CoreData: Crash purging caches after a Memory warning

Question

I've got a stack trace showing a crash responding to a memory warning:

Thu Oct 28 00:42:55 iPhone DTMobileIS[10853] : _memoryNotification : {name = (null), num = 1}


Thu Oct 28 00:42:55 iPhone DTMobileIS[10853] : _memoryNotification : {
        OSMemoryNotificationLevel = 1;
        timestamp = "2010-10-28 07:42:55 GMT";
    }


Thu Oct 28 00:42:55 iPhone MyApp[11059] : Received memory warning. Level=1

Thu Oct 28 00:42:55 iPhone MyApp[11059] : -[__NSCFType tryLock]: unrecognized selector sent to instance 0x41bfd0

Thu Oct 28 00:42:55 iPhone MyApp[11059] : *** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '-[__NSCFType tryLock]: unrecognized selector sent to instance 0x41bfd0'

*** Call stack at first throw:
(
    0   CoreFoundation                      0x30897ed3 __exceptionPreprocess + 114
    1   libobjc.A.dylib                     0x3002f811 objc_exception_throw + 24
    2   CoreFoundation                      0x30899683 -[NSObject(NSObject) doesNotRecognizeSelector:] + 102
    3   CoreFoundation                      0x308411d9 ___forwarding___ + 508
    4   CoreFoundation                      0x30840f90 _CF_forwarding_prep_0 + 48
    5   CoreData                            0x316cc261 -[_NSSQLCoreConnectionObsever _purgeCaches:] + 124
    6   Foundation                          0x349bb6b3 _nsnote_callback + 142
    7   CoreFoundation                      0x3081e713 __CFXNotificationPost_old + 402
    8   CoreFoundation                      0x3081e3b3 _CFXNotificationPostNotification + 118
    9   Foundation                          0x349aadb7 -[NSNotificationCenter postNotificationName:object:userInfo:] + 70
    10  Foundation                          0x349b42d1 -[NSNotificationCenter postNotificationName:object:] + 24
    11  UIKit                               0x31ed9431 -[UIApplication _performMemoryWarning] + 48
    12  UIKit                               0x31ec7383 -[UIApplication _receivedMemoryNotification] + 126
    13  UIKit                               0x31ec72c7 _memoryStatusChanged + 42
    14  CoreFoundation                      0x3084e825 __CFNotificationCenterDarwinCallBack + 24
    15  CoreFoundation                      0x30835313 __CFMachPortPerform + 210
    16  CoreFoundation                      0x3082cce3 __CFRUNLOOP_IS_CALLING_OUT_TO_A_SOURCE1_PERFORM_FUNCTION__ + 26
    17  CoreFoundation                      0x3082cca7 __CFRunLoopDoSource1 + 166
    18  CoreFoundation                      0x3081f56d __CFRunLoopRun + 520
    19  CoreFoundation                      0x3081f277 CFRunLoopRunSpecific + 230
    20  CoreFoundation                      0x3081f17f CFRunLoopRunInMode + 58
    21  GraphicsServices                    0x31e445f3 GSEventRunModal + 114
    22  GraphicsServices                    0x31e4469f GSEventRun + 62
    23  UIKit                               0x31e51123 -[UIApplication _run] + 402
    24  UIKit                               0x31e4f12f UIApplicationMain + 670
    25  MyApp                            0x00052c81 main + 72
    26  MyApp                            0x00002f18 start + 52
)

I'm not really sure what core data was doing that it tried to obtain a lock from this object, however I feel like this is indicative of a threading issue somewhere inside my code.

Can anyone think of a good way to debug this or possible root causes?

Answer 1

I actually found, after a bit of code analysis, the root cause of this appears to be crossing threads with CoreData. I had a background thread from an operation queue manipulating a NSManagedObjectContext which originated on the main thread (and caused a save! ouch!).

After a few of these manipulations if I sent a memory warning in the simulator the app would crash.