in Sources/SPTPersistentCacheGarbageCollector.m [136:141]
static BOOL SPTPersistentCacheGarbageCollectorSchedulerIsInMainQueue(void)
{
NSString *currentQueueLabelString = [NSString stringWithUTF8String:dispatch_queue_get_label(DISPATCH_CURRENT_QUEUE_LABEL)];
NSString *mainQueueLabelString = [NSString stringWithUTF8String:dispatch_queue_get_label(dispatch_get_main_queue())];
return [currentQueueLabelString isEqualToString:mainQueueLabelString];
}