NSURLSession Upload task does not return server response for large uploads

Hi,

The problem I am seeing occurs only for large file uploads (300-500 MBs). What I am observing is that iOS sometimes does NOT call didCompleteWithError even after my server sends a response. In the client logs I can see that the didSendBodyData method gets called several times. Which is expected, however it does not seem to stop even after sending all the bytes. I have also noticed that sometimes the taskId changes or the totalBytesSent gets reset. Which does not make any sense since I have not queued up another upload task. I only called resume once.

Also note: This behavior happens when the app is in the foreground. I have not attempted testing background uploads yet. Additionally I have tired using the default session and still observe the same behavior.

Everything works as expected the file is small.(~50 Mbs). The didRecieveData method gets called along with the didCompleteWithError method.

Below are some code snippets of the background task:

//FileOperationsDelegate.mm 
- (void)URLSession:(NSURLSession *)session 
              task:(NSURLSessionTask *)task 
   didSendBodyData:(int64_t)bytesSent 
    totalBytesSent:(int64_t)totalBytesSent 
totalBytesExpectedToSend:(int64_t)totalBytesExpectedToSend 
{ 
    NSNumber* taskId = [NSNumber numberWithUnsignedInteger:task.taskIdentifier]; 
    AwLog(@"DidSendBodyData for taskID: %@  BytesSent: %lld TotalBytesSent: %lld ExpectedToSend: %lld\n", taskId, bytesSent, totalBytesSent, totalBytesExpectedToSend); 
    if (updateFileProgressCallback != NULL) 
    { 
        updateFileProgressCallback(bytesSent, totalBytesSent); 
    } 
}

Here is by backgroundSession.

//FileOperationsDelegate.mm 
- (NSURLSession *)backgroundSession 
{ 
    static NSURLSession *session = nil; 
    static dispatch_once_t onceToken; 
    dispatch_once(&onceToken, ^{ 
       NSURLSessionConfiguration *configuration = [NSURLSessionConfiguration backgroundSessionConfigurationWithIdentifier: KEY_BACKGROUND_SESSSION_ID]; 
        configuration.discretionary = YES; 
        configuration.sessionSendsLaunchEvents = YES; 
        configuration.URLCache = nil; 
        bool wifiOnly = [ClientConfiguration.sharedInstance isWifiOnlyEnabled]; 
        if (wifiOnly) 
        { 
            configuration.allowsCellularAccess = false; 
        } 
        NSOperationQueue* delegateMainQueue = [NSOperationQueue mainQueue]; 
        delegateMainQueue.maxConcurrentOperationCount = 1; 
        session = [NSURLSession sessionWithConfiguration:configuration delegate:self delegateQueue: delegateMainQueue]; 
    }); 
    return session; 
}

//FileUploader.mm 
NSURLSession* backgroundSession = [FileOperationsSessionDelegate.sharedInstance backgroundSession]; 
                       NSMutableString *file = [[NSMutableString alloc] initWithString:absLocalPath]; 
                       [file insertString:@"file://" atIndex:0]; 
                       NSURL* fileToBeUploaded = [NSURL URLWithString:file]; 
                       NSURLSessionUploadTask* uploadTask = [backgroundSession uploadTaskWithRequest:urlRequest fromFile:fileToBeUploaded]; 
                       uploadTask.countOfBytesClientExpectsToSend = self.uploadSizeBytes; 
                       uploadTask.countOfBytesClientExpectsToReceive = 450; 
ConnectionCallback uploadCompleteCallback = ^(NSURLResponse *  response, NSData * data,  NSError *  error) { 
// A callback to complete the file upload. It will update the UI and notify the user  
// on how the upload went.  
} 
  BytesUploadedCallback updateFileProgressCallback = ^(long bytesWritten, long totalBytesWritten) 
// A callback that will update the progress bar. 
                       }; 
                       [FileOperationsSessionDelegate.sharedInstance setUploadCallback:uploadCompleteCallback]; 
                       [FileOperationsSessionDelegate.sharedInstance setConnectionCallback:updateFileProgressCallback];; 
                       [uploadTask resume];

Any help with this will be greatly apperiated.

Thanks : )

Specs:

iOS version 12.3

Device: iPad Pro 11

Xcode: 10.1