CoreData, child MOC on separate thread, unexpected: error: NULL _cd_rawData but the object is not being turned into a fault

Question

Ok, im a bit lost with this one, i am currently trying to run a background core data operation using a second ManagedObjectContext with its type set to NSPrivateQueueConcurrencyType and failing miserably with the above error.

I have a custom subclass of NSOperation, which is being passed an NSArray of strings, and the PersistentStoreCoordinator from the main thread, it then creates its own ManagedObjectContext, runs a query and performs and operation.

Here is the code from the class:

//
//  ProcessProfanity.m
//  Hashtag Live Desktop
//
//  Created by Gareth Jeanne on 24/03/2014.
//  Copyright (c) 2014 Gareth Jeanne. All rights reserved.
//

#import "ProcessProfanity.h"
#import "Tweet.h"

static const int ImportBatchSize = 250;

@interface ProcessProfanity ()
@property (nonatomic, copy) NSArray* badWords;
@property (nonatomic, strong) NSManagedObjectContext* backgroundContext;
@property (nonatomic, strong) NSPersistentStoreCoordinator* persistentStoreCoordinator;
@end

@implementation ProcessProfanity


{

}


- (id)initWithStore:(NSPersistentStoreCoordinator*)store badWords:(NSArray*)words
{
self = [super init];
if(self) {
    self.persistentStoreCoordinator = store;
    self.badWords = words;
}
return self;
}


- (void)main
{
_backgroundContext = [[NSManagedObjectContext alloc] initWithConcurrencyType:NSPrivateQueueConcurrencyType];
_backgroundContext.persistentStoreCoordinator = [self persistentStoreCoordinator];
_backgroundContext.undoManager = nil;
[_backgroundContext performBlockAndWait:^
{
    [self import];
}];
}

- (void)import
{

//Create new fetch request
NSFetchRequest *request = [[NSFetchRequest alloc] init];

//Setup the Request
[request setEntity:[NSEntityDescription entityForName:@"Tweet" inManagedObjectContext:self.backgroundContext]];

NSError *error = nil;

//Create an array from the returned objects
NSArray* tweetsToProcess = [self.backgroundContext executeFetchRequest:request error:&error];
NSAssert2(tweetsToProcess != nil && error == nil, @"Error fetching events: %@\n%@", [error localizedDescription], [error userInfo]);

for (Tweet* tweetToCheck in tweetsToProcess){
    __block NSString *result = nil;
    [self.badWords indexOfObjectWithOptions:NSEnumerationConcurrent
                                   passingTest:^(NSString *obj, NSUInteger idx, BOOL *stop)
     {
         if (tweetToCheck){
             if ([tweetToCheck.text rangeOfString:obj].location != NSNotFound)
             {
                 result = obj;
                 *stop = YES;
                 //return YES;
             }
         }
         return NO;
     }];

    if (!result){
        //DDLogVerbose(@"The post does not contain any of the words from the naughty list");
        if(tweetToCheck){
            tweetToCheck.profanity = [NSNumber numberWithBool:false];
        }
    }
    else{
        if(tweetToCheck){
            //DDLogVerbose(@"The string contains '%@' from the the naughty list", result);
            tweetToCheck.profanity = [NSNumber numberWithBool:true];
        }
    }

}
[self.backgroundContext save:NULL];
}

@end

And this is how i am calling it:

-(void)checkForProfanity{

if(!self.operationQueue){
self.operationQueue = [[NSOperationQueue alloc] init];
}

NSArray* termsToPass = [self.filterTerms copy];
ProcessProfanity* operation = [[ProcessProfanity alloc] initWithStore:self.persistentStoreCoordinator badWords:termsToPass];
[self.operationQueue addOperation:operation];


}

Edit 1

The specific line i seem to be getting the error on, or at least where Xcode is breaking is:

if ([tweetToCheck.text rangeOfString:obj].location != NSNotFound)

I have managed to narrow this down a bit, the NSArray that contains the list of terms to search the strings for is potentially quite large, possibly over a 1,000 NSStrings. If i test with an array of that size, i get the issue. However if i reduce the array to around 15 NSStrings, i do not get the error, so i don't think this is necessarily a thread related issue, i'm wondering if the array is getting released in the main thread. I have modified the code to make a deep copy, and then a __block copy as follows, but it doesn't seem to have helped.

self.badWords = [[NSArray alloc] initWithArray:words copyItems:YES];

and

for (Tweet* tweetToCheck in tweetsToProcess){
    __block NSArray *array = [[NSArray alloc] initWithArray:self.badWords copyItems:YES];
    __block NSString *result = nil;
    [array indexOfObjectWithOptions:NSEnumerationConcurrent

In fact, at the point where Xcode breaks, if i PO array, i get an object not found message, but if i po result, i correct get an object returned that is nil.

Edit 2

So i have made the following changes, with no change:

Made the NSArray strong rather than copy:

@property (nonatomic, strong) NSArray* badWords;

And made it a copy when allocated:

self.badWords = [[NSArray alloc] initWithArray:words copyItems:YES];

And created a local copy of the NSArray with the ___block declaration inside the actual method processing the objects:

__block NSArray *array = [[NSArray alloc] initWithArray:self.badWords copyItems:YES];

Which should surely mean it sticks around for the life of the ProcessProfanity object?

Am i wrong in expecting to be able to PO the array from the breakpoint within the block?

Answer 1

In this instance the error message "error: NULL _cd_rawData but the object is not being turned into a fault" indicates that you are accessing a managed object outside of its context. Basically your fetch returns all the Tweets from your persistent store as faults. Once you try and access a property on the Managed Object, Core Data will fire a fault and fetch the full object from the store.

By calling the NSArray method indexOfObjectWithOptions:passingTest: with an option of NSEnumerationConcurrent you are implying that you want to perform asynchronous execution on the elements in your array. The keyword concurrent indicates that multiple threads can be used to operate on the array elements.

In your context this means that accessing a managed object inside this block might result in accessing it on a different thread from the managed object context that owns the object. So when you access tweetToCheck.text in your conditional check - if ([tweetToCheck.text rangeOfString:obj].location != NSNotFound), under the hood Core Data is fetching that managed object from the persistent store and returning it to a thread that is not part of the managed object contexts thread.

Furthermore, it is not necessary to use the method indexOfObjectWithOptions:passingTest: since you are not actually interested in the result of this operation.

It seems to me that it might be more convenient for you to use an NSSet as you are only testing to see whether or not a given tweet word exists in your profane words. Quoting the documentation for NSSet: "You can use sets as an alternative to arrays when the order of elements isn’t important and performance in testing whether an object is contained in the set is a consideration". Clearly this seems to meet your criteria.

So your init would look like:

 -(id)initWithStore:(NSPersistentStoreCoordinator*)store 
           badWords:(NSSet*)badWords
{
   self = [super init];
   if(self) {
     self.persistentStoreCoordinator = store;
     self.badWords = [words copy];
   }
   return self;
}

Since you are only interested in updating tweets that have not yet been tagged for profanity you would probably only want to fetch tweets that haven't been flagged profane:

//Create new fetch request
NSFetchRequest *request = [[NSFetchRequest alloc] init];

//Setup the Request
[request setEntity:[NSEntityDescription entityForName:@"Tweet" inManagedObjectContext:self.backgroundContext]];
[request setPredicate:[NSPredicate predicateWithFormat:@"profanity = NO"]];

Now that you have an array of tweets that are not profane you could iterate through your tweets and check each word if it contains a profane word. The only thing you will need to deal with is how to separate your tweet into words (ignoring commas and exclamation marks etc). Then for each word you are going to need to strip it of diacritics and probably ignore the case. So you would end up with someone along the lines of:

if([self.badWords containsObject:badWordString]) {
    currentTweet.profanity = [NSNumber numberWithBOOL:YES];
}

Remember, you can run predicates on an NSSet so you could actually perform a case and diacritic insensitive query:

NSPredicate *searchPredicate = [NSPredicate predicateWithFormat:@"SELF = %@[cd]",wordToCheck];
BOOL foundABadWord = ([[[self.badWords filteredSetUsingPredicate:searchPredicate] allObjects] count] > 0);

Another thing you might want to consider is removing duplicate words in your tweets, you don't really want to perform the same check multiple times. So depending on how you find the performance you could place each word of your tweet into an NSSet and simply run the query on the unique words in your tweet:

if([[self.badWords intersectsSet:tweetDividedIntoWordsSet]) {
    //we have a profane tweet here!
}

Which implementation you choose is up to you but assuming you are only using english in your app you are definitely going to want to run a case and diacritic insensitive search.

EDIT

One final thing to note is that no matter how much you try, people will always be the best means of detecting profane or abusive language. I encourage you to read this SO's post on detecting profanity - How do you implement a good profanity filter?

Answer 2

Ok, so still not quite sure what was going on, but i followed Daniels advice and re-wrote the indexOfObjectWithOptions method and now it's working. For completeness, and so it hopefully helps someone else, this is what i ended up doing.

    DDLogInfo(@"Processing posts to check for bad language");
for (Tweet* tweetToCheck in tweetsToProcess){
    __block NSArray *array = [[NSArray alloc] initWithArray:self.badWords copyItems:YES];
    __block NSString *result = nil;

    NSRange tmprange;
    for(NSString *string in array) {
        tmprange = [tweetToCheck.text rangeOfString:[NSString stringWithFormat:@" %@ ", string]];
        if (tmprange.location != NSNotFound) {
            result = string;
            DDLogVerbose(@"Naughty Word Found: %@", string);
            break;
        }
    }

    if (!result){
        //DDLogVerbose(@"The post does not contain any of the words from the naughty list");
        if(tweetToCheck){
            tweetToCheck.profanity = [NSNumber numberWithBool:false];
        }
    }
    else{
        if(tweetToCheck){
            //DDLogVerbose(@"The string contains '%@' from the the naughty list", result);
            tweetToCheck.profanity = [NSNumber numberWithBool:true];
        }
    }