CoreData asynchronous fetch causes concurrency debugger error

Question

I'm using the

-com.apple.CoreData.ConcurrencyDebug

argument on launch to debug concurrency in my CoreData app.

During app launch, I perform an asynchronous fetch on the main thread's managed object context.

// set up the async request
   NSError * error = nil;
        [MOC executeRequest:asyncFetch error:&error];
        if (error) {
            NSLog(@"Unable to execute fetch request.");
            NSLog(@"%@, %@", error, error.localizedDescription);
        }

This code is called from the main thread, but executeRequest: enqueues it to another thread, which I understand to be the correct behavior.

The concurrency debugger doesn't like this, saying (I reckon) that I'm doing something wrong here. I've also tried wrapping this in [MOC performBlock:] which also works, but also causes a multithreading violation. In both cases I get this :

[NSManagedObjectContext __Multithreading_Violation_AllThatIsLeftToUsIsHonor__

Am I using async fetches incorrectly, or is the concurrency debugger wrong here?

EDIT : I've also tried wrapping it in MOC performBlock which should ensure that it gets called from the main thread. In any case, the call is enqueued from the main thread, but executed elsewhere.

EDIT : here's the fetch request:

 NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] initWithEntityName:@"MyEntity"];

 NSPredicate * pred = [NSPredicate predicateWithFormat:@"boolProperty == YES"];
    fetchRequest.predicate = pred;
 NSSortDescriptor * sort = [NSSortDescriptor sortDescriptorWithKey:@"name" ascending:YES];
 fetchRequest.sortDescriptors = @[sort];      fetchRequest.propertiesToFetch = @[@"prop1", @"prop2", @"prop3", @"prop4"];

 NSPersistentStoreAsynchronousFetchResultCompletionBlock resultBlock = ^(NSAsynchronousFetchResult *result) {
        dispatch_async(dispatch_get_main_queue(), ^{
            [[NSNotificationCenter defaultCenter] postNotificationName:kFetchCompleteNotification object:result];
        });
    };

 NSAsynchronousFetchRequest *asyncFetch = [[NSAsynchronousFetchRequest alloc]
                                              initWithFetchRequest:fetchRequest
                                              completionBlock:resultBlock];

Then I receive the results from the notification:

- (void)fetchCompletedNote:(NSNotification *)note {
    NSAsynchronousFetchResult * result = note.object;
    if (![cachedResults isEqualToArray:result.finalResult]){
        cacheResults = result.finalResult;
        [self.collectionView reloadData];
    }
}

Answer 1

I think this is Apple's bug.

I filed bug report: https://openradar.appspot.com/30692722

and added example project that reproduces issue: https://github.com/jcavar/examples/tree/master/TestAsyncFetchCoreData

Also, if you don't want to disable flag just because of this issue you may want to swizzle __Multithreading_Violation_AllThatIsLeftToUsIsHonor__ method on NSManagedObjectContext just for this part of code. You need to revert that after request is executed so you get violations for real issues.

Answer 2

The concurrency debugger is telling you that you are accessing MOC from the wrong thread/queue. You can only call -executeRequest: error: on the thread/queue that the context belongs to. If this is a NSMainQueueConcurrencyType then you need to be on the main thread. Otherwise, if it is a NSPrivateQueueConcurrencyType you need to use either -performBlock: or -performBlockAndWait: to run the execute on the correct queue.

I added a screenshot, see above. The request is enqueued from the main thread, but executed on another.

Ok, a couple of things:

1. Is that the line that is breaking/crashing or are you seeing the error output?
2. Your error handling is incorrect. You should be looking at the result of the -executeRequest: error: and if the result is nil then you are in an error state. The error variable can be populated even with a success.

I note that the code you posted in your screenshot is different than the code you posted previously. Did you add the -performBlock: or just not include it originally?