Error when disconnecting from the server

[JayExtra]

Hello

I am trying to build an Android client for my Springboot backend and one of the features requires Websockets. I implemented Websockets with STOMP and I have run into a small issue. On initial connection, everything is well. The app subscribes to the intended topic and receives the payload. The issue arises when I diconnect. It throws this error:

FATAL EXCEPTION: DefaultDispatcher-worker-2
Process: com.example.zmeasy, PID: 19836
org.hildan.krossbow.stomp.WebSocketClosedUnexpectedly: the WebSocket was closed while subscriptions were still active. Code: 1000 Reason:
at org.hildan.krossbow.stomp.StompSocketKt.decodeToStompEvent(StompSocket.kt:89)
at org.hildan.krossbow.stomp.StompSocketKt.access$decodeToStompEvent(StompSocket.kt:1)
at org.hildan.krossbow.stomp.StompSocket.decodeStomp(StompSocket.kt:38)
at org.hildan.krossbow.stomp.StompSocket.access$decodeStomp(StompSocket.kt:13)
at org.hildan.krossbow.stomp.StompSocket$special$$inlined$map$1$2.emit(Emitters.kt:220)
at kotlinx.coroutines.flow.FlowKt__ErrorsKt$catchImpl$2.emit(Errors.kt:154)
at kotlinx.coroutines.flow.FlowKt__ChannelsKt.emitAllImpl$FlowKt__ChannelsKt(Channels.kt:33)
at kotlinx.coroutines.flow.FlowKt__ChannelsKt.access$emitAllImpl$FlowKt__ChannelsKt(Channels.kt:1)
at kotlinx.coroutines.flow.FlowKt__ChannelsKt$emitAllImpl$1.invokeSuspend(Unknown Source:14)
at kotlin.coroutines.jvm.internal.BaseContinuationImpl.resumeWith(ContinuationImpl.kt:33)
at kotlinx.coroutines.DispatchedTask.run(DispatchedTask.kt:104)
at kotlinx.coroutines.scheduling.CoroutineScheduler.runSafely(CoroutineScheduler.kt:585)
at kotlinx.coroutines.scheduling.CoroutineScheduler$Worker.executeTask(CoroutineScheduler.kt:802)
at kotlinx.coroutines.scheduling.CoroutineScheduler$Worker.runWorker(CoroutineScheduler.kt:706)
at kotlinx.coroutines.scheduling.CoroutineScheduler$Worker.run(CoroutineScheduler.kt:693)
Suppressed: kotlinx.coroutines.internal.DiagnosticCoroutineContextException: [StandaloneCoroutine{Cancelling}@3b251c1, Dispatchers.IO]

So I was wondering whether Krossbow has some function that can allow me to unsubscribe the user from the topic first before using the disconnect() function. I expected when disconnect() is called, the client would be unsubscribed automatically. Or is this a server issue? Any help would be appreciated.

[joffrey-bion]

Hi, thanks for reaching out!

I was wondering whether Krossbow has some function that can allow me to unsubscribe the user from the topic first

When a subscription flow collection ends (either the collecting coroutine is cancelled, or the flow uses some terminal operator like first(), count(), etc.), the client automatically unsubscribes from the topic. There is no unsubscribe function because of this, it's all handled for you.
If you still have active coroutines collecting subscription flows, it should still be ok to call disconnect() and you shouldn't have to worry about it. I know understand that the error message is confusing in this respect.

I expected when disconnect() is called, the client would be unsubscribed automatically

Yes, this is how it should work already. If you call disconnect(), you initiate a "proper" STOMP disconnection and you shouldn't get an error regarding unexpected web socket closure.

This error is meant for cases when the server drops the web socket connection without proper STOMP closing handshake. I'm surprised you're facing this in your case. I need to further investigate this exception anyway, to properly define when it should occur and reflect this in the error message. You can track this in #397.

Or is this a server issue?

It might be that the server is immediately closing the web socket without closure at STOMP level, but that would be surprising from a Spring server (and I have been using Krossbow with Spring in the past). Also, even if that were the case, Krossbow shouldn't fail with unexpected WS closure if we're calling disconnect(), hence why I'm rather suspecting a bug in Krossbow here.

[JayExtra]

Thank you, @joffrey-bion. I will be following issue #397 close. By the time a fix is published, do you have any suggestions I may use to stop the client app from crashing when disconnect() is called? A try-and-catch block isn't particularly useful.