Fix empty payload handling in RSocketRequester

Closes gh-24088

spring-messaging/src/main/java/org/springframework/messaging/rsocket/DefaultRSocketRequester.java

@@ -57,7 +57,7 @@ final class DefaultRSocketRequester implements RSocketRequester {
 
 	private final RSocketStrategies strategies;
 
-	private final DataBuffer emptyDataBuffer;
+	private final Mono<DataBuffer> emptyBufferMono;
 
 
 	DefaultRSocketRequester(
@@ -73,7 +73,7 @@ final class DefaultRSocketRequester implements RSocketRequester {
 		this.dataMimeType = dataMimeType;
 		this.metadataMimeType = metadataMimeType;
 		this.strategies = strategies;
-		this.emptyDataBuffer = this.strategies.dataBufferFactory().wrap(new byte[0]);
+		this.emptyBufferMono = Mono.just(this.strategies.dataBufferFactory().wrap(new byte[0]));
 	}
 
 
@@ -193,7 +193,7 @@ final class DefaultRSocketRequester implements RSocketRequester {
 			}
 
 			if (isVoid(elementType) || (adapter != null && adapter.isNoValue())) {
-				this.payloadMono = firstPayload(Mono.when(publisher).then(Mono.just(emptyDataBuffer)));
+				this.payloadMono = Mono.when(publisher).then(firstPayload(emptyBufferMono));
 				this.payloadFlux = null;
 				return;
 			}
@@ -204,7 +204,7 @@ final class DefaultRSocketRequester implements RSocketRequester {
 			if (adapter != null && !adapter.isMultiValue()) {
 				Mono<DataBuffer> data = Mono.from(publisher)
 						.map(value -> encodeData(value, elementType, encoder))
-						.defaultIfEmpty(emptyDataBuffer);
+						.switchIfEmpty(emptyBufferMono);
 				this.payloadMono = firstPayload(data);
 				this.payloadFlux = null;
 				return;
@@ -213,7 +213,7 @@ final class DefaultRSocketRequester implements RSocketRequester {
 			this.payloadMono = null;
 			this.payloadFlux = Flux.from(publisher)
 					.map(value -> encodeData(value, elementType, encoder))
-					.defaultIfEmpty(emptyDataBuffer)
+					.switchIfEmpty(emptyBufferMono)
 					.switchOnFirst((signal, inner) -> {
 						DataBuffer data = signal.get();
 						if (data != null) {
@@ -250,12 +250,7 @@ final class DefaultRSocketRequester implements RSocketRequester {
 
 		@Override
 		public Mono<Void> send() {
-			return getPayloadMonoRequired().flatMap(rsocket::fireAndForget);
-		}
-
-		private Mono<Payload> getPayloadMonoRequired() {
-			Assert.state(this.payloadFlux == null, "No RSocket interaction model for Flux request to Mono response.");
-			return this.payloadMono != null ? this.payloadMono : firstPayload(Mono.just(emptyDataBuffer));
+			return getPayloadMono().flatMap(rsocket::fireAndForget);
 		}
 
 		@Override
@@ -268,19 +263,9 @@ final class DefaultRSocketRequester implements RSocketRequester {
 			return retrieveMono(ResolvableType.forType(dataTypeRef));
 		}
 
-		@Override
-		public <T> Flux<T> retrieveFlux(Class<T> dataType) {
-			return retrieveFlux(ResolvableType.forClass(dataType));
-		}
-
-		@Override
-		public <T> Flux<T> retrieveFlux(ParameterizedTypeReference<T> dataTypeRef) {
-			return retrieveFlux(ResolvableType.forType(dataTypeRef));
-		}
-
 		@SuppressWarnings("unchecked")
 		private <T> Mono<T> retrieveMono(ResolvableType elementType) {
-			Mono<Payload> payloadMono = getPayloadMonoRequired().flatMap(rsocket::requestResponse);
+			Mono<Payload> payloadMono = getPayloadMono().flatMap(rsocket::requestResponse);
 
 			if (isVoid(elementType)) {
 				return (Mono<T>) payloadMono.then();
@@ -291,11 +276,22 @@ final class DefaultRSocketRequester implements RSocketRequester {
 					.map(dataBuffer -> decoder.decode(dataBuffer, elementType, dataMimeType, EMPTY_HINTS));
 		}
 
+		@Override
+		public <T> Flux<T> retrieveFlux(Class<T> dataType) {
+			return retrieveFlux(ResolvableType.forClass(dataType));
+		}
+
+		@Override
+		public <T> Flux<T> retrieveFlux(ParameterizedTypeReference<T> dataTypeRef) {
+			return retrieveFlux(ResolvableType.forType(dataTypeRef));
+		}
+
 		@SuppressWarnings("unchecked")
 		private <T> Flux<T> retrieveFlux(ResolvableType elementType) {
-			Flux<Payload> payloadFlux = this.payloadMono != null ?
-					this.payloadMono.flatMapMany(rsocket::requestStream) :
-					rsocket.requestChannel(this.payloadFlux);
+
+			Flux<Payload> payloadFlux = (this.payloadFlux != null ?
+					rsocket.requestChannel(this.payloadFlux) :
+					getPayloadMono().flatMapMany(rsocket::requestStream));
 
 			if (isVoid(elementType)) {
 				return payloadFlux.thenMany(Flux.empty());
@@ -306,6 +302,11 @@ final class DefaultRSocketRequester implements RSocketRequester {
 					(T) decoder.decode(dataBuffer, elementType, dataMimeType, EMPTY_HINTS));
 		}
 
+		private Mono<Payload> getPayloadMono() {
+			Assert.state(this.payloadFlux == null, "No RSocket interaction with Flux request and Mono response.");
+			return this.payloadMono != null ? this.payloadMono : firstPayload(emptyBufferMono);
+		}
+
 		private DataBuffer retainDataAndReleasePayload(Payload payload) {
 			return PayloadUtils.retainDataAndReleasePayload(payload, bufferFactory());
 		}

spring-messaging/src/test/java/org/springframework/messaging/rsocket/DefaultRSocketRequesterTests.java

@@ -145,15 +145,6 @@ public class DefaultRSocketRequesterTests {
 		assertThat(this.rsocket.getSavedPayload().getDataUtf8()).isEqualTo("");
 	}
 
-	@Test
-	public void sendMonoWithoutData() {
-		this.requester.route("toA").retrieveMono(String.class).block(Duration.ofSeconds(5));
-
-		assertThat(this.rsocket.getSavedMethodName()).isEqualTo("requestResponse");
-		assertThat(this.rsocket.getSavedPayload().getMetadataUtf8()).isEqualTo("toA");
-		assertThat(this.rsocket.getSavedPayload().getDataUtf8()).isEqualTo("");
-	}
-
 	@Test
 	public void testSendWithAsyncMetadata() {
 
@@ -205,6 +196,15 @@ public class DefaultRSocketRequesterTests {
 		assertThat(this.rsocket.getSavedMethodName()).isEqualTo("requestResponse");
 	}
 
+	@Test
+	public void retrieveMonoWithoutData() {
+		this.requester.route("toA").retrieveMono(String.class).block(Duration.ofSeconds(5));
+
+		assertThat(this.rsocket.getSavedMethodName()).isEqualTo("requestResponse");
+		assertThat(this.rsocket.getSavedPayload().getMetadataUtf8()).isEqualTo("toA");
+		assertThat(this.rsocket.getSavedPayload().getDataUtf8()).isEqualTo("");
+	}
+
 	@Test
 	public void retrieveFlux() {
 		String[] values = new String[] {"bodyA", "bodyB", "bodyC"};
@@ -227,11 +227,20 @@ public class DefaultRSocketRequesterTests {
 		assertThat(this.rsocket.getSavedMethodName()).isEqualTo("requestStream");
 	}
 
+	@Test
+	public void retrieveFluxWithoutData() {
+		this.requester.route("toA").retrieveFlux(String.class).blockLast(Duration.ofSeconds(5));
+
+		assertThat(this.rsocket.getSavedMethodName()).isEqualTo("requestStream");
+		assertThat(this.rsocket.getSavedPayload().getMetadataUtf8()).isEqualTo("toA");
+		assertThat(this.rsocket.getSavedPayload().getDataUtf8()).isEqualTo("");
+	}
+
 	@Test
 	public void fluxToMonoIsRejected() {
 		assertThatIllegalStateException()
 				.isThrownBy(() -> this.requester.route("").data(Flux.just("a", "b")).retrieveMono(String.class))
-				.withMessage("No RSocket interaction model for Flux request to Mono response.");
+				.withMessage("No RSocket interaction with Flux request and Mono response.");
 	}
 
 	private Payload toPayload(String value) {