Core Data concurrency issue and how to fix

Question

I use multiple contexts in my Core Data app, and have recently had some core data concurrency crashes. I have added -com.apple.CoreData.ConcurrencyDebug 1 to help track these down, but I am not understanding how to fix the issue that is shown.

Here is what I am doing:

- (void)getEvents:(void (^)(NSArray *fetchedItems))completionBlock {

    // Initialize Fetch Request
    NSFetchRequest *request = [[NSFetchRequest alloc] initWithEntityName:@"ZSSCDEvent"];

    // Initialize Asynchronous Fetch Request
    NSAsynchronousFetchRequest *asynchronousFetchRequest = [[NSAsynchronousFetchRequest alloc] initWithFetchRequest:request completionBlock:^(NSAsynchronousFetchResult *result) {

        dispatch_async(dispatch_get_main_queue(), ^{

            // Dismiss Progress HUD
            [SVProgressHUD dismiss];

            // Process Asynchronous Fetch Result
            if (result.finalResult) {

                NSArray *results = result.finalResult;

                completionBlock(results);

                // Reload Table View
                [self.activityIndicator stopAnimating];
                [self.tableViewList reloadData];

            }

        });
    }];

    // Execute Asynchronous Fetch Request
    [self.managedObjectContext performBlock:^{

        // Execute Asynchronous Fetch Request
        NSError *asynchronousFetchRequestError = nil;
        NSAsynchronousFetchResult *asynchronousFetchResult = (NSAsynchronousFetchResult *)[self.managedObjectContext executeRequest:asynchronousFetchRequest error:&asynchronousFetchRequestError];

        if (asynchronousFetchRequestError) {
            NSLog(@"Unable to execute asynchronous fetch result.");
            NSLog(@"%@, %@", asynchronousFetchRequestError, asynchronousFetchRequestError.localizedDescription);
        }

    }];

}

This gives me a Enqueued from com.apple.main-thread (Thread 1) error. This is where I am confused, since I am running this on the main thread and didn't think I needed to use my private context here.

Any ideas on why I am getting a concurrency issue here?

EDIT: It looks like someone else had the exact issue and thinks it is an Xcode bug: CoreData asynchronous fetch causes concurrency debugger error

Answer 1

Every managedObject has a context. Each context has one and only one thread that it can run on. ManagedObjects are NOT thread safe - not even for reading. Passing managedObjects around with completions blocks is a bad practice. It can be hard to figure out which managedObjects are supposed to be on which thread. Also, even if you are passing it around on the correct thread it is still a bad practice. When you do a dispatch_async the entity may have deleted from the database in the interim and accessing the managedObject will cause a crash. A good practice is that any method that does a fetch should be explicitly told which context to use and return synchronously. In your code the method is using self.managedObjectContext, but I have no way to know looking at you code what thread that is related to. Furthermore the pointer to the context may change and that can cause bugs that are very hard to track down.

NSAsynchronousFetchResult contains managedObjects, so is not thread safe and can only be used inside that completion block (which is running on the correct thread for the objects). You cannot pass them to another thread. If you return them in a block then that code must also not pass them to another thread. You should just do whatever you need to do with them inside the block and then discard them.

If you need to display the information to the user, then generally it is better to just do the fetch on the main thread synchronously and get managedObjects that are associated with a main thread context. If your fetch is taking to long then you should fix that - there is no reason for a fetch to take so long. In your case you appear to be fetching ALL the items in you database. That is a mistake. You should use a predicate to only get the ones that you need.

Another possibility is to copy the values of managedObjects into a thread safe object (a dictionary or a NSObject subclass).

TL;DR You probably don't need a NSAsynchronousFetchRequest. Use a regular fetch request on the main thread and return synchronously. Use a predicate to limit the results only to the objects you are actually displaying.