Spring 503 status code after completing SseEmitter in onTimeout

Hi all,

I experience an issue with SseEmitter in spring-framework 6.2.x vs 6.1.x. I am trying to complete an SseEmitter in the onTimeout callback in order to have a 200 http status code. This works fine in spring-framework 6.1.x and spring-boot 3.3.x, but breaks with spring-framework 6.2.x and spring-boot 3.4.x.

The commit that break this behaviour is the following https://github.com/spring-projects/spring-framework/commit/e67f892e44bab285ed7e2848f888ff897b0e6d0e. In the past even when completion was already set, the completion handlers were still executed, when explicitly calling the complete method, this is not the case anymore and in case of onTimeout the completion has been set to true already before calling the callback.

I have provided a sample project here https://github.com/stevijo/sse-emitter

The code looks something like the following, where I expect a 200 status code after the 1s timeout, because I explicitly complete the emitter in the onTimeout callback:

@Controller
class SseController {
    @GetMapping(path = "/", produces = TEXT_EVENT_STREAM_VALUE)
    public SseEmitter stream() {
        final var emitter = new SseEmitter(1000l);
        emitter.onTimeout(() -> {
            emitter.complete();
        });

        return emitter;
    }
}

Comment From: rstoyanchev

Indeed, ResponseBodyEmitter flips the completed flag before calling timeout handlers, which prevents them from being able to influence the response.