How to properly release an AVCaptureSession

Here's the best solution I've found so far. The basic idea is to use the finalizer of the dispatch queue. When the dispatch queue quits, we can be sure that there won't be any more action in the second thread where the sample buffers are processed.

static void capture_cleanup(void* p)
{
    AugmReality* ar = (AugmReality *)p; // cast to original context instance
    [ar release];  // releases capture session if dealloc is called
}

...

dispatch_queue_t queue = dispatch_queue_create("augm_reality", NULL);
dispatch_set_context(queue, self);
dispatch_set_finalizer_f(queue, capture_cleanup);
[output setSampleBufferDelegate: self queue: queue];
dispatch_release(queue);
[self retain];

...

Unfortunately, I now have to explicitly stop capturing. Otherwise releasing my instance won't free it because the second thread now increments and decrements the counter as well.

A further problem is that my class is now released from two different threads. Is this reliable or is it the next problem causing crashes?

As per current apple docs(1) [AVCaptureSession stopRunning] is a synchronous operation which blocks until the receiver has completely stopped running. So all these issues shouldn't happen any more.

I've posted a very similar question in the Apple Developer Forum and got an answer from an Apple employee. He says it's a known problem:

This is a problem with the AVCaptureSession / VideoDataOutput in iOS 4.0-4.1 that has been fixed and will appear in a future update. For the time being, you can work around it by waiting for a short period after stopping the AVCaptureSession, e.g. half a second, before disposing of the session and data output.

He/she proposes the following code:

dispatch_after(
    dispatch_time(0, 500000000),
    dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), // or main queue, or your own
    ^{
        // Do your work here.
        [session release];
        // etc.
    }
);

I still like the approach with the dispatch queue finalizer better because this code just guesses when the second thread might have finished.