Using GCD and Core Data causes crash - objective-c

Question

I retrieve data back from our server and I need to process it.
For each key, I create a NSManagedObject. Each object is created in the same context. I am using Magical Record.
-(id)init {
if (self = [super init]){
self.context = [NSManagedObjectContext MR_contextForCurrentThread];
}
return self;
}
Threading:
dispatch_queue_t queue = dispatch_get_global_queue(0, 0);
dispatch_group_t group = dispatch_group_create();
for (id key in boundariesDictionary) {
dispatch_group_async(group, queue, ^{
DLog(#"nsthread: %#", [NSThread currentThread]);
NSString *boundaryIDString;
if ([key isKindOfClass:[NSString class]]) {
boundaryIDString = key;
}
else if ([key isKindOfClass:[NSNumber class]]) {
boundaryIDString = [key stringValue];
}
if (boundaryIDString) {
DLog(#"boundaryIDString: %#", boundaryIDString)
NSDictionary *boundaryDictionary = [boundariesDictionary objectForKey:key];
Boundary *boundary = [Boundary MR_findFirstWithPredicate:[NSPredicate predicateWithFormat:#"boundaryID == %# AND api == %#", [NSNumber numberWithInteger:[boundaryIDString integerValue]], self.serverCall.API] inContext:self.context];
if ([boundaryDictionary objectForKey:AVI_NAME]) {
if (boundary == nil) {
DLog(#"creating boundary %#", boundaryIDString);
boundary = [Boundary MR_createInContext:self.context];
boundary.boundaryID = [NSNumber numberWithInteger:[boundaryIDString integerValue]];
}
}
boundary = [self processBoundary:boundary fromBoundaryDictionary:boundaryDictionary];
}
}
}
[self processBoundary] just takes the dictionary and sets it to the managed object's attributes.
if ([boundaryDictionary objectForKey:#"name"]) {
boundary.name = [boundaryDictionary objectForKey:#"name"];
}
//more data processing
This is causing an error though:
*** Terminating app due to uncaught exception 'NSGenericException', reason: '*** Collection <__NSCFSet: 0x1776f7f0> was mutated while being enumerated.'
It runs fine if I don't use the same context for each thread.
I don't understand what set other then the NSDictionary boundariesDictionary that i'm enumerating through. I am not mutating boundariesDictionary at all, only copying the data into core data.
When I PO the object (0x1776f7f0 in this case), I get a list of Boundary objects in a set. Those Boundary objects would only exist in the NSManagedObjectContext "set", I don't add them to an NSArray, NSDictionary, or NSSet. But I don't believe I enumerate over that set. I do mutate it by creating new boundary objects to it.
I think there is something going on that I don't understand or quite grasp yet.
Any ideas?
UPDATE:
for (id key in boundariesDictionary) {
NSString *boundaryIDString;
if ([key isKindOfClass:[NSString class]]) {
boundaryIDString = key;
}
else if ([key isKindOfClass:[NSNumber class]]) {
boundaryIDString = [key stringValue];
}
if (boundaryIDString) {
[MagicalRecord saveWithBlock:^(NSManagedObjectContext *localContext) {
DLog(#"saveWithBlock thread: %#", [NSThread currentThread]);
NSDictionary *boundaryDictionary = [boundariesDictionary objectForKey:key];
Boundary *boundary = [Boundary MR_findFirstWithPredicate:[NSPredicate predicateWithFormat:#"boundaryID == %# AND api == %#", [NSNumber numberWithInteger:[boundaryIDString integerValue]], self.serverCall.API] inContext:localContext];
if ([boundaryDictionary objectForKey:AVI_NAME]) {
if (boundary == nil) {
boundary = [Boundary MR_createInContext:localContext];
boundary.boundaryID = [NSNumber numberWithInteger:[boundaryIDString integerValue]];
}
boundary = [self processBoundary:boundary fromBoundaryDictionary:boundaryDictionary];
if (boundary == nil) {
//Prompt Error
}
else {
for (NSNumber *groupID in groupIDs) {
if ([groupID isKindOfClass:[NSNumber class]]) {
Group *group = [Group MR_findFirstWithPredicate:[NSPredicate predicateWithFormat:#"groupID == %# OR groupID == 0 AND api == %#", groupID, self.serverCall.API]];
if (group != nil) {
group.lastUpdated = [NSDate date];
[group addBoundariesObject:boundary];
}
else {
DLog(#"group %# DNE", groupID);
}
}
}
}
}
} completion:^(BOOL success, NSError *error) {
DLog(#"saveWithBlock completion Block | time: %f", [[NSDate date] timeIntervalSinceDate:startTime]);
}];
}
}
So for my Group 29 should see all the boundaries I'm creating, but its not. Its inconsistent. Sometimes sees all, sometimes some, and sometimes none.
Also, I often see
NO CHANGES IN ** BACKGROUND SAVING (ROOT) ** CONTEXT - NOT SAVING
in the log. Its also inconsistent on how many of these messages I see, while the context that do save will insert more then 1 object.
Not sure if that is how it should be behaving, seems like it should be a 1-to-1 ratio if each block has its own context and each block only creates 1 object.
I log each thread ID, and it is creating a new thread for each block. No thread ID is being logged twice, so the threads shouldn't be being reused.

Answer 1

Managed object contexts are not thread safe, you must not use them or any managed objects/sets/data structures/whatever they might return from different queues or threads.
I don't know if magical record supports it, but you should definitely be using queue containment for your contexts, and then you have to do everything within the context of the MOC's private queue.
If you use thread containment, you must guarantee that the context and any managed objects created by the context are always serially accessed, which you're absolutely not doing in the code above.

Answer 2

You're issue is that gcd queues a threads are not a one to one mapping. GCD reuses threads, and thus your are likely crossing thread boundaries unknowingly here. My suggestion is to simply create a new context and stop using contextForCurrentThread. I wrote more details about the issues on my blog. ContextForCurrentThread will be deleted in an upcoming release.