MultipartHttpMessageWriter consumes source once only

The previous fix #09f1f7 did not actually address the issue but only
moved it further down, so instead of the subscribe(), it was consuming
it inside the MultipartHttpMessageWriter#write method which returned
this.body.then(), and then again for the actual request body writing.

In this commit MultipartHttpMessageWriter#write returns Mono.empty()
since we don't actually want to write the part content from there, but
only want to access it as soon as it is availabele, for writing to
the request body.

Issue: SPR-16402

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

@@ -269,13 +269,18 @@ public class MultipartHttpMessageWriter implements HttpMessageWriter<MultiValueM
 		Publisher<T> bodyPublisher =
 				body instanceof Publisher ? (Publisher<T>) body : Mono.just(body);
 
-		Mono<Void> partWritten = ((HttpMessageWriter<T>) writer.get())
+		// The writer will call MultipartHttpOutputMessage#write which doesn't actually write
+		// but only stores the body Flux and returns Mono.empty().
+
+		Mono<Void> partContentReady = ((HttpMessageWriter<T>) writer.get())
 				.write(bodyPublisher, resolvableType, contentType, outputMessage, Collections.emptyMap());
 
-		return Flux.concat(
-				Mono.just(generateBoundaryLine(boundary)),
-				partWritten.thenMany(Flux.defer(outputMessage::getBody)),
-				Mono.just(generateNewLine()));
+		// After partContentReady, we can access the part content from MultipartHttpOutputMessage
+		// and use it for writing to the actual request body
+
+		Flux<DataBuffer> partContent = partContentReady.thenMany(Flux.defer(outputMessage::getBody));
+
+		return Flux.concat(Mono.just(generateBoundaryLine(boundary)), partContent, Mono.just(generateNewLine()));
 	}
 
 
@@ -353,7 +358,9 @@ public class MultipartHttpMessageWriter implements HttpMessageWriter<MultiValueM
 				return Mono.error(new IllegalStateException("Multiple calls to writeWith() not supported"));
 			}
 			this.body = Flux.just(generateHeaders()).concatWith(body);
-			return this.body.then();
+
+			// We don't actually want to write (just save the body Flux)
+			return Mono.empty();
 		}
 
 		private DataBuffer generateHeaders() {
@@ -387,8 +394,7 @@ public class MultipartHttpMessageWriter implements HttpMessageWriter<MultiValueM
 
 		@Override
 		public Mono<Void> setComplete() {
-			return (this.body != null ? this.body.then() :
-					Mono.error(new IllegalStateException("Body has not been written yet")));
+			return Mono.error(new UnsupportedOperationException());
 		}
 	}