Swift: downloadTaskWithURL sometimes "succeeds" with non-nil location even though file does not exist - ios

Question

The downloadTaskWithURL function sometimes returns a non-nil location for a file that does not exist.
There is no file at http://192.168.0.102:3000/p/1461224691.mp4 in the test environment.
Most of the time, invoking downloadTaskWithURL on this URL results in the expected error message:
Error downloading message during network operation. Download URL:
http://192.168.0.102:3000/p/1461224691.mp4. Location: nil. Error:
Optional(Error Domain=NSURLErrorDomain Code=-1100 "The requested URL
was not found on this server." UserInfo=0x17547b640
{NSErrorFailingURLKey=http://192.168.0.102:3000/p/1461224691.mp4,
NSLocalizedDescription=The requested URL was not found on this
server.,
NSErrorFailingURLStringKey=http://192.168.0.102:3000/p/1461224691.mp4})
Occasionally, and in a non-deterministic way, downloadTaskWithURL believes the file exists and writes something to the location variable. As a result, the guard condition does not fail, and the code continues to execute ... which it should not.
The permanent file created by fileManager.moveItemAtURL(location!, toURL: fileURL) is only 1 byte, confirming that the network file never existed in the first place.
Why does downloadTaskWithURL behave like this?
func download() {
// Verify <networkURL>
guard let downloadURL = NSURL(string: networkURL) where !networkURL.isEmpty else {
print("Error downloading message: invalid download URL. URL: \(networkURL)")
return
}
// Generate filename for storing locally
let suffix = (networkURL as NSString).pathExtension
let localFilename = getUniqueFilename("." + suffix)
// Create download request
let task = NSURLSession.sharedSession().downloadTaskWithURL(downloadURL) { location, response, error in
guard location != nil && error == nil else {
print("Error downloading message during network operation. Download URL: \(downloadURL). Location: \(location). Error: \(error)")
return
}
// If here, no errors so save message to permanent location
let fileManager = NSFileManager.defaultManager()
do {
let documents = try fileManager.URLForDirectory(.DocumentDirectory, inDomain: .UserDomainMask, appropriateForURL: nil, create: false)
let fileURL = documents.URLByAppendingPathComponent(localFilename)
try fileManager.moveItemAtURL(location!, toURL: fileURL)
self.doFileDownloaded(fileURL, localFilename: localFilename)
print("Downloaded file # \(localFilename). Download URL: \(downloadURL)")
} catch {
print("Error downloading message during save operation. Network URL: \(self.networkURL). Filename: \(localFilename). Error: \(error)")
}
}
// Start download
print("Starting download # \(downloadURL)")
task.resume()
}

Answer 1

Just to clarify, the server is returning a 404, but the download task is returning a basically-empty file? And you're certain that the server actually returned an error code (by verifying the server logs)?
Either way, I would suggest checking the status code in the response object. If it isn't a 200, then the download task probably just downloaded the response body of an error page. Or, if the status code is 0, the connection timed out. Either way, treat it as a failure.
You might also try forcing this to all happen on a single thread and see if the nondeterminism goes away.