Add DataBuffer BodyInserter/BodyExtractor

Added a BodyExtractor for Flux<DataBuffer>, and a BodyInserter for
Publisher<DataBuffer>

Issue: SPR-14918

spring-web/src/main/java/org/springframework/http/codec/BodyExtractors.java

@@ -28,6 +28,7 @@ import reactor.core.publisher.Flux;
 import reactor.core.publisher.Mono;
 
 import org.springframework.core.ResolvableType;
+import org.springframework.core.io.buffer.DataBuffer;
 import org.springframework.http.HttpMessage;
 import org.springframework.http.MediaType;
 import org.springframework.http.ReactiveHttpInputMessage;
@@ -85,12 +86,24 @@ public abstract class BodyExtractors {
 	 */
 	public static <T> BodyExtractor<Flux<T>, ReactiveHttpInputMessage> toFlux(ResolvableType elementType) {
 		Assert.notNull(elementType, "'elementType' must not be null");
-		return (request, context) -> readWithMessageReaders(request, context,
+		return (inputMessage, context) -> readWithMessageReaders(inputMessage, context,
 				elementType,
-				reader -> reader.read(elementType, request, Collections.emptyMap()),
+				reader -> reader.read(elementType, inputMessage, Collections.emptyMap()),
 				Flux::error);
 	}
 
+	/**
+	 * Return a {@code BodyExtractor} that returns the body of the message as a {@link Flux} of
+	 * {@link DataBuffer}s.
+	 * <p><strong>Note</strong> that the returned buffers should be released after usage by calling
+	 * {@link org.springframework.core.io.buffer.DataBufferUtils#release(DataBuffer)}
+	 * @return a {@code BodyExtractor} that returns the body
+	 * @see ReactiveHttpInputMessage#getBody()
+	 */
+	public static BodyExtractor<Flux<DataBuffer>, ReactiveHttpInputMessage> toDataBuffers() {
+		return (inputMessage, context) -> inputMessage.getBody();
+	}
+
 	private static <T, S extends Publisher<T>> S readWithMessageReaders(
 			ReactiveHttpInputMessage inputMessage,
 			BodyExtractor.Context context,

spring-web/src/main/java/org/springframework/http/codec/BodyInserters.java

@@ -28,6 +28,7 @@ import reactor.core.publisher.Mono;
 
 import org.springframework.core.ResolvableType;
 import org.springframework.core.io.Resource;
+import org.springframework.core.io.buffer.DataBuffer;
 import org.springframework.http.MediaType;
 import org.springframework.http.ReactiveHttpOutputMessage;
 import org.springframework.http.server.reactive.ServerHttpResponse;
@@ -182,16 +183,33 @@ public abstract class BodyInserters {
 		Assert.notNull(eventsPublisher, "'eventsPublisher' must not be null");
 		Assert.notNull(eventType, "'eventType' must not be null");
 		return BodyInserter.of(
-				(response, context) -> {
+				(outputMessage, context) -> {
 					HttpMessageWriter<T> messageWriter = sseMessageWriter(context);
 					return messageWriter.write(eventsPublisher, eventType,
-							MediaType.TEXT_EVENT_STREAM, response, Collections.emptyMap());
+							MediaType.TEXT_EVENT_STREAM, outputMessage, Collections.emptyMap());
 
 				},
 				() -> eventsPublisher
 		);
 	}
 
+	/**
+	 * Return a {@code BodyInserter} that writes the given {@code Publisher<DataBuffer>} to the
+	 * body.
+	 * @param publisher the data buffer publisher to write
+	 * @param <T> the type of the publisher
+	 * @return a {@code BodyInserter} that writes directly to the body
+	 * @see ReactiveHttpOutputMessage#writeWith(Publisher)
+	 */
+	public static <T extends Publisher<DataBuffer>> BodyInserter<T, ReactiveHttpOutputMessage> fromDataBuffers(T publisher) {
+		Assert.notNull(publisher, "'publisher' must not be null");
+
+		return BodyInserter.of(
+				(outputMessage, context) -> outputMessage.writeWith(publisher),
+				() -> publisher
+		);
+	}
+
 	private static <T> HttpMessageWriter<T> sseMessageWriter(BodyInserter.Context context) {
 		return context.messageWriters().get()
 				.filter(messageWriter -> messageWriter