NSURLSessionDownloadTask finishes with no file at the location URL

Question

I fire a bunch of downloads using NSURLSessionDownload tasks and NSURLSession with background configuration. Most of the time everything works fine but sometimes there is no temp file at the location specified in the delegate callback method. It happens on both simulator and device. Here is the code, but everything looks pretty straight forward, just like they have it SimpleBackgroundTransfer sample.

 ....
for (NSUInteger i = 0; i < 10; i++) {
    NSString *downloadURLString = ...;
    NSURLSessionDownloadTask *task = [[self backgroundURLSession] downloadTaskWithURL:[NSURL URLWithString:downloadURLString]];
    [task resume];
}
 ....


- (NSURLSession *)backgroundURLSession
{
    static NSURLSession *session = nil;
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        NSString *identifier = @"com.yyy.backgroundTransfer";
        NSURLSessionConfiguration* sessionConfig = [NSURLSessionConfiguration         backgroundSessionConfiguration:identifier];
        session = [NSURLSession sessionWithConfiguration:sessionConfig
                                                delegate:self
                                           delegateQueue:[NSOperationQueue mainQueue]];
    });    

    return session;
}
- (void)URLSession:(NSURLSession *)session downloadTask:(NSURLSessionDownloadTask *)downloadTask didFinishDownloadingToURL:(NSURL *)location
{
    NSLog(@"%s", __PRETTY_FUNCTION__);
    NSAssert([[NSFileManager defaultManager] fileExistsAtPath:[location path]], nil);
    /// This assert gets triggered every now and then. Any idea why? 
    /// And downloadTask.originalRequest in fact has a valid download URL and the status code for the response in downloadTask it is 200
}

- (void)URLSession:(NSURLSession *)session downloadTask:(NSURLSessionDownloadTask *)downloadTask didWriteData:(int64_t)bytesWritten totalBytesWritten:(int64_t)totalBytesWritten totalBytesExpectedToWrite:(int64_t)totalBytesExpectedToWrite
{
     NSLog(@"%s", __PRETTY_FUNCTION__);
}

- (void)URLSession:(NSURLSession *)session task:(NSURLSessionTask *)task didCompleteWithError:(NSError *)error
{
     NSLog(@"%s", __PRETTY_FUNCTION__);
}

Also, the strange thing is that when this weird thing occurs no other NSURLSessionDelegate methods are called, just URLSession:downloadTask:downloadTask:location with no file at the location URL (and downloadTask.originalRequest in fact has a valid download URL and the status code for the response in downloadTask it is 200).
If this weird thing happens it happens for all scheduled downloads, just like the system can not create files at all. URLSession:downloadTask:downloadTask:location is called immediately.
Restarting device/simulator seems to always help. Relaunching the app never helps.
Did anyone experience anything like this?

Do you control the server? I'm curious as to what happens if a server sends a 200 response but doesn't send any actual data. I'd expect a zero-length file, but no file at all seems like a possibility, too... — Matt Gibson, Apr 10 '14 at 07:32
I'm late to the party but, when you get this issue again, check your logs to see if you're getting a duplicated URLSession:downloadTask:didFinishDownloadingToURL callback for the same task. That's what's happening on my code (I don't really know why - suppose it's an iOS bug), and since the first callback moves the downloaded file to a different location, the second one will be considered as failed because the file is missing. — Oscar Hierro, Feb 06 '15 at 12:39

NSURLSessionDownloadTask finishes with no file at the location URL

0 Answers0