecked")
+		private Publisher postProcessFunction(Publisher result, AtomicReference<Message<?>> firstInputMessage) {
+			if (FunctionTypeUtils.isPublisher(this.inputType) && FunctionTypeUtils.isPublisher(this.outputType)) {
+				if (!FunctionTypeUtils.getRawType(FunctionTypeUtils.getImmediateGenericType(this.inputType, 0))
+						.isAssignableFrom(Void.class)
+					&& !FunctionTypeUtils.getRawType(FunctionTypeUtils.getImmediateGenericType(this.outputType, 0))
+						.isAssignableFrom(Void.class)) {
+
+					if (result instanceof Mono) {
+						return Mono.from((result)).map(v -> {
+							if (firstInputMessage.get() != null && CloudEventMessageUtils
+									.isCloudEvent(firstInputMessage.get())) {
+								return functionInvocationHelper.postProcessResult(v,
+										firstInputMessage.get());
+							}
+							return v;
+						});
+					}
+					else {
+						return Flux.from((result)).map(v -> {
+							if (firstInputMessage.get() != null && CloudEventMessageUtils
+									.isCloudEvent(firstInputMessage.get())) {
+								return functionInvocationHelper.postProcessResult(v,
+										firstInputMessage.get());
+							}
+							return v;
+						});
+					}
+				}
+			}
+
+			return result;
+		}
+
 		/*
 		 *
 		 */

spring-cloud-function-context/src/test/java/org/springframework/cloud/function/cloudevent/CloudEventFunctionTests.java

@@ -23,6 +23,7 @@ import java.util.function.Function;
 
 import org.junit.jupiter.api.Test;
 import reactor.core.publisher.Flux;
+import reactor.core.publisher.Mono;
 
 import org.springframework.boot.autoconfigure.EnableAutoConfiguration;
 import org.springframework.boot.builder.SpringApplicationBuilder;
@@ -153,6 +154,42 @@ public class CloudEventFunctionTests {
 		assertThat(message.getHeaders().get("ce_source")).isEqualTo(URI.create("http://spring.io/"));
 	}
 
+	@SuppressWarnings("unchecked")
+	@Test
+	public void testBinaryPojoToPojoDefaultOutputHeaderProviderReactiveMono() {
+		Function<Object, Object> function = this.lookup("springReleaseReactiveMono", TestConfiguration.class);
+
+		String id = UUID.randomUUID().toString();
+
+		String payload = "{\n" +
+				"        \"version\" : \"1.0\",\n" +
+				"        \"releaseName\" : \"Spring Framework\",\n" +
+				"        \"releaseDate\" : \"24-03-2004\"\n" +
+				"    }";
+
+		Message<String> inputMessage = CloudEventMessageBuilder
+			.withData(payload)
+			.setId(id)
+			.setSource("https://spring.io/")
+			.setType("org.springframework")
+			.setHeader(MessageUtils.TARGET_PROTOCOL, CloudEventMessageUtils.Protocols.KAFKA)
+			.build(CloudEventMessageUtils.AMQP_ATTR_PREFIX);
+
+		assertThat(CloudEventMessageUtils.isCloudEvent(inputMessage)).isTrue();
+
+		Message<?> message = ((Mono<Message<?>>) function.apply(Mono.just(inputMessage))).block();
+
+		/*
+		 * Validates that although user only deals with POJO, the framework recognizes
+		 * both on input and output that it is dealing with Cloud Event and generates
+		 * appropriate headers/attributes
+		 */
+
+		assertThat(CloudEventMessageUtils.isCloudEvent(message)).isTrue();
+		assertThat(CloudEventMessageUtils.getType(message)).isEqualTo(SpringReleaseEvent.class.getName());
+		assertThat(CloudEventMessageUtils.getSource(message)).isEqualTo(URI.create("http://spring.io/"));
+		assertThat(message.getHeaders().get("ce_source")).isEqualTo(URI.create("http://spring.io/"));
+	}
 
 
 	// this kind of emulates that message came from Kafka
@@ -336,6 +373,20 @@ public class CloudEventFunctionTests {
 			});
 		}
 
+		@Bean
+		Function<Mono<SpringReleaseEvent>, Mono<SpringReleaseEvent>> springReleaseReactiveMono() {
+			return mono -> mono.map(event -> {
+				try {
+					event.setReleaseDate(new SimpleDateFormat("dd-MM-yyyy").parse("01-10-2006"));
+					event.setVersion("2.0");
+					return event;
+				}
+				catch (Exception e) {
+					throw new IllegalArgumentException(e);
+				}
+			});
+		}
+
 		@Bean
 		Function<Message<SpringReleaseEvent>, Message<SpringReleaseEvent>> springReleaseAsMessage() {
 			return message -> {