GH-1835 Fixed channel-to-publisher adapter

- Needed to copy one from SI to address the issue but the actual fix should go into SI

Resolves #1835

spring-cloud-stream/src/main/java/org/springframework/cloud/stream/function/FunctionConfiguration.java

@@ -34,7 +34,9 @@ import java.util.stream.Stream;
 import org.apache.commons.logging.Log;
 import org.apache.commons.logging.LogFactory;
 import org.reactivestreams.Publisher;
+import org.reactivestreams.Subscriber;
 import reactor.core.publisher.Flux;
+import reactor.core.publisher.FluxSink;
 import reactor.core.publisher.Mono;
 import reactor.core.publisher.MonoSink;
 import reactor.util.function.Tuples;
@@ -82,7 +84,6 @@ import org.springframework.core.annotation.AnnotationUtils;
 import org.springframework.core.env.Environment;
 import org.springframework.core.type.MethodMetadata;
 import org.springframework.integration.channel.AbstractMessageChannel;
-import org.springframework.integration.channel.MessageChannelReactiveUtils;
 import org.springframework.integration.dsl.IntegrationFlow;
 import org.springframework.integration.dsl.IntegrationFlowBuilder;
 import org.springframework.integration.dsl.IntegrationFlows;
@@ -91,6 +92,7 @@ import org.springframework.integration.support.MessageBuilder;
 import org.springframework.lang.Nullable;
 import org.springframework.messaging.Message;
 import org.springframework.messaging.MessageChannel;
+import org.springframework.messaging.MessageHandler;
 import org.springframework.messaging.MessageHeaders;
 import org.springframework.messaging.SubscribableChannel;
 import org.springframework.util.Assert;
@@ -364,7 +366,7 @@ public class FunctionConfiguration {
 
 			Publisher[] inputPublishers = inputBindingNames.stream().map(inputBindingName -> {
 				SubscribableChannel inputChannel = this.context.getBean(inputBindingName, SubscribableChannel.class);
-				return this.enhancePublisher(MessageChannelReactiveUtils.toPublisher(inputChannel), inputBindingName);
+				return this.enhancePublisher(new SubscribableChannelPublisherAdapter(inputChannel), inputBindingName);
 			}).toArray(Publisher[]::new);
 
 
@@ -425,7 +427,7 @@ public class FunctionConfiguration {
 			if (FunctionTypeUtils.isReactive(FunctionTypeUtils.getInputType(functionType, 0)) && StringUtils.hasText(outputChannelName)) {
 				MessageChannel outputChannel = context.getBean(outputChannelName, MessageChannel.class);
 				SubscribableChannel subscribeChannel = (SubscribableChannel) inputChannel;
-				Publisher<?> publisher = this.enhancePublisher(MessageChannelReactiveUtils.toPublisher(subscribeChannel),
+				Publisher<?> publisher = this.enhancePublisher(new SubscribableChannelPublisherAdapter<>(subscribeChannel),
 																((DirectWithAttributesChannel) inputChannel).getBeanName());
 				this.subscribeToInput(function, publisher, message -> outputChannel.send((Message<?>) message));
 			}
@@ -677,4 +679,27 @@ public class FunctionConfiguration {
 		}
 
 	}
+
+	private static final class SubscribableChannelPublisherAdapter<T> implements Publisher<Message<T>> {
+
+		private final SubscribableChannel channel;
+
+		SubscribableChannelPublisherAdapter(SubscribableChannel channel) {
+			this.channel = channel;
+		}
+
+		@Override
+		@SuppressWarnings("unchecked")
+		public void subscribe(Subscriber<? super Message<T>> subscriber) {
+			Flux.
+					<Message<?>>push(emitter -> {
+								MessageHandler messageHandler = emitter::next;
+								this.channel.subscribe(messageHandler);
+								emitter.onCancel(() -> this.channel.unsubscribe(messageHandler));
+							},
+							FluxSink.OverflowStrategy.BUFFER)
+					.subscribe((Subscriber<? super Message<?>>) subscriber);
+		}
+
+	}
 }