NSURLResponse returns Content-Range, ignoring the Range that I asked for

Question

When downloading a file, I want to be able to resume the download. For example:

NSFileManager *fm = [NSFileManager defaultManager];
NSString *downloadTempPath = [_tempPath path];
if ([fm fileExistsAtPath:downloadTempPath]) {
    // We have a partial download. Get the rest of the file starting at byteCount
    NSDictionary *dAttrib = [fm attributesOfItemAtPath:downloadTempPath error:nil];
    NSUInteger byteCount = [dAttrib fileSize];
    NSLog(@"Preparing to resume download: %@", downloadTempPath);
    NSString *requestRange = [NSString stringWithFormat:@"bytes=%d-", byteCount];
   [self.request setValue:requestRange forHTTPHeaderField:@"Range"];
}

This will produce a get request with a header like this:

Range:bytes=1000-

I would expect the first 206 response to give me a chunk where I asked it to start, having a header and value like this (dumped from allHeaderFields):

"Content-Range" = "bytes 1000-340197/340198";

But sometimes it jumps ahead and returns something like this instead:

"Content-Range" = "bytes 32000-340197/340198";

Which is really bad, because I need those bytes from 1000-32000, and it didn't give them to me.

I know that there is no server error, because when I curl to the server it returns the section that I asked for. Also, when I use wireshark, I don't see any packet going to the server. So this is a problem on the iOS side, possibly with the NSURLCache

Answer 1

It appears to be a bug in the NSURLCache. When I detect that the problem has happened, I do the following and try again.

    [[NSURLCache sharedURLCache] removeCachedResponseForRequest:self.request]; 

Once I have done that, the problem does not happen again. It seems to heal the cache. I saw this on iOS5.0, and since I can no longer reproduce, I don't know if it happens on newer OS'es.