Handle onError events that are sent by the servlet containers [SPR-15614]

[spring-projects-issues]

Violeta Georgieva opened SPR-15614 and commented

Hi,

Currently when an I/O error happens, completeWithError will be called:
https://github.com/spring-projects/spring-framework/blob/master/spring-webmvc/src/main/java/org/springframework/web/servlet/mvc/method/annotation/ResponseBodyEmitter.java#L172

At some point this call will result in:
https://github.com/spring-projects/spring-framework/blob/master/spring-web/src/main/java/org/springframework/web/context/request/async/WebAsyncManager.java#L357
Thus the implementation guarantees that the I/O error will be handled properly and the corresponding error handlers will be invoked.

On the other hand there is AsyncListener that will be notified by the servlet containers that an error occurred:
https://github.com/spring-projects/spring-framework/blob/master/spring-web/src/main/java/org/springframework/web/context/request/async/StandardServletAsyncWebRequest.java#L137
This does not trigger any error handling but just completes the async operation.

When the error is I/O then this is not a problem as the implementation handles it as described above, but if it is another type of error then it will not be handled properly.

What do you think?

Regards,
Violeta

---

Affects: 5.0 RC1

Issue Links:

• WebAsyncManager concurrency issue with SseEmitter when client disconnect [SPR-16058] #20607 WebAsyncManager concurrency issue with SseEmitter when client disconnect
• Improve docs and handling of send errors in ResponseBodyEmitter [SPR-16548] #21091 Improve docs and handling of send errors in ResponseBodyEmitter

Referenced from: commits e0678ba

[spring-projects-issues]

Rossen Stoyanchev commented

Taking a closer look, as far as I can see, if an onError AsyncEvent occurs when ResponseBodyEmitter is not trying to write, the AsyncListener in StandardServletAsyncWebRequest will result in a call to the onCompletion handler of ResponseBodyEmitter.

So the ResponseBodyEmitter will be notified at least and from what I can see after that Tomcat will complete the AsyncContext if neither dispatch nor complete where called.

Overall AsyncListener#onError is not exposed explicitly but treated as a completion callback instead. This the case of all the higher-level APIs like DeferredResult, DeferredResultProcessInterceptor, and ResponseBodyEmitter. The idea is that after a container-level exception the connection is practically over with. We just don't explicitly complete from onError but rather leave it to those higher level APIs (like DeferredResult) or to the container (in case of an error unrelated to writing) to do so. I also wonder what error can arise in the container prompting an onError but in any case I imagine that most often errors are related to attempts to write to the response.

So unless leaving it to the container to is an issue for, as it became with timeouts when containers started dispatching even if response status was set (#19233), then I don't see a strong reason for change. What do you think?

[spring-projects-issues]

Rossen Stoyanchev commented

After some more discussion and experiments (including on the Tomcat mailing list) it's clear that both onTimeout and onError notifications should only be handled from within a container thread, i.e. through the AsyncListener callbacks.

In case of a write error, the server is clearly aware of it and should turn it into a notification. So we can refrain from catching the write exception and calling SseEmitter.completeWithError(ex) and instead wait for the onError callback.

In order to do that we'll need addErrorHandler in StandardServletAsyncWebRequest next to similar options for timeout and complete. Then SseEmitter can register and call this::completeWithError from within the container thread.

[spring-projects-issues]

Violeta Georgieva commented

Here is the PR
#1463