Ktor Native (iOS) questions: We are using a precon...

# ktor

[Marc Lefrancois]

Ktor Native (iOS) questions:
We are using a preconfigured URLSession to do background downloads on iOS. We configure it that way:

private val sessionConfiguration = NSURLSessionConfiguration.backgroundSessionConfiguration(identifier)
    private val ktorDelegate = KtorNSURLSessionDelegate()
    private val session = NSURLSession.sessionWithConfiguration(
        sessionConfiguration,
        ktorDelegate,
        delegateQueue = null
    )

    protected val httpClient = HttpClient(Darwin) {
        engine {
            usePreconfiguredSession(session, ktorDelegate)
        }
        expectSuccess = true
    }

Everything works fine as long as requests succeed. If a request returns for example a 404 response, a simple httpClient.get(<RequestWithURLThatReturns404>) simply never returns

Changing sessionConfiguration to private val sessionConfiguration = NSURLSessionConfiguration.defaultSessionConfiguration fixes the problem

Question 1: Anyone has ever encountered this problem and come up with a solution
Question 2: I would like to contribute a fix if it’s not the case, any pointers on where I could start in the ktor code base

[Marc Lefrancois]

I did build my own SessionDelegate to make sure the URLSession was calling back in didReceiveData and didCompleteWithError

[Aleksei Tirman [JB]]

What version of Ktor do you use?

[Marc Lefrancois]

2.3.3

[Aleksei Tirman [JB]]

Unfortunately, I cannot reproduce your problem on MacosX64 target with the following code:

val sessionConfiguration = NSURLSessionConfiguration.backgroundSessionConfiguration("identifier")
val ktorDelegate = KtorNSURLSessionDelegate()
val session = NSURLSession.sessionWithConfiguration(
    sessionConfiguration,
    ktorDelegate,
    delegateQueue = null
)

val httpClient = HttpClient(Darwin) {
    engine {
        usePreconfiguredSession(session, ktorDelegate)
    }
    expectSuccess = true
}

@OptIn(DelicateCoroutinesApi::class)
fun main() {
    val job = GlobalScope.launch {
        httpClient.get("<https://example.com/404>")
    }
    while (job.isActive) {}
}

Is it only reproducible on iOS? Can you file an issue with a minimal sample project attached?

[Łukasz Zieliński]

@Marc Lefrancois I am just dealing with background downloads on iOS. Does “Everything works fine” mean that the downloads are processed even when the app is minimized (as described in https://developer.apple.com/documentation/foundation/url_loading_system/downloading_files_in_the_background)? I couldn’t make it so with similar config.

[Łukasz Zieliński]

I can confirm that requesting non-existing url on Android gets 404 error page and on iOS it hangs indefinetly also with this multiplatform config

val client = HttpClient() {
    install(Logging) {
        level = LogLevel.BODY
    }
    install(ContentNegotiation) {
        json()
    }
    install(HttpTimeout) {
        requestTimeoutMillis = 20000L
        connectTimeoutMillis = 20000L
        socketTimeoutMillis = 20000L
    }
}

[Marc Lefrancois]

@Łukasz Zieliński I think you are missing an important part: expectSuccess = true that should threat 404 response has errors for the flow ?

As for my problem @Aleksei Tirman [JB] I can now confirm that it was a setup problem on our side combine with an unexpected (bug?) behaviour of backgroundSessionConfiguration . I’ll try to explain clearly, do not hesitate if it is not clear enough

I was by error using 2 backgroundSessionConfiguration() with the same identifier in 2 different instance of HttpClient . So 2 URLSession instances , with 2 KtorNSURLSessionDelegate instances.

1. When the requests are successful, no problems.
2. When the requests have an error status code (like 404). The following happened using the second HttpClient:
a. When the request is initiated, read is called on the right KtorDelegate instance
b. When the urlSession calls back to the delegate for didReceiveData and didCompleteWithError it does so on the other KtorDelegate instance. So there is no NSURLSessionTask in the taskHandlers map and the flow never emits anything.

[Marc Lefrancois]

This leads u to believe that calling NSURLSessionConfiguration.backgroundSessionConfiguration returns shared configuration per identifier and that there is a mechanism to handle the multiple delegates

[Marc Lefrancois]

But there might be a bug in the multiple delegate handling for in error requests in Apple’s code

[Marc Lefrancois]

This is the farthest we went in our investigation since having different identifiers for each background configuration fixed the problem for us