Fix CompletableFuture exception handling

[sviperll]

CompletableFuture exceptions are always wrapped into CompletionException.
We need to unwrap CompletionException to properly handle exceptions in MVC handlers that return CompletableFuture.
See https://stackoverflow.com/questions/49676889/spring-controller-advice-does-not-correctly-handle-a-completablefuture-completed

#22475

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[jazdw]

Can we get this merged? FWIW, it looks good to me.

[rstoyanchev]

I've resolved this in a separate commit also adding a check to ensure the cause is not null. Thanks for submitting a PR and for pointing this out!

[whiskeysierra]

Is this fix already part of a release?

[bclozel]

Yes, Spring Framework 5.1.7

[Blackdread]

@bclozel weird, I am using Spring Boot v2.1.9.RELEASE but still have this error (dependency of spring is 5.1.10)

[maverick1601]

I have the same problem, when returning e.g. CompletableFuture<ResponseEntity<Void>>from a controller method. The debugger revealed, that the DeferredResultMethodReturnValueHandlerisn't used in that case, since ResponseBodyEmitterReturnValueHandleris kicking in instead.

[rstoyanchev]

@maverick1601, ResponseBodyEmitterReturnValueHandleris delegates to ReactiveTypeHandler which handles adapts the future via a Reactor-based adapter registered through the ReactiveAdapterRegistry. So this comes from Project Rector and I see a fix there reactor/reactor-core#1652 related to this for version 3.3. Spring Framework 5.2 is based on that version of Reactor but you can try independently upgrading Reactor first.

[maverick1601]

Never mind, I figured, that it works as expected, just not via the DeferredResultMethodReturnValueHandler, as you've mentioned. The problem was on my side, testing with a mis-configured ExceptionHandler.