Code review comment for lp:~vanvugt/mir/fix-1339700-alarm

Thread A is calling BufferQueue::release, owns BufferQueue::guard, and it's about to call TimeoutFrameDroppingPolicy::swap_unblocked() gets pre-empted

Thread B is executing the alarm handler in TimeoutFrameDroppingPolicy, which invokes the callback installed by BufferQueue to drop a frame, attempts to own BufferQueue::guard so it blocks.

Thread A resumes, calls TimeoutFrameDroppingPolicy::swap_unblocked() and calls alarm->cancel(), it deadlocks, since data->callbacks_running will never be decremented (waiting for the alarm handler invocation to return).