URLSessionUploadTask getting automatically cancelled instantly

Question

I'm having this weird issue in which a newly created URLSessionUploadTask gets cancelled instantly. I'm not sure if it's a bug with the current beta of Xcode 8.

I suspect it might be a bug because the code I'm about to post ran fine exactly once. No changes were made to it afterwards and then it simply stopped working. Yes, it literally ran once, and then it stopped working. I will post the error near the end.

I will post the code below, but first I will summarize how the logic here works.

My test, or user-exposed API (IE for use in Playgrounds or directly on apps), calls the authorize method. This authorize method will in turn call buildPOSTTask, which will construct a valid URL and return a URLSessionUploadTask to be used by the authorize method.

With that said, the code is below:

The session:

internal let urlSession = URLSession(configuration: .default)

Function to create an upload task:

internal func buildPOSTTask(onURLSession urlSession: URLSession, appendingPath path: String, withPostParameters postParams: [String : String]?, getParameters getParams: [String : String]?, httpHeaders: [String : String]?, completionHandler completion: URLSessionUploadTaskCompletionHandler) -> URLSessionUploadTask {
    let fullURL: URL
    if let gets = getParams {
        fullURL = buildURL(appendingPath: path, withGetParameters: gets)
    } else {
        fullURL = URL(string: path, relativeTo: baseURL)!
    }

    var request = URLRequest(url: fullURL)
    request.httpMethod = "POST"

    var postParameters: Data? = nil

    if let posts = postParams {
        do {
            postParameters = try JSONSerialization.data(withJSONObject: posts, options: [])
        } catch let error as NSError {
            fatalError("[\(#function) \(#line)]: Could not build POST task: \(error.localizedDescription)")
        }
    }

    let postTask = urlSession.uploadTask(with: request, from: postParameters, completionHandler: completion)
    return postTask
}

The authentication function, which uses a task created by the above function:

    public func authorize(withCode code: String?, completion: AccessTokenExchangeCompletionHandler) {

// I have removed a lot of irrelevant code here, such as the dictionary building code, to make this snippet shorter.

        let obtainTokenTask = buildPOSTTask(onURLSession: self.urlSession, appendingPath: "auth/access_token", withPostParameters: nil, getParameters: body, httpHeaders: nil) { (data, response, error) in
            if let err = error {
                completion(error: err)
            } else {
                print("Response is \(response)")
                completion(error: nil)
            }
        }

        obtainTokenTask.resume()
    }

I caught this error in a test:

let testUser = Anilist(grantType: grant, name: "Test Session")

let exp = expectation(withDescription: "Waiting for authorization")

testUser.authorize(withCode: "a valid code") { (error) in
    if let er = error {
        XCTFail("Authentication error: \(er.localizedDescription)")
    }
    exp.fulfill()
}
self.waitForExpectations(withTimeout: 5) { (err) in
    if let error = err {
        XCTFail(error.localizedDescription)
    }
}

It always fails instantly with this error:

Error Domain=NSURLErrorDomain Code=-999 "cancelled" UserInfo={NSErrorFailingURLKey=https://anilist.co/api/auth/access_token?client_secret=REMOVED&grant_type=authorization_code&redirect_uri=genericwebsitethatshouldntexist.bo&client_id=ibanez-hod6w&code=REMOVED, NSLocalizedDescription=cancelled, NSErrorFailingURLStringKey=https://anilist.co/api/auth/access_token?client_secret=REMOVED&grant_type=authorization_code&redirect_uri=genericwebsitethatshouldntexist.bo&client_id=ibanez-hod6w&code=REMOVED}

Here's a few things to keep in mind:

• The URL used by the session is valid.
• All credentials are valid.
• It fails instantly with a "cancelled" error, that simply did not happen before. I am not cancelling the task anywhere, so it's being cancelled by the system.
• It also fails on Playgrounds with indefinite execution enabled. This is not limited to my tests.

Here's a list of things I have tried:

• Because I suspect this is a bug, I first tried to clean my project, delete derived data, and reset all simulators. None of them worked.
• Even went as far restarting my Mac...
• Under the small suspicion that the upload task was getting deallocated due to it not having any strong pointers, and in turn calling cancel, I also rewrote authorize to return the task created by buildPOSTTask and assigned it to a variable in my test. The task was still getting cancelled.

Things I have yet to try (but I will accept any other ideas as I work through these):

• Run it on a physical device. Currently downloading iOS 10 on an iPad as this is an iOS 10 project. EDIT: I just tried and it's not possible to do this.

I'm out of ideas of what to try. The generated logs don't seem to have any useful info.

EDIT:

I have decided to just post the entire project here. The thing will be open source anyway when it is finished, and the API credentials I got are for a test app.

ALCKit

Urethra

Right, sorry, guess that is important, haha. Yeah the test does have a waitForExpectationsWithTimeout call. I will edit my question to add that. I haven't tried it in an actual app yet (this is a framework I'm building, but an app that consumes will be built shortly afterwards), but it does fail in a Swift playground with indefinite execution enabled. – Urethra

Can you try: pastebin.com/B7nZFY1n Does it print the response? Gives an error? – Jonejonell

Thanks for your input Brandon. I ran it on a Playground and it worked fine. Printed the response as expected. – Urethra

Writing a test for the code you gave me is working fine as well. Something must be happening between my method calls. – Urethra

pastebin.com/ijshDH6Q Seems to be the same code you're using. I was able to deduce it based on your error code. – Jonejonell

I just went to buildPOSTTask and hard coded the StackOverflow URL there. So instead of var request = URLRequest(url: fullURL), I had var request = URLRequest(url: URL(string: "http://www.stackoverflow.com")!). It still failed. It also fails too soon, so it doesn't really look like it gets to make the request. I just tried doing it in the Playground as well, and also got cancelled there. – Urethra

Answer 1

After struggling non-stop with this for 6 days, and after googling non-stop for a solution, I'm really happy to say I have finally figured it out.

Turns out that, for whatever mysterious reason, the from: parameter in uploadTask(with:from:completionHandler) cannot be nil. Despite the fact that the parameter is marked as an optional Data, it gets cancelled instantly when it is missing. This is probably a bug on Apple's side, and I opened a bug when I couldn't get this to work, so I will update my bug report with this new information.

With that said, everything I had to do was to update my buildPOSTTask method to account for the possibility of the passed dictionary to be nil. With that in place, it works fine now:

internal func buildPOSTTask(onURLSession urlSession: URLSession, appendingPath path: String, withPostParameters postParams: [String : String]?, getParameters getParams: [String : String]?, httpHeaders: [String : String]?, completionHandler completion: URLSessionUploadTaskCompletionHandler) -> URLSessionUploadTask {
    let fullURL: URL
    if let gets = getParams {
        fullURL = buildURL(appendingPath: path, withGetParameters: gets)
    } else {
        fullURL = URL(string: path, relativeTo: baseURL)!
    }

    var request = URLRequest(url: fullURL)
    request.httpMethod = "POST"

    var postParameters: Data

    if let posts = postParams {
        do {
            postParameters = try JSONSerialization.data(withJSONObject: posts, options: [])
        } catch let error as NSError {
            fatalError("[\(#function) \(#line)]: Could not build POST task: \(error.localizedDescription)")
        }
    } else {
        postParameters = Data()
    }

    let postTask = urlSession.uploadTask(with: request, from: postParameters, completionHandler: completion)
    return postTask
}

Urethra

Just struggled with this for over 2 hours. Did apple ever reply to your bug report? The cancelled error for not having post parameters is nonsensical (and infuriating) – Herod

They didn't reply. It hasn't been marked as duplicate, either. It has been open since July 19, 2016. – Urethra

It is 2019. And still need to pass non-nil data. – Tarnopol

@Tarnopol yep. And the bug report has been ignored for over 3 years too. – Urethra

Still an issue in 2024. Very disappointing. Thank you for saving me a lot of time with this Q&A! – Offshoot

Answer 2

Are you by any chance using a third party library such as Ensighten? I had the exact same problem in XCode 8 beta (works fine in XCode 7) and all of my blocks with nil parameters were causing crashes. Turns out it was the library doing some encoding causing the issue.

Hypothesis

How did you determine this? – Photosensitive

I through trial and error. I disabled the library and everything ran fine. In my particular case they have since updated the library and everything runs fine. – Hypothesis

Answer 3

For me, this was a weak reference causing the issue, so I changed

completion: { [weak self] (response: Result<ResponseType, Error>)

to

completion: { [self] (response: Result<ResponseType, Error>)

Enrique