iOS10 Open Beta - iOS Core Data Crash on save - ios

Question

I am assuming it is ok to post beta iOS10 issues here, since it is an open beta. The apple forums don't come close to the help one gets on Stackoverflow.
Since the update to iOS10, our users are getting a constant crash when saving the managedObjectContext in CoreData.
This is something that has worked flawlessly since iOS 5.
It only happens on the 64 bit version of iOS10. The 32 bit version works fine.
Here is my simple save code snip:
[self.managedObjectContext performBlockAndWait:^{
NSError *error;
if (![self.managedObjectContext save:&error])
{
NSLog(#"\n\nerror in save 1 %#\n\n", error);
}
}];
[self.managedObjectContext performBlock:^{
NSError *error;
if (![_privateWriterContext save:&error])
{
NSLog(#"\n\nerror in save 2 %#\n\n", error);
}
}];
(Above NSLog reports nothing, as it works in simulator).
This only fails on a 64bit device. I cannot replicate on the simulator. I also cannot replicate in debug on a device due to issues with my only 64 bit device not connecting to Xcode correctly. So I am a bit stuck.
Below is a sample stack trace reported from a user:
Incident Identifier: 7A8DD23B-48A2-4ABF-88E7-67F1E6CDA8D5
CrashReporter Key: 5454e7c7b99a94cd75b6adfa8334bbdf10b859eb
Hardware Model: iPhone8,1
Code Type: ARM-64 (Native)
Role: Foreground
Parent Process: launchd [1]
Date/Time: 2016-07-20 22:23:00.1429 -0400
Launch Time: 2016-07-20 22:21:30.6058 -0400
OS Version: iPhone OS 10.0 (14A5309d)
Report Version: 104
Exception Type: EXC_BAD_ACCESS (SIGSEGV)
Exception Subtype: KERN_INVALID_ADDRESS at 0x0000000985e4beb8
Triggered by Thread: 0
Thread 0 name:
Thread 0 Crashed:
0 libobjc.A.dylib 0x00000001850e6eb0 objc_msgSend + 16
1 CoreFoundation 0x0000000185a76740 __CFNOTIFICATIONCENTER_IS_CALLING_OUT_TO_AN_OBSERVER__ + 20 (CFNotificationCenter.c:650)
2 CoreFoundation 0x0000000185a75e44 _CFXRegistrationPost + 400 (CFNotificationCenter.c:164)
3 CoreFoundation 0x0000000185a75bc0 ___CFXNotificationPost_block_invoke + 60 (CFNotificationCenter.c:1031)
4 CoreFoundation 0x0000000185ae40cc -[_CFXNotificationRegistrar find:object:observer:enumerator:] + 1504 (CFXNotificationRegistrar.m:163)
5 CoreFoundation 0x00000001859b9484 _CFXNotificationPost + 376 (CFNotificationCenter.c:1028)
6 Foundation 0x000000018644e9dc -[NSNotificationCenter postNotificationName:object:userInfo:] + 68 (NSNotification.m:482)
7 CoreData 0x0000000187e1dbf4 -[NSManagedObjectContext(_NSInternalNotificationHandling) _postContextDidSaveNotificationWithUserInfo:] + 880 (NSManagedObjectContext.m:7381)
8 CoreData 0x0000000187dabb4c -[NSManagedObjectContext(_NSInternalAdditions) _didSaveChanges] + 2156 (NSManagedObjectContext.m:5827)
9 CoreData 0x0000000187d98570 -[NSManagedObjectContext save:] + 3440 (NSManagedObjectContext.m:1532)
10 App 0x000000010008db34 __29-[RPSAppDelegate saveContext]_block_invoke227 + 52 (RPSAppDelegate.m:338)
11 CoreData 0x0000000187e17c68 developerSubmittedBlockToNSManagedObjectContextPerform + 168 (NSManagedObjectContext.m:3529)
12 libdispatch.dylib 0x0000000185525784 _dispatch_client_callout + 16 (object.m:455)
13 libdispatch.dylib 0x000000018552a330 _dispatch_main_queue_callback_4CF + 1000 (inline_internal.h:2421)
14 CoreFoundation 0x0000000185a8a0cc __CFRUNLOOP_IS_SERVICING_THE_MAIN_DISPATCH_QUEUE__ + 12 (CFRunLoop.c:1793)
15 CoreFoundation 0x0000000185a87cb8 __CFRunLoopRun + 1660 (CFRunLoop.c:3004)
16 CoreFoundation 0x00000001859b78d8 CFRunLoopRunSpecific + 444 (CFRunLoop.c:3113)
17 GraphicsServices 0x00000001873be198 GSEventRunModal + 180 (GSEvent.c:2245)
18 UIKit 0x000000018b9617c8 -[UIApplication _run] + 664 (UIApplication.m:2651)
19 UIKit 0x000000018b95c534 UIApplicationMain + 208 (UIApplication.m:4088)
20 App 0x000000010008c138 main + 152 (main.m:27)
21 libdyld.dylib 0x00000001855585b8 start + 4
Thank you for any ideas.

Answer 1

You are saving _privateWriterContext in a block call of managedObjectContext. This is not allowed. The fact that it worked before is a "false positive".
Instead, you should always only use at the same context that creates the block.