[SOLVED] Unexpected EXC_BAD_ACCESS while setting weak property at BG global queue

Unexpected EXC_BAD_ACCESS while setting weak property at BG global queue

In my ChatMessage class I have the weak reference to Chat class

@interface ChatMessage : NSObject

@property (nonatomic, weak) Chat *chat;

I make the following initialization within Chat implementation on the global queue

ChatMessage *chatMessage = [[ChatMessage alloc] initWithDictionary:dictionary];
chatMessage.chat = self;

and get very strange error at the second line

Exception Type:  EXC_BAD_ACCESS (SIGSEGV)
Exception Subtype: KERN_INVALID_ADDRESS at 0x4c008be5
Triggered by Thread:  17

Thread 17 Crashed:
0   libobjc.A.dylib                 0x39f535d2 cache_getImp + 18
1   libobjc.A.dylib                 0x39f4e9a8 lookUpImpOrForward + 28
2   libobjc.A.dylib                 0x39f4e942 lookUpImpOrNil + 22
3   libobjc.A.dylib                 0x39f48aca class_getMethodImplementation + 30
4   libobjc.A.dylib                 0x39f5833a weak_register_no_lock + 38
5   libobjc.A.dylib                 0x39f586fa objc_storeWeak + 106
6   MyMessenger                     0x000d366c -[ChatMessage setChat:]
7   MyMessenger                 0x001173bc -[Chat getMessagesForPage:]
8   libdispatch.dylib               0x3a432d50 _dispatch_call_block_and_release + 8
9  libdispatch.dylib                0x3a438684 _dispatch_root_queue_drain + 224
10  libdispatch.dylib               0x3a4388d8 _dispatch_worker_thread2 + 52
11  libsystem_pthread.dylib         0x3a563c14 _pthread_wqthread + 296
12  libsystem_pthread.dylib         0x3a563ad8 start_wqthread + 4

Did someone come across such problems with setting weak properties?

Solution

It appears that the problem is in simultaneous access to this weak property from different threads.

Weakness of the property doesn't allow us to neglect multithread assigning.

The safe rule to work with datasources is to change it synchronously within one selected thread.