再看Core Data中PSC陷入死锁的问题

在《Core Data Programming Guide》文档的Concurrency with Core Data这一章节中提到了“Use Thread Confinement to Support Concurrency”，这里的“Thread”还包含serial operation queue和dispatch queue。

我目前的理解是MOC会负责对PSC进行加锁解锁，这些动作需要保证有序地得到执行，如果两个thread同时访问一份context，尤其是其中一个进行写操作，另一个进行读操作，就很容易发生死锁：

- (void)runSave
{
    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
        AppDelegate *appDelegate = [[UIApplication sharedApplication] delegate];
        NSManagedObjectContext *moc = appDelegate.managedObjectContext;
        
        if (moc) {
            for (int j = 0; j < 10000; ++j) {
                
                NSString *threadPtr = [NSString stringWithFormat:@"%p", [NSThread currentThread]];
                [_threadDict setObject:@"Running Thread" forKey:threadPtr];
                
                People *people = [NSEntityDescription insertNewObjectForEntityForName:@"People" inManagedObjectContext:moc];
                people.name = @"noname";
            }
            
            NSLog(@"**********IN SAVE %@", [NSThread currentThread]);
            NSError *error = nil;
            if ([moc save:&error]) {
                ;
            }

            NSLog(@"**********OUT SAVE %@", [NSThread currentThread]);
        }
    });
}

- (void)runFetch
{
    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0), ^{
        AppDelegate *appDelegate = [[UIApplication sharedApplication] delegate];
        NSManagedObjectContext *moc = appDelegate.managedObjectContext;
        
        if (moc) {
            NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
            [fetchRequest setEntity:[NSEntityDescription entityForName:@"People" inManagedObjectContext:moc]];
            
            NSLog(@"~~~~~~~~~~IN FETCH %@", [NSThread currentThread]);
            NSError *error = NULL;
            NSArray *array = [moc executeFetchRequest:fetchRequest error:&error];
            if (array) {
                ;
            }
            NSLog(@"~~~~~~~~~~OUT FETCH %@", [NSThread currentThread]);
        }
    });
}

死锁的表现为：