defunct NSURLConnection never executes or times out - ios

Question

I'm running a LOT of asynchronous (delegate, not block) NSURLConnections simultaneously, and they all come back very quickly as I'm hitting a LAN server.
Every so often, one NSURLConnection will go defunct and never return.
connection:willSendRequest: is called but connection:didReceiveResponse: (and failure) is not.
Any ideas? I'm wondering if I should make a simple drop-in replacement using CFNetwork instead.
Edit: There's really not much code to show. What I've done is created a wrapper class to download files. I will note that the problem happens less when I run the connection on a separate queue - but still happens.
The general gist of what I'm doing is creating a download request for each cell as a tableview scrolls (in cellForRowAtIndexPath) and then asynchronously loading in an image file to the table cell if the cell is still visible.
_request = [NSMutableURLRequest requestWithURL:_URL];
_request.cachePolicy = NSURLRequestReloadIgnoringCacheData;
_request.timeoutInterval = _timeoutInterval;
if(_lastModifiedDate) {
[_request setValue:[_lastModifiedDate RFC1123String] forHTTPHeaderField:#"If-Modified-Since"];
}
_connection = [[NSURLConnection alloc] initWithRequest:_request
delegate:self
startImmediately:NO];
[_connection start];
As requested, instance variables:
NSMutableURLRequest *_request;
NSURLConnection *_connection;
And delegate methods:
- (NSURLRequest *)connection:(NSURLConnection *)connection willSendRequest:(NSURLRequest *)request redirectResponse:(NSURLResponse *)response {
NSLog(#"%# send", _URL);
return request;
}
- (void)connection:(NSURLConnection *)connection didReceiveResponse:(NSURLResponse *)response {
NSLog(#"%# response", _URL);
_response = (id)response;
// create output stream
}
- (void)connection:(NSURLConnection *)connection didReceiveData:(NSData *)data {
_receivedLength += data.length;
_estimatedProgress = (Float32)_receivedLength / (Float32)_response.expectedContentLength;
[_outputStream write:data.bytes maxLength:data.length];
// notify delegate
}
- (void)connectionDidFinishLoading:(NSURLConnection *)connection {
// close output stream
// notify delegate
}
- (void)connection:(NSURLConnection *)connection didFailWithError:(NSError *)error {
NSLog(#"%# failure", _URL);
// notify delegate
}
- (void)connection:(NSURLConnection *)connection
didReceiveAuthenticationChallenge:(NSURLAuthenticationChallenge *)challenge {
if(_credential && challenge.previousFailureCount == 0) {
[[challenge sender] useCredential:_credential forAuthenticationChallenge:challenge];
}
}

Answer 1

After poking around in profiler, I found a lead, and it gave me a hunch.
My credentials were failing (not sure why...) and so previousFailureCount was not 0, and hence I wasn't using my credential object.
Changed the code to this and I have no problems:
- (void)connection:(NSURLConnection *)connection didReceiveAuthenticationChallenge:(NSURLAuthenticationChallenge *)challenge {
if(_credential) {
[[challenge sender] useCredential:_credential forAuthenticationChallenge:challenge];
}
}

Answer 2

A NSURLConnection will send either didReceiveResponse or didFailWithError.
Often, you're dealing with timeouts before didFailWithError occurs.