Merging Changes between NSManagedObjectContexts (Multithreaded)

Question

I have a problem/crash merging the data of different NSManagedObjectContexts (iOS 6.1, Xcode 4.6).

Most of the time the error that rises is the following:

CoreData: error: Serious application error. Exception was caught during Core Data change processing. This is usually a bug within an observer of NSManagedObjectContextObjectsDidChangeNotification. statement is still active with userInfo (null)

One time i got this error: An observer of NSManagedObjectContextDidSaveNotification illegally threw an exception. Objects saved = { $OBJLIST } and exception = statement is still active with userInfo = (null)

Sadly there is no value in the stacktraces i got. They just show symbols that are CoreData internal (if any).

Our CoreData stack:

1 NSPersistentStoreLocator shared by all threads
1 unique MOC per thread ( created on first need )
- All MOCs are saved in a Dictionary

An observer is added for the notification NSManagedObjectContextDidSaveNotification to update the MOCs when one is saving to the store. The defined selector is calling mergeChangesFromContextDidSaveNotification on every other thread/context except the one that did the save operation.

+ (void)mergeChanges:(NSNotification *)notification {
    NSManagedObjectContext *ctx;
    for ( NSNumber *threadId in [__managedObjectContexts keyEnumerator] ) {
        ctx = [__managedObjectContexts objectForKey:threadId];
        if ( notification.object != ctx ) {
            [ctx mergeChangesFromContextDidSaveNotification:notification];
        }
    }
}

Steps to produce the error:

In a background thread CoreData data objects that arent needed anymore (unreferenced by other objects) are being deleted.

[[CDUtils managedObjectContext] deleteObject:obj];
[[CDUtils managedObjectContext] save:&error];

While this is happening the user can navigate throughout the application. Userinteraction (i.e. opening a tableview) can trigger executeFetch calls on the moc of the main thread. Every thread uses the same NSPersistentStoreLocator but a different/unique MOC.

We tried different methods of locking with NSLocks and the lock on the NSPersistentStoreLocator for threadsafety. I.e. enclosing the mergeChanges Method and the save operation each by a lock/unlock or enclosing both methods in the same lock/unlock. Sadly we had no succes thus far.

[__storeCoordinator lock];
[__storeCoordinator unlock];

I'd be thankful for every piece of advice you can give me to approach a solution. Thank you for your time!

Are multiple contexts using the same persistant store coordinator? Only one context should be linked to the persistent store. You can create child contexts to process and import data, but all of the save operations should merge upwards to save from a single persistant store. — mrosales
Yes all MOCs use the same PSC (should normally not be a problem according to developer.apple.com/library/mac/#documentation/Cocoa/Conceptual/…). Do we really need child contexts? — frested
For the people interested. I managed to make things work with multiple threads / MOCs. I basicly solved the original problem / those errors i had by locking PSC and MOCs correctly. The next problem that arose was how to know if its save to mergeChanges on a context. I cant lock nor should mergeChanges on a MOC that has no running thread anymore. But how do i know if the thread is running or not? If i just check for NSThreads "isExecuting"-Method it might happen that the thread exits just after i checked the BOOL. Im trying an easier approach now where i just merge into the main thread. — frested

frested frested · Accepted Answer · 2013-07-05T15:46:47

For the people interested. I managed to make things work with multiple threads / MOCs. I basicly solved the original problem / those errors i had by locking PSC and MOCs correctly. The next problem that arose was how to know if its save to mergeChanges on a context. I cant lock nor should mergeChanges on a MOC that has no running thread anymore. But how do i know if the thread is running or not? If i just check for NSThreads "isExecuting"-Method it might happen that the thread exits just after i checked the BOOL. Im trying an easier approach now where i just merge into the main thread.

Merging Changes between NSManagedObjectContexts (Multithreaded)

Our CoreData stack:

Steps to produce the error:

1 Answers