performBlockAndWait在iOS 7上使用专用队列死锁的子上下文

我有两个名为importContext和childContext的NSManagedobjectContexts. childContext是importContextand的子节点,它们都是nsprivateQueueConcurrencyType.

为了避开主线程,我在importContext的队列上做了很多工作.这项工作涉及大量的提取和保存,因此将整个事物包装在importContext的performBlockAndWait中是很方便的(它确实需要通过同步操作,因为performBlockAndWait之后的代码依赖于它的结果).

在这项工作中的某些时候,我可能需要从JSON结果创建新的托管对象.这些JSON值可能无效并且我的验证失败,因此在创建对象后,如果它们不好,我需要能够抛弃它们.这就是childContext的用武之地.我将新对象插入其中,如果它的JSON属性最终没有意义,我就抛弃了childContext.

当我需要保存childContext时出现问题.我希望它有自己的私有队列,与父队列分开.但是,这会导致iOS 7(不是iOS 8)上的死锁.当我在iOS 8模拟器和设备上运行相同的代码时,childContext会在单独​​的线程上创建自己的队列并正确进行保存.

看起来当我运行iOS 7时,childContext正在尝试执行save：在父队列中,但父进程正在等待其子进程导致死锁.在iOS 8中,这不会发生.有谁知道为什么？

这是简化的代码：

-(NSManagedobjectContext *)importContext
   {
       NSManagedobjectContext* moc = [[NSManagedobjectContext alloc] initWithConcurrencyType:nsprivateQueueConcurrencyType];
       moc.persistentStoreCoordinator = [self storeCoordinator];
       return moc;
   }

   -(void)updateItems:(NSArray*)ItemDescriptions
   {
      [self.importContext performBlockAndWait:^{
           //get info and update
           ...
           ...

       if(needToCreateNewItem){
          NSManagedobjectContext* childContext = [[NSManagedobjectContext alloc] initWithConcurrencyType:nsprivateQueueConcurrencyType];
          childContext.parentContext = self.importedContext;

          //Insert and create new item 
          ...
          [childContext performBlockAndWait:^{
              id newObject = [NSEntityDescription insertNewObjectForEntityForName:[self entityName]
                                                 inManagedobjectContext:childContext];
          }];
          ...

          // Do something with this object

          if([newObject isReadyToSave])
              __block NSError* e = nil;
              __block BOOL saveSucceeded = NO;

              [childContext performBlockAndWait:^{
                 saveSucceeded = [childContext save:&e]; // DEADLOCK ON iOS 7!!!!
              }];
          }
          ....

       }
  }];  
}

一个简单的解决方法是将工作保持在一个单独的调度队列(而不是importContext的队列),但我问这个问题的原因是因为我想了解发生这种情况的根本原因.我认为孩子的保存应该只发生在自己的队列中.

更新1

回覆.马库斯的问题：

> updateItems：从操作队列中的NSInvocationoperation调用,因此它不在主队列中.
>在iOS 7上,我可以随时暂停应用程序并查看堆栈,并且托管对象上下文的队列将被死锁：

(lldb) bt

* thread #7: tid = 0xed07,0x38546aa8 libsystem_kernel.dylib`semaphore_wait_trap + 8,queue = 'NSManagedobjectContext Queue'
frame #0: 0x38546aa8 libsystem_kernel.dylib`semaphore_wait_trap + 8
frame #1: 0x385bbbac libsystem_platform.dylib`_os_semaphore_wait + 12
frame #2: 0x3848461a libdispatch.dylib`_dispatch_barrier_sync_f_slow + 138
frame #3: 0x2d4f3df2 CoreData`_perform + 102
frame #4: 0x2d4fe1ac CoreData`-[NSManagedobjectContext(_nestedContextSupport) executeRequest:withContext:error:] + 240
frame #5: 0x2d492f42 CoreData`-[NSManagedobjectContext save:] + 826
  * frame #6: 0x000c1c96 DBDevApp`__69+[DBManagedobject createWithAttributes:inManagedobjectContext:error:]_block_invoke77(.block_descriptor=<unavailable>) + 118 at DBManagedobject.m:117
frame #7: 0x2d4f6934 CoreData`developerSubmittedBlockToNSManagedobjectContextPerform + 88
frame #8: 0x3847e81e libdispatch.dylib`_dispatch_client_callout + 22
frame #9: 0x384847ca libdispatch.dylib`_dispatch_barrier_sync_f_invoke + 26
frame #10: 0x2d4f6a72 CoreData`-[NSManagedobjectContext performBlockAndWait:] + 106
frame #11: 0x000c1916 DBDevApp`+[DBManagedobject createWithAttributes:inManagedobjectContext:error:](self=0x005c1790,_cmd=0x0054a033,attributes=0x188e    context=0x17500800,error=0x02e68ae8) + 658 at DBManagedobject.m:116
frame #12: 0x000fe138 DBDevApp`-[DBAPIController createOrUpdateItems:withIDs:IDKeys:ofClass:amongExistingItems:withFindByIDPredicate:](self=0x17775de0,_cmd=0x0054de   newItemDescriptions=0x188eada0,itemIDs=0x18849580,idKey=0x0058e290,class=0x005c1790,existingItems=0x1756b560,findByID=0x18849c80) + 2472 at DBAPIController.m:972
frame #13: 0x00100ca0 DBDevApp`__39-[DBAPIController updatePatientGroups:]_block_invoke(.block_descriptor=0x02e68ce0) + 476 at DBAPIController.m:1198
frame #14: 0x2d4f6934 CoreData`developerSubmittedBlockToNSManagedobjectContextPerform   
frame #15: 0x3847e81e libdispatch.dylib`_dispatch_client_callout + 22
frame #16: 0x384847ca libdispatch.dylib`_dispatch_barrier_sync_f_invoke + 26
frame #17: 0x2d4f6a72 CoreData`-[NSManagedobjectContext performBlockAndWait:] + 106
frame #18: 0x00100a96 DBDevApp`-[DBAPIController updatePatientGroups:](self=0x17775de0,_cmd=0x0054dfcd,groupsArray=0x188eada0) + 214 at DBAPIController.m:1191
frame #19: 0x2d721584 CoreFoundation`__invoking___ + 68
frame #20: 0x2d66c0da CoreFoundation`-[NSInvocation invoke] + 282
frame #21: 0x2e0f3d2c Foundation`-[NSInvocationoperation main] + 112
frame #22: 0x2e0515aa Foundation`-[__NSOperationInternal _start:] + 770
frame #23: 0x2e0f576c Foundation`__NSOQSchedule_f + 60
frame #24: 0x38484f10 libdispatch.dylib`_dispatch_queue_drain$VARIANT$mp + 488
frame #25: 0x38484c96 libdispatch.dylib`_dispatch_queue_invoke$VARIANT$mp + 42
frame #26: 0x38485a44 libdispatch.dylib`_dispatch_root_queue_drain + 76
frame #27: 0x38485d28 libdispatch.dylib`_dispatch_worker_thread2 + 56
frame #28: 0x385c0bd2 libsystem_pthread.dylib`_pthread_wqthread + 298

我上面展示的代码是一个简化版本.我创建一个新子上下文的部分位于一个名为DBManagedobject的类中.这是整个堆栈的屏幕截图：

更新2 – 解释DBManagedobject

DBManagedobject是我所有核心数据类的基类.它基本上处理与JSON解析的字典之间的转换.它有3个主要方法：createWithAttributes：inManagedobjectContext：error：,– updateWithAttributes：error：和attributes.

> createWithAttributes：inManagedobjectContext：error ::创建提供的托管对象上下文的子上下文,在子上下文中插入一个新对象,并在该对象上调用updateWithAttributes：error：.如果更新成功(即,我们想要在此对象上设置的所有值都有意义),它将保存子上下文,获取对作为参数作为参数的MOC中的新对象的引用,并返回该引用：

NSManagedobjectContext* childContext = [[NSManagedobjectContext alloc] initWithConcurrencyType:nsprivateQueueConcurrencyType];
childContext.parentContext = context;
__block id newObject;
[childContext performBlockAndWait:^{
    newObject = [NSEntityDescription insertNewObjectForEntityForName:[self entityName] inManagedobjectContext:childContext];
}];

if ([newObject updateWithAttributes:attributes error:error])
{
    NSError* e = nil;
    if ([childContext save:&e])
    {
        id parentContextObject = [context objectWithID:[(NSManagedobject*)newObject objectID]];
        return parentContextObject;
    }
    else
    {
         if (error != NULL) {
            *error = e;
        }
        return nil;
    }
}
else
    return nil;

> updateWithAttributes：error ::大量将JSON键之间的键转换为我在数据模型中使用的键作为实体上的属性. (即’first_name’变为’firstName’).如果需要,它还格式化JSON值(日期字符串变为NSDates).它还建立了关系.