Use DataBufferUtils.write in DefaultFilePart.transferTo

This commit makes sure that in DefaultMultipartMessageReader's
DefaultFilePart, the file is not closed before all bytes are written,
by using DataBufferUtils.write (see c1b6885191d6a50347aeaa14da994f0db88f26fe).

The commit also improves on the logging of the
DefaultMultipartMessageReader.

Closes gh-23130

spring-webflux/src/test/java/org/springframework/web/reactive/result/method/annotation/MultipartIntegrationTests.java

@@ -16,6 +16,10 @@
 
 package org.springframework.web.reactive.result.method.annotation;
 
+import java.io.IOException;
+import java.nio.file.Files;
+import java.nio.file.Path;
+import java.nio.file.Paths;
 import java.util.List;
 import java.util.stream.Collectors;
 
@@ -31,6 +35,7 @@ import org.springframework.context.annotation.AnnotationConfigApplicationContext
 import org.springframework.context.annotation.Bean;
 import org.springframework.context.annotation.Configuration;
 import org.springframework.core.io.ClassPathResource;
+import org.springframework.core.io.Resource;
 import org.springframework.http.HttpEntity;
 import org.springframework.http.HttpStatus;
 import org.springframework.http.client.MultipartBodyBuilder;
@@ -145,6 +150,34 @@ public class MultipartIntegrationTests extends AbstractHttpHandlerIntegrationTes
 				.verifyComplete();
 	}
 
+	@Test
+	public void transferTo() {
+		Flux<String> result = webClient
+				.post()
+				.uri("/transferTo")
+				.syncBody(generateBody())
+				.retrieve()
+				.bodyToFlux(String.class);
+
+		StepVerifier.create(result)
+				.consumeNextWith(filename -> verifyContents(Paths.get(filename), new ClassPathResource("foo.txt", MultipartHttpMessageReader.class)))
+				.consumeNextWith(filename -> verifyContents(Paths.get(filename), new ClassPathResource("logo.png", getClass())))
+				.verifyComplete();
+
+	}
+
+	private static void verifyContents(Path tempFile, Resource resource) {
+		try {
+			byte[] tempBytes = Files.readAllBytes(tempFile);
+			byte[] resourceBytes = Files.readAllBytes(resource.getFile().toPath());
+			assertThat(tempBytes).isEqualTo(resourceBytes);
+		}
+		catch (IOException ex) {
+			throw new AssertionError(ex);
+		}
+	}
+
+
 	@Test
 	public void modelAttribute() {
 		Mono<String> result = webClient
@@ -217,6 +250,21 @@ public class MultipartIntegrationTests extends AbstractHttpHandlerIntegrationTes
 			return partFluxDescription(Flux.from(parts));
 		}
 
+		@PostMapping("/transferTo")
+		Flux<String> transferTo(@RequestPart("fileParts") Flux<FilePart> parts) {
+			return parts.flatMap(filePart -> {
+				try {
+					Path tempFile = Files.createTempFile("MultipartIntegrationTests", filePart.filename());
+					return filePart.transferTo(tempFile)
+							.then(Mono.just(tempFile.toString() + "\n"));
+
+				}
+				catch (IOException e) {
+					return Mono.error(e);
+				}
+			});
+		}
+
 		@PostMapping("/modelAttribute")
 		String modelAttribute(@ModelAttribute FormBean formBean) {
 			return formBean.toString();