KotlinJS Ktor server "Connection closed without response" when deployed to Heroku - kotlin

Question

I'm new to kotlin and having some trouble running my Ktor server on heroku
I'm setting up the server as follows:
val port = System.getenv("PORT")?.toInt() ?: 9090
embeddedServer(Netty, port = port) {
routing {
get("/") {
call.respondText(
this::class.java.classLoader.getResource("index.html")!!.readText(),
ContentType.Text.Html
)
}
static("/static") {
resources("")
}
}
}.start(wait = true)
When running on my localhost, this responds just fine. And when running in heroku, I get the log output that tells me it did start up the server on the port heroku defines:
[main] INFO ktor.application - Responding at http://0.0.0.0:55278
But then when I try to load the heroku app from the browser, I get an error, and the heroku logs say: "Connection closed without response"
heroku[router]: at=error code=H13 desc="Connection closed without response" method=GET path="/" host=melomidi.herokuapp.com request_id=e0cdaa83-eaa4-4733-a51f-947de463c433 fwd="174.246.82.169" dyno=web.1 connect=0ms service=13ms status=503 bytes=0 protocol=http
I have tried with both http and https. I have also tried loading /static/index.html directly and got the same result.
I've also tried adding a SERVER_HOST env var and using that to change the default 0.0.0.0 host to various things, like localhost, 127.0.0.1 and melomidi.herokuapp.com but those resulted in other errors where the server failed to start up.
There are no other details or additional logs being output in the heroku logs when I make the request. Just the one error line per request as displayed above.
Anyone have any suggestions? Thanks in advance!

Answer 1

After shuffling the order of operations around so that my static resources declaration came first, I started seeing the following error in the logs:
2022-01-08T00:19:36.616138+00:00 app[web.1]: Caused by: java.lang.NoSuchMethodError: java.nio.ByteBuffer.clear()Ljava/nio/ByteBuffer;
2022-01-08T00:19:36.616138+00:00 app[web.1]: at io.ktor.utils.io.jvm.javaio.ReadingKt$toByteReadChannel$1.invokeSuspend(Reading.kt:53)
2022-01-08T00:19:36.616138+00:00 app[web.1]: at io.ktor.utils.io.jvm.javaio.ReadingKt$toByteReadChannel$1.invoke(Reading.kt)
2022-01-08T00:19:36.616139+00:00 app[web.1]: at io.ktor.utils.io.jvm.javaio.ReadingKt$toByteReadChannel$1.invoke(Reading.kt)
2022-01-08T00:19:36.616139+00:00 app[web.1]: at io.ktor.utils.io.CoroutinesKt$launchChannel$job$1.invokeSuspend(Coroutines.kt:132)
2022-01-08T00:19:36.616139+00:00 app[web.1]: at kotlin.coroutines.jvm.internal.BaseContinuationImpl.resumeWith(ContinuationImpl.kt:33)
2022-01-08T00:19:36.616139+00:00 app[web.1]: at kotlinx.coroutines.DispatchedTask.run(DispatchedTask.kt:106)
2022-01-08T00:19:36.616139+00:00 app[web.1]: at kotlinx.coroutines.scheduling.CoroutineScheduler.runSafely(CoroutineScheduler.kt:571)
2022-01-08T00:19:36.616140+00:00 app[web.1]: at kotlinx.coroutines.scheduling.CoroutineScheduler$Worker.executeTask(CoroutineScheduler.kt:750)
2022-01-08T00:19:36.616140+00:00 app[web.1]: at kotlinx.coroutines.scheduling.CoroutineScheduler$Worker.runWorker(CoroutineScheduler.kt:678)
2022-01-08T00:19:36.616140+00:00 app[web.1]: at kotlinx.coroutines.scheduling.CoroutineScheduler$Worker.run(CoroutineScheduler.kt:665)
That got me to this post:https://youtrack.jetbrains.com/issue/KTOR-3358 where I found there is a binary compatibility issue with Ktor server 1.6.5 and jdk 1.8
I updated my jvmTarget to 11 and now it's up and running!
Strange that I only encountered the issue on heroku though when both my localhost and heroku were each using jdk 1.8 previously.