Multipart messages with empty parts are not correctly parsed in WebFlux

[hopefulwalker]

I create a demo project using https://start.spring.io/, when upgrading to springboot3.1.2(springframework6.0.11), , the unit test failed. I'm not sure it's a bug or I need to change the code?

It seems the uploading source code by zip is not support, the Github library address is: https://github.com/hopefulwalker/learning.git
The controller code:

@Slf4j
@RestController
@RequestMapping("/api/v1alpha1/files")
public class FileController {
    @PostMapping("/")
    @ResponseStatus(HttpStatus.CREATED)
    public Flux<FileInfo> uploadFile(@RequestPart("files") Flux<FilePart> files) {
        return files.log().filter(part -> StringUtils.isNotBlank(part.filename()))
                .flatMap(part -> Mono.just(new FileInfo(part.filename())))
                .doOnNext(fileInfo -> log.debug(fileInfo.toString()));
    }
}

The unit test code :

    @Test
    void uploadFile() throws IOException {
        MultipartBodyBuilder builder = new MultipartBodyBuilder();
        List<Path> paths = addTempFiles(builder, "files", 2);
        webTestClient.post()
                .uri("/api/v1alpha1/files/")
                .body(BodyInserters.fromMultipartData(builder.build()))
                .accept(MediaType.APPLICATION_JSON)
                .exchange()
                .expectStatus().isCreated()
                .expectBodyList(FileInfo.class)
                .hasSize(paths.size());
        deleteTempFiles(paths);
    }

when running with 3.1.2, It seems only send the fist FilePart

2023-07-26T12:31:42.080+08:00  INFO 56104 --- [     parallel-1] reactor.Flux.MonoFlattenIterable.1       : | onSubscribe([Fuseable] FluxFlattenIterable.FlattenIterableSubscriber)
2023-07-26T12:31:42.081+08:00  INFO 56104 --- [     parallel-1] reactor.Flux.MonoFlattenIterable.1       : | request(256)
2023-07-26T12:31:42.133+08:00  INFO 56104 --- [      Thread-13] reactor.Flux.MonoFlattenIterable.1       : | onNext(DefaultFilePart{files (file13136779453907688947.tmp)})
2023-07-26T12:31:42.144+08:00  INFO 56104 --- [      Thread-13] reactor.Flux.MonoFlattenIterable.1       : | onComplete()
2023-07-26T12:31:42.185+08:00 ERROR 56104 --- [    Test worker] o.s.t.w.reactive.server.ExchangeResult   : Request details for assertion failure:
> POST /api/v1alpha1/files/
> WebTestClient-Request-Id: [1]
> Accept: [application/json]
> Content-Type: [multipart/form-data;boundary=oZMAASAjMOjgus2_uZdRe-B3I3rKkw22-]
412 bytes of content.
< 201 CREATED Created
< Content-Type: [application/json]
[{"name":"file13136779453907688947.tmp"}]
Response body does not contain 2 elements expected:<2> but was:<1>

when running with 2.7.14, it send 2 FilePart

2023-07-26 12:32:49.910  INFO 33252 --- [     parallel-1] reactor.Flux.MonoFlattenIterable.1       : | onSubscribe([Fuseable] FluxFlattenIterable.FlattenIterableSubscriber)
2023-07-26 12:32:49.912  INFO 33252 --- [     parallel-1] reactor.Flux.MonoFlattenIterable.1       : | request(256)
2023-07-26 12:32:49.959  INFO 33252 --- [      Thread-15] reactor.Flux.MonoFlattenIterable.1       : | onNext(DefaultFilePart{files (file5549318982591335212.tmp)})
[learning.zip](https://github.com/spring-projects/spring-boot/files/12167963/learning.zip)

2023-07-26 12:32:49.964  INFO 33252 --- [      Thread-15] reactor.Flux.MonoFlattenIterable.1       : | onNext(DefaultFilePart{files (file4020380138397462466.tmp)})
2023-07-26 12:32:49.964  INFO 33252 --- [      Thread-15] reactor.Flux.MonoFlattenIterable.1       : | onComplete()

[wilkinsona]

Thanks for the sample. I've reproduced the problem. It doesn't appear to have anything to do with Spring Boot as it fails when Spring Boot is only used for dependency management as shown by the changes in my fork. We'll transfer this to the Framework team so that they can take a look.

[poutsma]

Thanks again for the sample. The problem is due to the fact that the added parts are empty, which is an uncommon scenario that was not supported by our multipart parser.

Preparing a fix now.