Change adaptPollableChannelToPublisher to Mono

We can use a `Mono.fromCallable()` in combination with `repeat()`
instead of possible race condition deal in the `Flux.create()`

Author: artembilan

spring-integration-core/src/main/java/org/springframework/integration/channel/MessageChannelReactiveUtils.java

@@ -16,8 +16,9 @@
 
 package org.springframework.integration.channel;
 
+import java.time.Duration;
+
 import org.reactivestreams.Publisher;
-import org.reactivestreams.Subscriber;
 
 import org.springframework.messaging.Message;
 import org.springframework.messaging.MessageChannel;
@@ -27,6 +28,7 @@
 
 import reactor.core.publisher.EmitterProcessor;
 import reactor.core.publisher.Flux;
+import reactor.core.publisher.Mono;
 import reactor.core.scheduler.Schedulers;
 
 /**
@@ -69,34 +71,12 @@ private static <T> Publisher<Message<T>> adaptSubscribableChannelToPublisher(Sub
 		});
 	}
 
+	@SuppressWarnings("unchecked")
 	private static <T> Publisher<Message<T>> adaptPollableChannelToPublisher(PollableChannel inputChannel) {
-		return new PollableChannelPublisherAdapter<>(inputChannel);
-	}
-
-	private static final class PollableChannelPublisherAdapter<T> implements Publisher<Message<T>> {
-
-		private final PollableChannel channel;
-
-		PollableChannelPublisherAdapter(final PollableChannel channel) {
-			this.channel = channel;
-		}
-
-		@Override
-		@SuppressWarnings("unchecked")
-		public void subscribe(Subscriber<? super Message<T>> subscriber) {
-			Flux
-					.<Message<T>>create(sink ->
-							sink.onRequest(n -> {
-								Message<?> m;
-								while (!sink.isCancelled() && n-- > 0
-										&& (m = this.channel.receive()) != null) { // NOSONAR
-									sink.next((Message<T>) m);
-								}
-							}))
-					.subscribeOn(Schedulers.elastic())
-					.subscribe(subscriber);
-		}
-
+		return Mono.fromCallable(() -> (Message<T>) inputChannel.receive())
+				.subscribeOn(Schedulers.boundedElastic())
+				.repeatWhenEmpty(it -> it.delayElements(Duration.ofMillis(10)))
+				.repeat();
 	}
 
 }