Crashlytics Crash with NSOperationQueue (QOS: UNSPECIFIED) - ios

Question

I'm getting a crash, reported in Crashlytics, and I have no idea how to reproduce the error, It happens randomly so it's hard to debug it with Xcode. Any ideas?
Crashed: NSOperationQueue 0x280419200 (QOS: UNSPECIFIED)
0 libobjc.A.dylib 0x22c471430 objc_retain + 16
1 CoreFoundation 0x22d2b5888 __CFBasicHashAddValue + 1480
2 CoreFoundation 0x22d1e64ac CFDictionarySetValue + 260
3 Foundation 0x22dd04888 _encodeObject + 732
4 myAPI 0x1062b44b0 -[DataCore encodeWithCoder:] (DataCore.m:236)
5 myAPI 0x1062909c4 -[DataHandle encodeWithCoder:] (DataHandle.m:53)
6 Foundation 0x22dd04aa8 _encodeObject + 1276
7 Foundation 0x22dc69c6c +[NSKeyedArchiver archivedDataWithRootObject:] + 168
8 myAPI 0x106288a34 __77+[CachableObject addObjectToCache:withCacheName:withTTL:withCompletionBlock:]_block_invoke (CachableObject.m:162)
9 Foundation 0x22dd198bc NSBLOCKOPERATION_IS_CALLING_OUT_TO_A_BLOCK + 16
10 Foundation 0x22dc21ab8 -[NSBlockOperation main] + 72
11 Foundation 0x22dc20f8c -[__NSOperationInternal _start:] + 740
12 Foundation 0x22dd1b790 __NSOQSchedule_f + 272
13 libdispatch.dylib 0x22ccc16c8 _dispatch_call_block_and_release + 24
14 libdispatch.dylib 0x22ccc2484 _dispatch_client_callout + 16
15 libdispatch.dylib 0x22cc6582c _dispatch_continuation_pop$VARIANT$mp + 412
16 libdispatch.dylib 0x22cc64ef4 _dispatch_async_redirect_invoke + 600
17 libdispatch.dylib 0x22cc71a18 _dispatch_root_queue_drain + 376
18 libdispatch.dylib 0x22cc722c0 _dispatch_worker_thread2 + 128
19 libsystem_pthread.dylib 0x22cea517c _pthread_wqthread + 472
20 libsystem_pthread.dylib 0x22cea7cec start_wqthread + 4
Here is what the code in DataCore.m looks like
- (void)encodeWithCoder:(NSCoder *)coder {
[super encodeWithCoder:coder];
[coder encodeObject:programFormatPlayInfo forKey:#"ProgramFormatPlayInfo"];
[coder encodeObject:bigScreenPlayInfo forKey:#"BigScreenPlayInfo"];
[coder encodeObject:pivotHandle forKey:#"PivotHandle"];
[coder encodeInteger:pivotDataLinkId forKey:#"PivotDataLinkId"];
[coder encodeInteger:viewContextId forKey:#"ViewContextId"];
[coder encodeBool:suppressImagePivot forKey:#"SuppressImagePivot"];
[coder encodeObject:attributeIds forKey:#"AttributeIds"];
[coder encodeObject:self.overflow forKey:#"Overflow"];
[coder encodeObject:self.cacheNameWithUser forKey:#"CacheNameWithUser"];
[coder encodeObject:self.metaData forKey:#"Metadata"];
}
and here is where I try to add an object to the cache,
not sure if it's failing to decode or something related to the background queue.
+ (void)addObjectToCache:(CachableObject*)object withCacheName:(NSString*)cacheName withTTL:(CacheTime)cacheTimeSeconds withCompletionBlock:(void(^)()) block {
CachableObject* theObject = object;
[_backgroundQueue addOperationWithBlock:^{
#autoreleasepool {
#try {
NSString * path = [CachableObject pathForCachedObject:cacheName];
NSDate * date = [NSDate date];
[object setCacheDate:date];
[object setTtlSeconds:[NSNumber numberWithInteger:cacheTimeSeconds]];
[object setApiVersion:APIVERSION];
// Add to NSCache
[[CachableObject objectCache] setObject:theObject forKey:cacheName];
// Add to file system
NSError* err = nil;
NSData * data = [NSKeyedArchiver archivedDataWithRootObject:theObject];
if (data) {
[data writeToFile:path options:NSDataWritingAtomic error:&err];
}
// Add to dynamic cache
unwrapObjectAndComplyWithClass(object, [DataHandle class], ^(id unwrappedObject) {
DataHandle *objectUnwrapped = unwrappedObject;
DataFrame *objectFrame = objectUnwrapped.frame;
for (NSString *eachDependencyName in objectFrame.dependencies) {
[[VVIDynamicCacheManager sharedManager]addDependencyToStore:eachDependencyName withCacheName:cacheName];
}
}, ^{
/*Not a data handle*/
});
} #catch (NSException* ex) {
NSLog(#"CachableObject: got an exception %#", ex);
} #finally {
if (block) {
block();
}
}
}
}];
}

Answer 1

Some thoughts here.
First, you're using #try/#catch. I believe that NSKeyedArchiver throws exceptions when it should actually return NSError objects. So, perhaps that's why you are doing it. But, you have to keep in mind that none of Apple's frameworks are guaranteed to be exception-safe. This means that catching exceptions can leave Apple's code (and, of course, your own) in inconsistent states that will prevent it from working correctly in the future.
I would strongly recommend either removing the #catch, or scoping it extremely tightly around the NSKeyedArchiver code, if that's why you are using it. This could be unintentionally introducing a variety of other bugs into your app.
Now, on to the specific crash. It's occurring in the runtime's memory management code. This points very strongly at heap corruption. This means you've got a pointer that doesn't point to a valid Objective-C object in memory. This can happen for many reasons, and is extremely common. The most common cause is known as a dangling pointer. However, it could also be caused by an over-release. And, I wouldn't be at all surprised if it is possible to use an #catch to trigger an over-release. (I know, I'm harping, but I've seen so many issues caused by this pattern)
What I generally recommend in these situations:
Look for other crashes that look heap-corruption-related
Try out Zombies in Instruments
Try malloc scribble, or guardmalloc, two other good memory debugging tools
It's hard, and often even impossible, to reason about heap corruption. Replicating the bug can also be impossible, as memory corruption is typically not deterministic.
So, just try to find and fix as many issues as you can. It's totally possible that one of them is responsible for a variety of crashes, one of which could be this one.