Add ReactiveMessageSourceProducer (#3254)

* Add `ReactiveMessageSourceProducer`

The `ReactiveMessageSourceProducer` wraps a provided `MessageSource`
into a `Flux` for subscription in the `subscribeToPublisher(Publisher<? extends Message<?>>)`
to make a source polling feature fully based on a reactive, on demand solution

* Introduce a `IntegrationReactiveUtils` replacing existing `MessageChannelReactiveUtils`
with more functionality
* Replace a deprecated `MessageChannelReactiveUtils` with a new `IntegrationReactiveUtils`
* Test and document the feature

* * Fix Docs typos

* * Remove unused imports from `MessageChannelReactiveUtils`

* * Fix JavaDoc copy/paste artifact

src/reference/asciidoc/reactive-streams.adoc

@@ -89,6 +89,17 @@ The channel adapter goes to a stopped state when a subscription to the `Publishe
 Calling `stop()` on such a channel adapter completes the producing from the source `Publisher`.
 The channel adapter can be restarted with automatic subscription to a newly created source `Publisher`.
 
+=== Message Source to Reactive Streams
+
+Starting with version 5.3, a `ReactiveMessageSourceProducer` is provided.
+It is a combination of a provided `MessageSource` and event-driven production into the configured `outputChannel`.
+Internally it wraps a `MessageSource` into the repeatedly resubscribed `Mono` producing a `Flux<Message<?>>` to be subscribed in the `subscribeToPublisher(Publisher<? extends Message<?>>)` mentioned above.
+The subscription for this `Mono` is done using `Schedulers.boundedElastic()` to avoid possible blocking in the target `MessageSource`.
+When the message source returns `null` (no data to pull), the `Mono` is turned into a `repeatWhenEmpty()` state with a `delay` for a subsequent re-subscription based on a `IntegrationReactiveUtils.DELAY_WHEN_EMPTY_KEY` `Duration` entry from the subscriber context.
+By default it is 1 second.
+If the `MessageSource` produces messages with a `IntegrationMessageHeaderAccessor.ACKNOWLEDGMENT_CALLBACK` information in the headers, it is acknowledged (if necessary) in the `doOnSuccess()` of the original `Mono` and rejected in the `doOnError()` if the downstream flow throws a `MessagingException` with the failed message to reject.
+This `ReactiveMessageSourceProducer` could be used for any use-case when a a polling channel adapter's features should be turned into a reactive, on demand solution for any existing `MessageSource<?>` implementation.
+
 === Splitter and Aggregator
 
 When an `AbstractMessageSplitter` gets a `Publisher` for its logic, the process goes naturally over the items in the `Publisher` to map them into messages for sending to the `outputChannel`.

src/reference/asciidoc/whats-new.adoc

@@ -27,6 +27,13 @@ See its JavaDocs and <<./graph.adoc#integration-graph,Integration Graph>> for mo
 The `ReactiveMessageHandler` is now natively supported in the framework.
 See <<./reactive-streams.adoc#reactive-message-handler,ReactiveMessageHandler>> for more information.
 
+
+[[x5.3-reactive-message-source-producer]]
+==== `ReactiveMessageSourceProducer`
+
+The `ReactiveMessageSourceProducer` is a reactive implementation of the `MessageProducerSupport` to wrap a provided `MessageSource` into a `Flux` for on demand `receive()` calls.
+See <<./reactive-streams.adoc#reactive-streams,Reactive Streams Support>> for more information.
+
 [[x5.3-java-dsl-extensions]]
 ==== Java DSL Extensions