Inconsistency in `SseEmitter.onCompletion()` behavior between Spring 6.2.3 and 6.2.5

[jrpedrianes]

Summary

There is an inconsistency in the invocation behaviour of the SseEmitter.onCompletion() callback between Spring Framework versions 6.2.3 and 6.2.5. Previously, calling emitter.complete() would immediately trigger the callback registered via onCompletion(). However, starting with Spring Framework 6.2.5 (used in Spring Boot 3.4), this callback no longer appears to execute after complete() method is invoqued.

Current Behaviour

When using Spring Boot 3.4 (with Spring Framework 6.2.5), calling emitter.complete() does not cause an immediate invocation of the onCompletion() callback. This behaviour differs from that in Spring Boot 3.3 (with Spring Framework 6.2.3), where emitter.complete() immediately triggered the callback.

Expected behaviour

The expected behaviour is that calling emitter.complete() should trigger the onCompletion() callback immediately, just as it did in Spring Boot 3.3 (Spring Framework 6.2.3).

Steps to Reproduce

You can reproduce the issue with the following setup:

1. Implement the controller below in your application:

   @RestController
   public class SseController {
   
       @GetMapping("/sse")
       public SseEmitter handleSse() {
           SseEmitter emitter = new SseEmitter();
   
           emitter.onCompletion(() -> {
               System.out.println("onCompletion callback executed");
           });
   
           ScheduledExecutorService scheduler = Executors.newScheduledThreadPool(1);
           scheduler.schedule(() -> {
               emitter.complete();
           }, 1, TimeUnit.SECONDS);
   
           return emitter;
       }
   }

2. Run the application with Spring Boot 3.3 (which uses Spring Framework 6.2.3) and observe that the log shows "onCompletion callback executed" immediately after calling complete().
3. Update the application to Spring Boot 3.4 (with Spring Framework 6.2.5) and notice that the callback is not executed immediately.

Additional Notes

• The code to reproduce this behaviour is available in the [spring-sse-emitter GitHub repository https://github.com/jrpedrianes/spring-sse-emitter.

• This behavioural change appears to be related to 503 status code after completing SseEmitter in onTimeout #34426 in the Spring Framework repository.

[rstoyanchev]

The #34426 fix in 6.2.4 was to address a regression introduced by a performance improvement #33831 in 6.2.0. However, that in turn led to this issue.

Looking at it more deeply, the intent of synchronized methods in ResponseBodyEmitter was to prevent concurrent use of the response, which is not allowed by the Servlet API, but the AtomicBoolean checks that replaced them in #33831 also had the effect of skipping the calls to Handler#complete and Handler#completeWithError, and that caused the regression reported in #34426.

I will update ResponseBodyEmitter accordingly to fix this in 6.2.x by matching behavior before the performance improvement.

I have also realized that protection against concurrent use of the response should no longer be necessary at the level of ResponseBodyEmitter, so I have created #34767 to address in 7.0.

[rstoyanchev]

I have reverted both #34426 and #33831, in effect undoing the performance optimization since for 7.0 we're going to address the same.