NSOperation crash on isCancelled - objective-c

Question

I have implemented a concurrent nsoperation and have ARC enabled. Now my customer is experiencing a crash which I cannot reproduce. He sent me the follow crash log :
Date/Time: 2013-04-24 12:23:34.925 -0400
OS Version: Mac OS X 10.8.3 (12D78)
Report Version: 10
Interval Since Last Report: 30946 sec
Crashes Since Last Report: 1
Per-App Interval Since Last Report: 33196 sec
Per-App Crashes Since Last Report: 1
Anonymous UUID: FB8460EE-5199-C6FB-55DC-F927D7F81A80
Crashed Thread: 15 Dispatch queue: com.apple.root.default-priority
Exception Type: EXC_BAD_ACCESS (SIGSEGV)
Exception Codes: EXC_I386_GPFLT
Application Specific Information:
objc_msgSend() selector name: isCancelled
Thread 15 Crashed:: Dispatch queue: com.apple.root.default-priority
0 libobjc.A.dylib 0x00007fff877f1250 objc_msgSend + 16
1 Myapp 0x000000010a608807 0x10a601000 + 30727
2 Myapp 0x000000010a650575 0x10a601000 + 324981
3 com.apple.Foundation 0x00007fff8b66212f -[NSBlockOperation main] + 124
4 com.apple.Foundation 0x00007fff8b638036 -[__NSOperationInternal start] + 684
5 com.apple.Foundation 0x00007fff8b63f861 __block_global_6 + 129
6 libdispatch.dylib 0x00007fff832d0f01 _dispatch_call_block_and_release + 15
7 libdispatch.dylib 0x00007fff832cd0b6 _dispatch_client_callout + 8
8 libdispatch.dylib 0x00007fff832ce1fa _dispatch_worker_thread2 + 304
9 libsystem_c.dylib 0x00007fff87d19d0b _pthread_wqthread + 404
10 libsystem_c.dylib 0x00007fff87d041d1 start_wqthread + 13
My code looks like this:
-(void)start
{
// Always check for cancellation before launching the task.
if ([self isCancelled])
{
// Must move the operation to the finished state if it is canceled.
[self onCancelSyncOperation];
return;
}
// If the operation is not canceled, begin executing the task.
[self willChangeValueForKey:#"isExecuting"];
[NSThread detachNewThreadSelector:#selector(main) toTarget:self withObject:nil];
executing = YES;
[self didChangeValueForKey:#"isExecuting"];
}
- (void)onCancelSyncOperation
{
[self willChangeValueForKey:#"isFinished"];
[self willChangeValueForKey:#"isExecuting"];
executing = NO;
finished = YES;
[self didChangeValueForKey:#"isExecuting"];
[self didChangeValueForKey:#"isFinished"];
}
It seems like the nsoperation is already released? when it tries to check for isCancelled?
Is this possible?

Answer 1

I don't think anyone can tell you why your app crashes by looking at this log. I haven't looked at your code, but you cut your crash-log, so it only shows system modules (lib.dispatch..., com.apple...). Typically the error is in the first occurrence of "com.myname...".
If this kind of crash EXC_BAD_ACCESS (SIGSEGV) appears along with an objc_msgSend(), it probably means you're trying to message to an object (or in other words: call a method of an object) that isn't there anymore. If you call that object, chances are very good that you'll find it, if you call it delayed or on another Thread or from a block, it will be a bit more complicated.
Your best chance to find the cause of this is to inspect your app with Instrument, using the Allocations or Leaks tool with NSZombies enabled (which is the default). You can launch Instruments from within Xcode. Then try to reproduce your crash. If you succeed, you might be able to find the class and location where the crash occurs.
If you don't know what to do with this answer, then check out the WWDC Developer Videos from Apple, there are some that show how to profile your app with Instruments (you'll need a [free] Dev Account to access the videos).
Good luck!