Upload file error handling using URLSessionConfiguration.background and uploadTask

I am using URLSessionConfiguration.background and uploadTask to upload a file from an iOS app.

The upload session is configured in the following way:

let configuration = URLSessionConfiguration.background(withIdentifier: "com.mycompany.myapp.fileUploader")
configuration.isDiscretionary = false
configuration.allowsCellularAccess = true
uploadURLSession = URLSession(configuration: configuration, delegate: self, delegateQueue: nil)

and the request is:

request.httpMethod = "POST"
request.setValue("application/octect-stream", forHTTPHeaderField: "Content-Type")


let task = uploadURLSession.uploadTask(with: request, fromFile: fileURL)

I'd like to understand how to manage the error handling.

How the http errors 4xx or 5xx are handled by the URLSession.uploadTask?

How can I trigger the retrying on 5xx errors?

URLSession.uploadTask does not handle the http server side errors. It handles retrying and errors only for client side or network issues.

The http status/error has to be retrieved from the task response casting it to an HTTPURLResponse.

Directly from the uploadTask call (not supported by background URLSession):

let task = uploadSession?.uploadTask(with: request, fromFile: fileURL, completionHandler: { data, response, error in
    if let error = error {
        print("Upload error:\(error)")
        //Client side error
        return
    }
    
    guard let res = response as? HTTPURLResponse else {
        print("Upload completed with response:\(response.description ?? "undefined")")
        //It should not happen at all
        return
    }
    
    if (200...299).contains(res.statusCode) {
        print("Upload completed successfully. Status code:\(res.statusCode)")
    }
    else if (400...499).contains(res.statusCode) {
        print("Upload fatal issue. Status code:\(res.statusCode)")
        //Fatal issue, do not retry the upload
    }
    else if (500...599).contains(res.statusCode) {
        print("Upload issue. Status code:\(res.statusCode)")
        //Schedules a new uploading task for the file
    }
    else {
        print("Upload completed with status code:\(res.statusCode)")
    }
})

or, if you are using a background URLSession, from the URLSessionTaskDelegate:

func urlSession(_ session: URLSession, task: URLSessionTask, didCompleteWithError error: Error?) {
    if let error = error {
        print("Upload error:\(error)")
        //Client side error
        return
    }

    guard let res = task.response as? HTTPURLResponse else {
        print("Upload completed with response:\(task.response?.description ?? "undefined")")
        //It should not happen at all
        return
    }

    if (200...299).contains(res.statusCode) {
        print("Upload completed successfully. Status code:\(res.statusCode)")
    }
    else if (400...499).contains(res.statusCode) {
        print("Upload fatal issue. Status code:\(res.statusCode)")
        //Fatal issue, do not retry the upload
    }
    else if (500...599).contains(res.statusCode) {
        print("Upload issue. Status code:\(res.statusCode)")
        //Schedules a new uploading task for the file
    }
    else {
        print("Upload completed with status code:\(res.statusCode)")
    }
}