Using NSURLSession delegate with BackgroundTasks

It seems that apps using background processing are required to implement BackgroundTasks, but I am struggling to figure out how to do that when continuing an URLSession upload task when a device enters the background.

Currently, I use BGTaskScheduler to register a new task, and schedule that task when I enter the background:

Code Block 
BGTaskScheduler.shared.register(forTaskWithIdentifier: backgroundTaskIdentifier, using: nil) { task in
      self.handleUploadTask(task)
}

The actual content of the task uses a stored uploadIdentifier to recreate the session configuration and get the relevant upload tasks. Then I add a cancel call for each to when the BGTask expires, and (unnecessarily?) resume each of those ongoing upload tasks:

Code Block 
func handleUploadTask(_ task: BGTask) {
     let uploader = UploadService()
     if let identifier = uploader.uploadIdentifier {
         let sessionConfig = URLSessionConfiguration.background(withIdentifier: identifier)
         let session = URLSession(configuration: sessionConfig, delegate: firebase, delegateQueue: OperationQueue.main)
         task.expirationHandler = {
             session.getTasksWithCompletionHandler { (_, uploadTasks, _) in
                 for uploadTask in uploadTasks {
                        uploadTask.cancel()
                 }
             }
         }
         session.getTasksWithCompletionHandler { (_, uploadTasks, _) in
             for uploadTask in uploadTasks {
                    uploadTask.resume()
             }
         }
     }
 }

How do I complete the BGTask when I get a callback from my URLSession delegate in my UploadService? Are there other pieces of the puzzle I am missing?