TaskExecutors.IO is not honored with Server-Sent Event when client is disconnected (blocking netty event loop)

[fhussonnois]

Expected Behavior

The @ExecuteOn(TaskExecutors.IO) annotation should be honored even in the case a Server-Sent Event Flux is terminated proactively by Micronaut (i.e., when client is disconnected)

Actual Behaviour

Using Server-Sent-Event, when the client disconnects, then the Flux is terminated by Micronaut, any doFinally operation will be executed by the netty event loop even if the method is annotated by @ExecuteOn(TaskExecutors.IO).

As the method is annotated as blocking I would expect the flux to be published on io-executors before being competed.

Steps To Reproduce

Reproducer

• Create an SSE endpoint like below:

@Controller("/test")
@Secured(SecurityRule.IS_ANONYMOUS)
public class BlockingSseController {

    @ExecuteOn(TaskExecutors.IO)
    @Get(uri = "/sse", produces = MediaType.TEXT_EVENT_STREAM)
    public Flux<Event<String>> sse() {
        return Flux.<Event<String>>create(emitter -> {
                // Emit next
                for (int i = 0; i < 10; i++) {
                    System.out.printf("[%s] emitter%n", Thread.currentThread());
                    emitter.next(Event.of("data-"+i));
                    doSleep(1000);
                }
                // Complete
                emitter.complete();
            }, FluxSink.OverflowStrategy.BUFFER)
            .doFinally(ignored -> {
                 System.out.printf("[%s] doFinally%n", Thread.currentThread());
                 doSleep(1000000000000L);
            });
    }

• Request the endpoint (For: http://localhost:8080/api/v1/test/sse)
• Interrupt the connection before the Flux is completed

logs:

[Thread[#252,io-executor-thread-4,5,main]] emitter
...
[Thread[#252,io-executor-thread-4,5,main]] emitter
[Thread[#206,default-nioEventLoopGroup-1-4,5,main]] doFinally

When the Flux is completed by the emitter:

logs:

[Thread[#218,io-executor-thread-3,5,main]] emitter
...
[Thread[#218,io-executor-thread-3,5,main]] emitter
[Thread[#218,io-executor-thread-3,5,main]] doFinally

Environment Information

• Linux
• Java Version: 21

Example Application

No response

Version

4.7.3