How to catch ktor server exception properly. ```su...

# ktor

[Venkat]

How to catch ktor server exception properly.

suspend fun main() {
    val scope = CoroutineScope(<http://Dispatchers.IO|Dispatchers.IO>)

    val validServer = embeddedServer(CIO, port = 8080){}.start(false)

    val addressReuseFailureServer = scope.async(start = CoroutineStart.LAZY) {
        embeddedServer(
            factory = CIO,
            environment = applicationEngineEnvironment {
                connector { port = 8080 }
            },
            configure = { reuseAddress = false }
        ).start(wait = false)
    }
    runCatching { addressReuseFailureServer.await() }.also { println(it) }
}

when I run the above code, the line runCatching {} doesn't;t catch the address reuse exception, instead, it propagates and crashes the application. Could anyone help me with this?

[Sam]

Exceptions from async always propagate to the outer scope in addition to being thrown from await(). In your example code, since you are using suspend fun main you do not need to create a separate CoroutineScope.

[Venkat]

Exceptions from async always propagate to the outer scope in addition to being thrown from await()

But I'm catching that with runCathcing {}

[Sam]

You are catching the exception thrown from await(), but the exception will also be propagated to the parent scope. That’s just how async works.

[Venkat]

How can I stop propagating it to parents, sorry I'm not good at coroutine concepts. Do I need to make the scope SuperVisor?

[Venkat]

My intention is to not crash when this exception occur

[Sam]

In your example code, since you are using suspend fun main you do not need to create a separate CoroutineScope. If your actual application does need the additional scope, then yes, a supervisor scope could be a good solution 👍.

[Venkat]

suspend fun main() {
    val scope = CoroutineScope(<http://Dispatchers.IO|Dispatchers.IO> + SupervisorJob())
    val validServer = embeddedServer(CIO, port = 8080){}.start(false)

    val addressReuseFailureServer = scope.async(start = CoroutineStart.LAZY) {
        embeddedServer(
            factory = CIO,
            environment = applicationEngineEnvironment {
                connector { port = 8080 }
            },
            configure = { reuseAddress = false }
        ).start(wait = false)
    }
    runCatching { addressReuseFailureServer.await() }.also { println(it) }
}

[Venkat]

I made the scope SupervisorJob, but still the same

[Venkat]

I think it's something to do with this https://github.com/Kotlin/kotlinx.coroutines/issues/3773#issuecomment-1602975278

[Venkat]

fun main() {
    val exceptionHandler = CoroutineExceptionHandler { _, throwable ->
        println("Caught an exception: ${throwable.message}")
    }

    val validServer = embeddedServer(CIO, port = 8080){}.start(false)
    val addressReuseFailureServer =
        embeddedServer(
            factory = CIO,
            environment = applicationEngineEnvironment { parentCoroutineContext = exceptionHandler;connector { port = 8080 } },
            configure = { reuseAddress = false }
    )
    runCatching { addressReuseFailureServer.start(wait = false) }.also { println(it) }
}

I ended up with this,

[Venkat]

Failure(kotlinx.coroutines.JobCancellationException: LazyStandaloneCoroutine is cancelling; job=LazyStandaloneCoroutine{Cancelling}@71423665)
Caught an exception: Address already in use

The above code catch all exception but don't whether it's the right way of doing it.

[Aleksei Tirman [JB]]

I can successfully catch the exception if the scope is defined as CoroutineScope(<http://Dispatchers.IO|Dispatchers.IO> + SupervisorJob()).