The gist of it is

val selectorManager = SelectorManager(<http://Dispatchers.IO|Dispatchers.IO>)
            val serverSocket = aSocket(selectorManager).tcp().bind("127.0.0.1", port)

            launch {
                val socket = serverSocket.accept()

                launch {
                    delay(15.seconds)
                    socket.close()
                    // hangs here
                    serverSocket.close()
                }

                socket.openReadChannel().readInt()
            }

Mmm, I don't have a

serverSocket.cancel()

, do you mean cancelling the scope?

Or just

socket.cancel()

? (trying now... but same thing)

Cancelling the scope is what I did initially but this seemed to keep the port & active sockets alive, which I don't want.

Filed this: https://youtrack.jetbrains.com/issue/KTOR-6406