I need to detect whether a MongoDB instance is run...

# ktor

[CLOVIS]

I need to detect whether a MongoDB instance is running on localhost:27017 or mongo:27017 . I thought this would be a good occasion to try out Ktor Network, so I wrote:

suspend fun findMongoAddress(): InetSocketAddress {	
    val manager = SelectorManager(currentCoroutineContext())
    val candidateHostNames = listOf("localhost", "mongo")

	println("Searching for the address of the running MongoDB instance…")
	for (hostName in candidateHostNames) {
		val address = InetSocketAddress(hostName, 27017)

		try {
			println("» Trying $address…")
			val opened = aSocket(manager).tcp().connect(address) {
				socketTimeout = 100
			}.use { println("  Connected successfully!") }
			return@shared address
		} catch (e: Exception) {
			println("  Could not connect to MongoDB on socket $address • $e")
		}
	}

	error("Could not find on which port MongoDB is running.")
}

but I'm seeing different behavior on different platforms.

On the JVM, I'm seeing:

Searching for the address of the running MongoDB instance…
» Trying localhost/127.0.0.1:27017…
  Connected successfully!

and then it hangs indefinitely.

I see the same behavior on linuxX64.

However, on wasmJs, it doesn't hang, and successfully continues to the rest of the program.

What is going on?

[Oliver.O]

Looks like its hanging on socket.close(). That might be a bug.

Note that you are also supposed to close the SelectorManager.

[Arjan van Wieringen]

The full example in the docs also does not close the SelectorManager.

[Arjan van Wieringen]

https://github.com/ktorio/ktor-documentation/blob/3.3.0/codeSnippets/snippets/sockets-server/src/main/kotlin/com/example/Application.kt

[CLOVIS]

^ that example is a server, I'm a client.

[CLOVIS]

Interesting. If I replace the dispatcher from currentCoroutineContext() to Dispatchers.Default , then it finishes as expected.

[CLOVIS]

I wonder if it's because the socket connection has an infinite loop somewhere that doesn't suspend, and since I'm running in the test dispatcher which is single-threaded

[CLOVIS]

If so, that's a shame, because using the test dispatcher would be quite convenient here

[CLOVIS]

If I give it a Job that is a child of the current job (but still use Dispatchers.Default) then it hangs too 🤔

[CLOVIS]

Ok after some setup to be able to use DebugProbes…

It's blocked on this line in ActorSelectorManager .

[CLOVIS]

Interesting. With this coroutine context, it still hangs on the JVM and passes on Wasm, but now it also passes on LinuxX64.

[CLOVIS]

This works on the three platforms without hanging:

val job = Job()
	currentCoroutineContext().job.invokeOnCompletion { e -> job.cancel("Finished searching for the address. (ended with: $e)")}
	val manager = SelectorManager(Dispatchers.Default + job + CoroutineName("FindMongoAddressManager"))

[Aleksei Tirman [JB]]

You can find the issue here.

[Oliver.O]

Wow! That's a concise reproducer. 👏

[CLOVIS]

Thanks 🙏 I'll add the other info I found there