NSManagedObjectContext executeFetchRequest:error: throws exception: OS_xpc_error

Question

I am experiencing an unusual error in my iOS app.

My app downloads a large number of JSON objects which are then inserted into Core Data. Each download takes the following steps:

1. Open network connection and get JSON object
2. One main NSManagedObject is created
3. Any number between 0 and around 20 additional NSManagedObjects are created
4. For some, a fetch request for existing NAManagedObjects is performed to find existing matching objects, and a relationship is created, or a new object is created if one isn't found.

Here is the relevant code snippet, where song is an instance of an NSManagedObject subclass, and Category is a different NSManagedObject subclass:

for (NSString *category in dict[@"categories"]) {
    NSError *error;
    NSFetchRequest *fetchRequest = [NSFetchRequest fetchRequestWithEntityName:@"Category"];
    fetchRequest.predicate = [NSPredicate predicateWithFormat:@"name like %@", category];
    NSArray *fetchResults = [_moc executeFetchRequest:fetchRequest error:&error];

    Category *c;

    if (fetchResults.count > 0) {
        c = fetchResults[0];
    } else {
        c = [NSEntityDescription insertNewObjectForEntityForName:@"Category" inManagedObjectContext:_moc];
        c.name = category;
    }
    [song addCategoriesObject:c];
}

This works fine, until after a few hundred download-and-inserts, the app will crash. The number of successful downloads will vary, seemingly at random.

The line causing the exception is NSArray *fetchResults = [_moc executeFetchRequest:fetchRequest error:&error]; and the log output is this line:

2014-11-17 10:05:54.265 MyApp[3211:1774124] Communications error: <OS_xpc_error: <error: 0x19ce62a80> { count = 1, contents =
    "XPCErrorDescription" => <string: 0x19ce62e78> { length = 22, contents = "Connection interrupted" }
}>
2014-11-17 10:06:03.631 MyApp[3211:1773135] *** Terminating app due to uncaught exception 'NSGenericException', reason: '*** Collection <__NSCFSet: 0x174075ac0> was mutated while being enumerated.'
*** First throw call stack:
(0x188111e48 0x198b140e4 0x1881117fc 0x187dc9cf8 0x1000965d8 0x100097de0 0x1003a4e30 0x1003a4df0 0x1003af854 0x1003a8120 0x1003b175c 0x1003b2f18 0x1993352e4 0x199334fa8)
libc++abi.dylib: terminating with uncaught exception of type NSException

My attempts to Google this error have proved fruitless. (Seriously - I've never searched for an error and got zero results:)

Has anyone else experienced a similar error and can point me in the right direction? For once I am at a complete loss as to where to even begin with my search here.

Answer 1

OK it looks like the crash and the OS_xpc_error are completely unrelated.

The OS_xpc_error appeared before the debugger hit the exception breakpoint. I didn't realise though that the exception message wasn't printed until after I hit continue in the debugger. The relevant line was about mutating __NSCFSet, and the problem was due to accessing my NSManagedObjectContext on a background thread.

Creating a new background NSManagedObjectContext, per this question, has solved my issue.