Get client disconnect notifications in bidirectional async gRPC

We don’t get any notification when the client closes the connection when using asynchronous bidirectional stream calls.

https://github.com/SourceForgery/tachikoma/blob/ddfb03ba5d25f3de61e8fe1e1b2cb9be2aba0eb4/tachikoma-grpc/src/main/kotlin/com/sourceforgery/tachikoma/mta/MTAEmailQueueService.kt#L31

We get neither onComplete(), nor onError() in the ~ResponseObserver~StreamObserver when the client closes the socket.

Just to clarify the code, the getEmails() method will return immediately, but will hook up a listener to the RabbitMQ which will then deliver messages via responseObserver.onNext(response). The RabbitMQ will stop delivering messages when the future is finished (which will only happen in response to exceptions or close).

The lifetime of this call is counted in months, as it’s more of a subscription than anything else.

[tobias-]

It is not.

~I’m really not sure about this, but it seems that when doing HttpServerHandler.handleRequest() there’s a lot of logic used to find the correct Service and call it, but when coming back asynchronously later in HttpServerHandler.channelInactive(), that context is gone, although the ChannelHandlerContext is still available.~

~It doesn’t seem that the HttpServerHandler is shut down properly in the case of closed socket, as unfinishedRequests is never counted down to 0 nor is gracefulShutdownSupport.~

All that said (and while true, I believe it to be irrelevant), I looked at how the stream objects/data is propagated, and that uses the Http2RequestDecoder. Armeria does get a callback when the client closes the socket as a Http2RequestDecoder.onStreamRemoved(), which imho should be propagated a lot more.

Could something as simple as this work? (cloning armeria and testing)

     @Override
     public void onStreamRemoved(Http2Stream stream) {
-        requests.remove(stream.id());
+        final DecodedHttpRequest req = requests.remove(stream.id());
+        if (req != null) {
+            req.close();
+        }
     }

[tobias-]

Ok, that fix works for me. I’m none too fond of signing license agreements, so if somebody who has signed it can open a PR with that, please do. I consider the above code trivial and therefore not copyrightable. Do what you want with it.

I’ve run all the tests and they pass. This code also makes sure that the HttpServerHandler mentioned above is closed seemingly properly, and that propagates into ResponseObserver.onComplete() which is exactly where I would expect it to end up, since it isn’t an error.

Example commit message:

Close the requests associated with killed stream

When a socket is closed, make sure that the outstanding requests
(unfinishedRequests) are closed/terminated as well by closing
the stream when removing it.

Closes #971