Platorm: iOS (mpp), kotlin: `1.3.71`, coroutines: ...

# kotlin-native

[Peter Samokhin]

Platorm: iOS (mpp), kotlin: 1.3.71, coroutines: 1.3.5-native-mt, ktor: 1.3.2.

Can someone explain to me, how is this working:
For now, this is not possible to do ktor calls in the background.
I know that this is ktor's issue, but only when you are calling HttpClient from different threads on iOS (according to the github issues), and ktor handles this (does background calls) by itself.
In my case, I do such calls only before, but anyway I got InvalidMutabilityException.
How this is possible?

A detailed description is below.

I have the code like this:

class IssueReproduce: CoroutineScope {
	private val dispatcherUi = // on iOS: Dispatchers.Main
	private val dispatcherDefault = // on iOS: Dispatchers.Default
	private val job = SupervisorJob()
	private val api = SomeApi()
	
	override val coroutineContext = dispatcherUi + job

	/* ... */

    fun checkAuth() {
	    launch(coroutineContext) {
	    	// make some call in a background BEFORE
	        val token = withContext(dispatcherDefault) {
	            repo.getToken()
	        }

	        val response = try {
	            api.doAuth(token)
	        } catch (e: Throwable) {
	            // error: 
	            // kotlin.native.concurrent.InvalidMutabilityException
	            // mutation attempt of frozen io.ktor.client.request.HttpRequestPipeline@838cd448
	        }
	    }
	}
}

Where:

class SomeApi {
	private val client = HttpClient(/* ... */)

	suspend fun doAuth(token: String): SomeDataClass {
		return <http://client.post|client.post> {
			url(/* ... */)
			body = /* ... */
		}
	}
}

But, when I change code to this:

val token = /* withContext(dispatcherDefault) { */
	repo.getToken()
/* } */

i.e. when I make no calls in the background before calling ktor.HttpClient, all is ok and there are no errors.

So, what I'm doing wrong? Removing withContext(dispatcherDefault) for precending code does the trick, but wrapping ktor calls with withContext(coroutineContext) or withContext(dispatcherUi) does not.

How preceding code with the call in the background can affect the thread of the following code?
And how to fix this? As I mentioned above, I tried to wrap ktor calls with the original thread, but it does not help.

[rudolf.hladik]

can you try to change the client initialization to lazy? private val client by lazy { HttpClient(/* … */) ?

[Peter Samokhin]

@rudolf.hladik it is lazy already, and it is injected by the Kodein. Removed this to make example simpler. The issue is reproduced in case of lazy client initialization too. Kodein (top level variable) is threadlocal and lazy too.

[Kris Wong]

ktor is not compatible with multithreaded coroutines

[Kris Wong]

it does not support freezing

[Kris Wong]

this is why i dropped ktor and rolled my own Http class. it was quite straightforward

[Peter Samokhin]

@Kris Wong it is completely incompatible with all -native-mt versions? But anyway, it is interesting for me, how calls in the background coroutine somewhere in the code can affect the thread of following code inside the “launch” block, and this thread has high priority and coroutineContext is ignored, and even force changing coroutine back is also does not help

[Kris Wong]

welcome to K/N concurrency

[Kris Wong]

this won't be the last time you'll be scratching your head

[Arkadii Ivanov]

Your background block captures this and freezes the entire class, and so SomeApi and its HttpClient. Try moving this call to a separate method and pass repo as argument.