Fix reactive save of Flux.

Original Pull Request #2581
Closes #2576

(cherry picked from commit d6b5540)

Author: sothawo

src/main/java/org/springframework/data/elasticsearch/core/AbstractReactiveElasticsearchTemplate.java

@@ -225,42 +225,42 @@ public <T> Flux<T> save(Flux<T> entities, IndexCoordinates index, int bulkSize)
 
 		return Flux.defer(() -> {
 			Sinks.Many<T> sink = Sinks.many().unicast().onBackpressureBuffer();
-			entities //
-					.bufferTimeout(bulkSize, Duration.ofMillis(200)) //
-					.subscribe(new Subscriber<List<T>>() {
-						private Subscription subscription;
-						private AtomicBoolean upstreamComplete = new AtomicBoolean(false);
-
-						@Override
-						public void onSubscribe(Subscription subscription) {
-							this.subscription = subscription;
-							subscription.request(1);
-						}
-
-						@Override
-						public void onNext(List<T> entityList) {
-							saveAll(entityList, index) //
-									.map(sink::tryEmitNext) //
-									.doOnComplete(() -> {
-										if (!upstreamComplete.get()) {
-											subscription.request(1);
-										} else {
-											sink.tryEmitComplete();
-										}
-									}).subscribe();
-						}
-
-						@Override
-						public void onError(Throwable throwable) {
-							subscription.cancel();
-							sink.tryEmitError(throwable);
-						}
-
-						@Override
-						public void onComplete() {
-							upstreamComplete.set(true);
-						}
-					});
+			entities.window(bulkSize) //
+							.concatMap(flux -> flux.collectList()) //
+							.subscribe(new Subscriber<List<T>>() {
+				private Subscription subscription;
+				private AtomicBoolean upstreamComplete = new AtomicBoolean(false);
+
+				@Override
+				public void onSubscribe(Subscription subscription) {
+					this.subscription = subscription;
+					subscription.request(1);
+				}
+
+				@Override
+				public void onNext(List<T> entityList) {
+					saveAll(entityList, index) //
+							.map(sink::tryEmitNext) //
+							.doOnComplete(() -> {
+								if (!upstreamComplete.get()) {
+									subscription.request(1);
+								} else {
+									sink.tryEmitComplete();
+								}
+							}).subscribe();
+				}
+
+				@Override
+				public void onError(Throwable throwable) {
+					subscription.cancel();
+					sink.tryEmitError(throwable);
+				}
+
+				@Override
+				public void onComplete() {
+					upstreamComplete.set(true);
+				}
+			});
 			return sink.asFlux();
 		});
 

src/test/java/org/springframework/data/elasticsearch/core/ReactiveElasticsearchIntegrationTests.java

@@ -28,6 +28,7 @@
 import java.lang.Integer;
 import java.lang.Long;
 import java.lang.Object;
+import java.time.Duration;
 import java.time.LocalDate;
 import java.time.format.DateTimeFormatter;
 import java.util.ArrayList;
@@ -1181,7 +1182,7 @@ void shouldWorkWithReadonlyId() {
 				}).verifyComplete();
 	}
 
-	@Test // #2496
+	@Test // #2496, #2576
 	@DisplayName("should save data from Flux and return saved data in a flux")
 	void shouldSaveDataFromFluxAndReturnSavedDataInAFlux() {
 
@@ -1190,9 +1191,11 @@ void shouldSaveDataFromFluxAndReturnSavedDataInAFlux() {
 				.mapToObj(SampleEntity::of) //
 				.collect(Collectors.toList());
 
-		var entityFlux = Flux.fromIterable(entityList);
+		// we add a random delay to make suure the underlying implementation handles irregular incoming data
+		var entities = Flux.fromIterable(entityList).concatMap(
+				entity -> Mono.just(entity).delay(Duration.ofMillis((long) (Math.random() * 10))).thenReturn(entity));
 
-		operations.save(entityFlux, SampleEntity.class).collectList() //
+		operations.save(entities, SampleEntity.class).collectList() //
 				.as(StepVerifier::create) //
 				.consumeNextWith(savedEntities -> {
 					assertThat(savedEntities).isEqualTo(entityList);