Exception / Crash When Merging CoreData Managed Contexts - iphone

Question

Im getting the following exception when attempting to merge a managed context (running on a background thread) with my main managed context (on mainthread). I cant seem to catch the exception in my own #try expression. Does anyone have any insight into this issue?
I'm using the default merge policy but im not sure this is correct - this issue is very intermittent - happens rarely but is causing my app to crash.
Exception Type: EXC_CRASH (SIGABRT)
Exception Codes: 0x00000000, 0x00000000
Crashed Thread: 0
Last Exception Backtrace:
0 CoreFoundation 0x37e3b8bf __exceptionPreprocess + 163
1 libobjc.A.dylib 0x319211e5 objc_exception_throw + 33
2 CoreData 0x344b7ea5 -[NSSQLiteStatement cachedSQLiteStatement] + 1
3 CoreData 0x344b774f -[NSSQLiteConnection prepareSQLStatement:] + 55
4 CoreData 0x3455b049 -[NSSQLChannel selectRowsWithCachedStatement:] + 61
5 CoreData 0x34586d63 newFetchedRowsForFetchPlan_MT + 783
6 CoreData 0x344bfb07 -[NSSQLCore newRowsForFetchPlan:] + 351
7 CoreData 0x34565011 -[NSSQLCore fetchRowForObjectID:] + 1005
8 CoreData 0x344d1a57 -[NSSQLCore newValuesForObjectWithID:withContext:error:] + 195
9 CoreData 0x344d0f83 _PFFaultHandlerLookupRow + 423
10 CoreData 0x3450e111 -[NSFaultHandler fulfillFault:withContext:] + 25
11 CoreData 0x34518999 -[NSManagedObject(_NSInternalMethods) _newPropertiesForRetainedTypes:andCopiedTypes:preserveFaults:] + 77
12 CoreData 0x345178ef -[NSManagedObject(_NSInternalMethods) _newAllPropertiesWithRelationshipFaultsIntact__] + 79
13 CoreData 0x345284db -[NSManagedObjectContext(_NSInternalChangeProcessing) _establishEventSnapshotsForObject:] + 47
14 CoreData 0x3452694b -[NSManagedObjectContext deleteObject:] + 155
15 CoreData 0x345238a1 -[NSManagedObjectContext _mergeChangesFromDidSaveDictionary:usingObjectIDs:] + 813
16 CoreData 0x34522c35 -[NSManagedObjectContext mergeChangesFromContextDidSaveNotification:] + 189
17 myapp 0x0008f0e9 0x8d000 + 8425
18 CoreFoundation 0x37d9a22b -[NSObject performSelector:withObject:] + 43
19 Foundation 0x31d75757 __NSThreadPerformPerform + 351
20 CoreFoundation 0x37e0fb03 __CFRUNLOOP_IS_CALLING_OUT_TO_A_SOURCE0_PERFORM_FUNCTION__ + 15
21 CoreFoundation 0x37e0f2cf __CFRunLoopDoSources0 + 215
22 CoreFoundation 0x37e0e075 __CFRunLoopRun + 653
23 CoreFoundation 0x37d914dd CFRunLoopRunSpecific + 301
24 CoreFoundation 0x37d913a5 CFRunLoopRunInMode + 105
25 GraphicsServices 0x3790ffcd GSEventRunModal + 157
26 UIKit 0x35221743 UIApplicationMain + 1091
I init the background context in start() of a nsoperation like so:
AppDelegate *appController = [[UIApplication sharedApplication] delegate];
_managedObjectContext = [[NSManagedObjectContext alloc] init];
[appController setPersistentStore:_managedObjectContext];
[[NSNotificationCenter defaultCenter] addObserver:self selector:#selector(receivedDeletedObjects:) name:NSManagedObjectContextDidSaveNotification object:_managedObjectContext];
I also set up a notification event which is called when objects are deleted on the background managed context, the callback then does:
-(void)receivedDeletedObjects:(NSNotification *)note
{
AppDelegate *appController = [[UIApplication sharedApplication] delegate];
NSManagedObjectContext *mainContext = [self managedObjectContext];
[mainContext performSelectorOnMainThread:#selector(mergeChangesFromContextDidSaveNotification:) withObject:note waitUntilDone:NO];
}
Thats pretty much the code. I have 4 different background threads each with its own managed context doing the same thing merging with the main context in this manner. Im wondering multiple threads are getting into mergeChangesFromContextDidSaveNotification at the same time but this shouldnt be the case as it is always called on the mainthread.

Answer 1

How about this:
(all in AppDelegate, call freshContextForBackgroundTask from a background Thread and use save: to trigger the merge.) Note syncObj is a plain NSObject instance needed for synchronization within the app delegate when using many background threads (or just have bad luck with scheduling)
-(NSManagedObjectContext*) freshContextForBackgroundTask {
#synchronized(syncObj) {
NSManagedObjectContext* r = [[NSManagedObjectContext alloc] init];
[r setPersistentStoreCoordinator:self.managedObjectContext.persistentStoreCoordinator];
[[NSNotificationCenter defaultCenter] addObserver:self
selector:#selector(backgroundContextDidSave:)
name:NSManagedObjectContextDidSaveNotification
object:r];
return r;
}
}
- (void)backgroundContextDidSave:(NSNotification *)notification {
/* Make sure we're on the main thread when updating the main context */
//NSLog(#"merging change: %#",notification);
dispatch_async(dispatch_get_main_queue(), ^{
NSManagedObjectContext *context = [self managedObjectContext];
// this for loop may not be needed for your purpose.
for(NSManagedObject *object in [[notification userInfo] objectForKey:NSUpdatedObjectsKey]) {
[[context objectWithID:[object objectID]] willAccessValueForKey:nil];
}
[self.managedObjectContext mergeChangesFromContextDidSaveNotification:notification];
});
}
Your approach seems strange. Especially when you set your app delegate's persistent store to the (nil) one of the freshly init'ed context.