Looking into using Ktor Server for websockets. Whi...

# ktor

[Trevor Stone]

Looking into using Ktor Server for websockets. While playing around with them I was testing how the coroutine scope works. WebSocketSession implements CoroutineScope so I assumed calling close on the websocket would cancel the coroutine scope and clean up children jobs. This doesn't seem to be the case, which I can work around, but I was wondering if this is the behavior for a specific reason?

[Aleksei Tirman [JB]]

Could you please share a code snippet to illustrate your issue?

[Trevor Stone]

Good idea. This is based upon the websocket sample, but simplified to show the point:

webSocket("/ws") { // websocketSession
    println("started")
    outgoing.send(Frame.Text("Hello Friend!"))

    launch {
        try {
            awaitCancellation()
        } catch (_: Exception) {
            println("complete")
        }
    }
    close(CloseReason(CloseReason.Codes.NORMAL, "Client said BYE"))
}

In this situation I would expect that upon calling close that the launched block would be cancelled and complete is printing. Similarly, it appears that if close isn't called here and instead the client disconnects the launch block is also NOT canceled

[Casey Brooks]

I’m wondering if it’s actually that launch is never actually started because you aren’t reading from the websocket. The whole scope is canceled before launch is actually dispatched, and therefore never actually enters the launch block. Since you’re not reading from incoming, I think it would just immedately return from websocket { }, thus cancelling the coroutineScope instead of suspending

Have you tried putting a println() inside launch before try to see if it even gets started? If you change it to launch(start = CoroutineStart.UNDISPATCHED) { } do you see the same thing?

[Trevor Stone]

Yes it does get started. And if I explicitly call cancel() after close it prints "complete"

routing {
    webSocket("/ws") { // websocketSession
        println("started")
        outgoing.send(Frame.Text("Hello Friend!"))

        launch {
            try {
                println("entered")
                awaitCancellation()
            } catch (_: Exception) {
                println("complete")
            }
        }
        close(CloseReason(CloseReason.Codes.NORMAL, "Client said BYE"))
        cancel()
}