I have a super simple embedded server. But running...

# ktor

[Sam]

I have a super simple embedded server. But running it on an iOS simulator I frequently get PosixException.AddressAlreadyInUseException: EADDRINUSE (48): Address already in use

Does anyone know if there's a way of registering an uncaught exception handler to the Ktor server so I can handle this gracefully. (Or a proper fix would be welcome).

I've tried wrapping the whole thing in a coroutine scope launch block with a coroutine exception handler but that doesn't work.

embeddedServer(CIO, port = 8080) {
        routing {
            get("/") {
                call.respondText("Hello, world!")
            }
        }
}.start(wait = true)

[ephemient]

if you use port = 0, it should automatically choose a free port. after it is started, you can find what port it ended up on through .resolvedConnectors()

[Sam]

Ah that should allow me to come up with a work around. Thanks!

Don't suppose you know how to catch that exception though?

[ephemient]

does a normal try-catch around .start() not work? (I don't know if there's any iOS-specific oddities there)

[Sam]

Sadly not. There's something strange happening, once I've patched this pretty critical issue I'll come up with a minimal reproducible example. What seems to happen is that I can catch and log the exception, but for some reason it also crashes the app. So I think what's happening is that it's somehow being thrown from multiple places

[ephemient]

the other solution would typically be to use the SO_REUSEADDR socket option (see https://stackoverflow.com/a/2270357 for explanation) but it doesn't look like embeddedServer has any way to customize how the server socket is bound 😞

[Sam]

Yeah I came to the same conclusion, do you know if it's possible to reproduce the embeddedServer behaviour at a lower level to access this api? From what I can see it only seems to be exposed to JVM? (If you don't know off the top of your head that's cool, just wondering)

[Sam]

(Also @ephemient you've saved my skin with that port=0 trick, thank you so much ❤️ )

[ephemient]

I suppose you could re-implement most of the embeddedServer behavior, I don't think it's using anything internal… but it looks like a pain :(

[ephemient]

luckily the port=0 trick is a fairly common use of the socket API, it's not just a ktor thing

[Sam]

Did not know that, noted! Yeah pain was my conclusion too, I think I might have to try, thanks for your help!

[ephemient]

FYI this is now possible in KTOR 2.3.0:

embeddedServer(CIO, port = 8080, configure = { reuseAddress = true }) {

[Sam]

I've been following the PR too! Thanks for the heads up tho!