Add support for returning Mono for gRPC requestReply

Resolves #776

spring-cloud-function-adapters/spring-cloud-function-grpc/src/test/java/org/springframework/cloud/function/grpc/GrpcInteractionTests.java

@@ -27,6 +27,7 @@ import org.junit.jupiter.api.BeforeEach;
 import org.junit.jupiter.api.Disabled;
 import org.junit.jupiter.api.Test;
 import reactor.core.publisher.Flux;
+import reactor.core.publisher.Mono;
 
 import org.springframework.boot.WebApplicationType;
 import org.springframework.boot.autoconfigure.EnableAutoConfiguration;
@@ -40,6 +41,7 @@ import org.springframework.util.MimeTypeUtils;
 import org.springframework.util.SocketUtils;
 
 import static org.assertj.core.api.Assertions.assertThat;
+import static org.junit.jupiter.api.Assertions.fail;
 
 /**
  *
@@ -79,6 +81,49 @@ public class GrpcInteractionTests {
 		}
 	}
 
+	@Test
+	public void testRequestReplyWithMonoReturn() {
+		int port = SocketUtils.findAvailableTcpPort();
+		try (ConfigurableApplicationContext context = new SpringApplicationBuilder(
+				SampleConfiguration.class).web(WebApplicationType.NONE).run(
+						"--spring.jmx.enabled=false",
+						"--spring.cloud.function.definition=uppercaseMonoReturn",
+						"--spring.cloud.function.grpc.port=" + port)) {
+
+			Message<byte[]> message = MessageBuilder.withPayload("\"hello gRPC\"".getBytes())
+					.setHeader("foo", "bar")
+					.setHeader(MessageHeaders.CONTENT_TYPE, MimeTypeUtils.TEXT_PLAIN)
+					.build();
+
+			Message<byte[]> reply = GrpcUtils.requestReply("localhost", port, message);
+
+			assertThat(reply.getPayload()).isEqualTo("\"HELLO GRPC\"".getBytes());
+		}
+	}
+
+	@Test
+	public void testRequestReplyWithFluxReturn() {
+		int port = SocketUtils.findAvailableTcpPort();
+		try (ConfigurableApplicationContext context = new SpringApplicationBuilder(
+				SampleConfiguration.class).web(WebApplicationType.NONE).run(
+						"--spring.jmx.enabled=false",
+						"--spring.cloud.function.definition=uppercaseFluxReturn",
+						"--spring.cloud.function.grpc.port=" + port)) {
+
+			Message<byte[]> message = MessageBuilder.withPayload("\"hello gRPC\"".getBytes())
+					.setHeader("foo", "bar")
+					.setHeader(MessageHeaders.CONTENT_TYPE, MimeTypeUtils.TEXT_PLAIN)
+					.build();
+			try {
+				GrpcUtils.requestReply("localhost", port, message);
+				fail();
+			}
+			catch (Exception e) {
+				assertThat(e.getMessage()).contains("Flux reply is not supported for `requestReply` mode");
+			}
+		}
+	}
+
 	@Test
 	public void testRequstReplyFunctionDefinitionInMessage() {
 		int port = SocketUtils.findAvailableTcpPort();
@@ -263,6 +308,16 @@ public class GrpcInteractionTests {
 			return v -> v.toUpperCase();
 		}
 
+		@Bean
+		public Function<String, Mono<String>> uppercaseMonoReturn() {
+			return v -> Mono.just(v.toUpperCase());
+		}
+
+		@Bean
+		public Function<String, Flux<String>> uppercaseFluxReturn() {
+			return v -> Flux.just(v.toUpperCase(), v.toUpperCase() + "-1", v.toUpperCase() + "-2");
+		}
+
 		@Bean
 		public Function<String, String> reverse() {
 			return v -> new StringBuilder(v).reverse().toString();