Synchronous NSURLConnection timeout

NSURLConnection+Timeout.m

@implementation NSURLConnection (Timeout)

+ (dispatch_queue_t)timeoutLockQueue {
  static dispatch_queue_t queue;
  static dispatch_once_t onceToken;
  dispatch_once(&onceToken, ^{
    queue = dispatch_queue_create("timeout lock queue", DISPATCH_QUEUE_SERIAL);
  });
  return queue;
}

+ (NSData *)sendSynchronousRequest:(NSURLRequest *)request returningResponse:(NSURLResponse **)response timeout:(NSTimeInterval)timeout error:(NSError **)error {
  // DEVNOTE: Timeout interval is quirky
  // https://devforums.apple.com/thread/25282
  //
  // The minimum timeout for NSURLConnection is 120/240 seconds, so we need this workaround
  // to get the synchronous mechanism to cancel before that fixed minimum.
  //
  // Set a min timeout of 5 seconds
  //
  timeout = MAX(5.0, timeout);
  
  dispatch_time_t popTime = dispatch_time(DISPATCH_TIME_NOW, timeout * NSEC_PER_SEC);
  
  // Use a serial queue for locking, it should be faster than a explicit lock
  dispatch_queue_t lockQueue = [self timeoutLockQueue];
  
  __block BOOL finished = NO;
  __block NSData *data = nil;
  __block NSURLResponse *internalResponse;
  __block dispatch_semaphore_t sema = dispatch_semaphore_create(0);
  
  dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
    data = [self sendSynchronousRequest:request returningResponse:&internalResponse error:error];
    
    // Use the locking queue
    dispatch_sync(lockQueue, ^{
      if (sema != NULL) {
        finished = YES;
        dispatch_semaphore_signal(sema);
      }
    });
  });
  dispatch_semaphore_wait(sema, popTime);
  
  // Release the semaphore inside the locking queue
  dispatch_sync(lockQueue, ^{
    dispatch_release(sema);
    sema = NULL;
  });
  
  if (finished) {
    if (response != NULL) {
      *response = internalResponse;
    }
    return data;
  }
  
  // If the request timed out, return an error
  if (error != NULL) {
    NSDictionary *infoDict = [NSDictionary dictionaryWithObject:NSLocalizedString(@"Request timed out", nil)
                                                         forKey:NSLocalizedDescriptionKey];
    *error = [NSError errorWithDomain:NSCocoaErrorDomain code:0x100 userInfo:infoDict];
  }
  
  return nil;
}

@end

I found this via a Google search, looks really nice. On a non-ARC iOS project I got bad access due to data not being retained outside the block so added the necessary memory management for non-ARC projects:

https://gist.github.com/2560473