Core Data EXC_BAD_ACCESS KERN_PROTECTION_FAILURE - ios

Question

I have multiple crash reports that point to the following:
Thread : Crashed: NSOperationQueue 0x18c7fba0
0 libsystem_platform.dylib 0x38665a36 OSAtomicCompareAndSwap32Barrier + 13
1 libobjc.A.dylib 0x3805694b realizeClass(objc_class*) + 78
2 libobjc.A.dylib 0x38058797 lookUpImpOrForward + 74
3 libobjc.A.dylib 0x3805102b _class_lookupMethodAndLoadCache3 + 34
4 libobjc.A.dylib 0x38050df9 _objc_msgSend_uncached + 24
5 CoreData 0x2da7b5bb -[_NSFaultingMutableSet copyWithZone:] + 238
6 MyApp 0x0027226f -[Zoo getSortedCats] (Zoo.m:63)
7 MyApp 0x00286955 -[BlockExecutionOperation main] (BlockExecutionOperation.m:30)
8 Foundation 0x2e627aa5 -[__NSOperationInternal _start:] + 772
9 Foundation 0x2e6cb96d __NSOQSchedule_f + 60
10 libdispatch.dylib 0x3853e4b7 _dispatch_async_redirect_invoke + 110
11 libdispatch.dylib 0x3853f7d9 _dispatch_root_queue_drain + 224
12 libdispatch.dylib 0x3853f9c5 _dispatch_worker_thread2 + 56
13 libsystem_pthread.dylib 0x38669dff _pthread_wqthread + 298
The getSortedCats method looks like this:
- (NSArray *)getSortedCats {
NSSet* cats = [self.cats copy]; //this is line 63, where the crash occurs
//do some sorting
return sortedCats;
}
And Zoo is an NSManagedObject subclass with a cats property:
#property (atomic, retain) NSSet *cats;
So why the crash on the self.cats line? What does this error mean? How to avoid? It only happens once in a while, and is not reproducible.

Answer 1

Peculiarity with Core Data. Sending copy would just return another NSFaultingMutableSet, and Core Data is too buggy to do that right.
Replace it with [NSSet setWithSet:self.cats] and you'd face lesser errors.