GH-8950: Fix PublisherIntegrationFlow for AbstractEndpoint

Fixes: #8950

The `IntegrationFlow.toReactivePublisher(true)` makes `PublisherIntegrationFlow`
to be stopped from the beginning and waiting for the `Publisher.subscribe()`.
However only `EndpointSpec` components are marked as `autoStartup(false)`.
The `MessageProducerSupport` is not included, therefore unexpected messages
are produced to the channel in the end of flow without subscribers.

* Check for `AbstractEndpoint` component type in the `PublisherIntegrationFlow`
and mark it as `setAutoStartup(false)`
* Ensure in a new `ReactiveStreamsTests.messageProducerIsNotStartedAutomatically()` test
that `MessageProducerSupport` is not started from the beginning,
but rather when we subscribe to the `Publisher` via `StepVerifier`

**Auto-cherry-pick to `6.2.x` & `6.1.x`**

Author: artembilan

spring-integration-core/src/main/java/org/springframework/integration/dsl/PublisherIntegrationFlow.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2016-2022 the original author or authors.
+ * Copyright 2016-2024 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.
@@ -22,6 +22,7 @@
 import org.reactivestreams.Subscriber;
 import reactor.core.publisher.Flux;
 
+import org.springframework.integration.endpoint.AbstractEndpoint;
 import org.springframework.messaging.Message;
 
 /**
@@ -48,8 +49,11 @@ class PublisherIntegrationFlow<T> extends StandardIntegrationFlow implements Pub
 		if (autoStartOnSubscribe) {
 			flux = flux.doOnSubscribe((sub) -> start());
 			for (Object component : integrationComponents.keySet()) {
-				if (component instanceof EndpointSpec) {
-					((EndpointSpec<?, ?, ?>) component).autoStartup(false);
+				if (component instanceof EndpointSpec<?, ?, ?> endpointSpec) {
+					endpointSpec.autoStartup(false);
+				}
+				else if (component instanceof AbstractEndpoint endpoint) {
+					endpoint.setAutoStartup(false);
 				}
 			}
 		}

spring-integration-core/src/test/java/org/springframework/integration/dsl/reactivestreams/ReactiveStreamsTests.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2016-2023 the original author or authors.
+ * Copyright 2016-2024 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.
@@ -34,6 +34,7 @@
 import reactor.core.Disposable;
 import reactor.core.publisher.Flux;
 import reactor.core.scheduler.Schedulers;
+import reactor.test.StepVerifier;
 
 import org.springframework.beans.factory.annotation.Autowired;
 import org.springframework.beans.factory.annotation.Qualifier;
@@ -45,8 +46,11 @@
 import org.springframework.integration.config.EnableIntegration;
 import org.springframework.integration.dsl.IntegrationFlow;
 import org.springframework.integration.dsl.MessageChannels;
+import org.springframework.integration.dsl.MessageProducerSpec;
 import org.springframework.integration.dsl.context.IntegrationFlowContext;
 import org.springframework.integration.endpoint.AbstractEndpoint;
+import org.springframework.integration.endpoint.MessageProducerSupport;
+import org.springframework.integration.endpoint.ReactiveMessageSourceProducer;
 import org.springframework.integration.endpoint.ReactiveStreamsConsumer;
 import org.springframework.messaging.Message;
 import org.springframework.messaging.MessageChannel;
@@ -240,6 +244,27 @@ void fixedSubscriberChannelFlowTest() throws InterruptedException {
 		assertThat(latch.await(10, TimeUnit.SECONDS)).isTrue();
 	}
 
+	@Autowired
+	MessageProducerSupport testMessageProducer;
+
+	@Autowired
+	Publisher<Message<String>> messageProducerFlow;
+
+	@Test
+	void messageProducerIsNotStartedAutomatically() {
+		assertThat(this.testMessageProducer.isRunning()).isFalse();
+
+		Flux<String> flux =
+				Flux.from(this.messageProducerFlow)
+						.map(Message::getPayload);
+
+		StepVerifier.create(flux)
+				.expectNext("test")
+				.expectNext("test")
+				.thenCancel()
+				.verify(Duration.ofSeconds(10));
+	}
+
 	@Configuration
 	@EnableIntegration
 	public static class ContextConfiguration {
@@ -287,6 +312,26 @@ public Publisher<Message<String>> fixedSubscriberChannelFlow() {
 					.toReactivePublisher();
 		}
 
+		@Bean
+		public Publisher<Message<String>> messageProducerFlow() {
+			TestMessageProducerSpec testMessageProducerSpec =
+					new TestMessageProducerSpec(new ReactiveMessageSourceProducer(() -> new GenericMessage<>("test")))
+							.id("testMessageProducer");
+
+			return IntegrationFlow
+					.from(testMessageProducerSpec)
+					.toReactivePublisher(true);
+		}
+
+	}
+
+	private static class TestMessageProducerSpec
+			extends MessageProducerSpec<TestMessageProducerSpec, ReactiveMessageSourceProducer> {
+
+		TestMessageProducerSpec(ReactiveMessageSourceProducer producer) {
+			super(producer);
+		}
+
 	}
 
 }