Is this crash in libdispatch caused by Reachability? - ios7

Question

Can anyone tell me if this crashing stack appears to be caused by Reachability? I am running apple's latest implementation of Reachability, downloadable at https://developer.apple.com/library/ios/samplecode/reachability/Introduction/Intro.html
I have an app where ARC is not enabled by default (it is a legacy app) but I enable it on a per-file basis. I have enabled -fobjc-arc for Reachability.m
When I look at these threads, my code is nowhere to be found. I see some activity happening because of a WebHTMLView (probably the ad network view code currently on screen; it's not mine).
On Thread 10, I see a SCNetworkReachabilityDeallocate, then a dispatch_semaphore_wait_slow, and the crash happens on Thread 14 in libdispatch.
Do you think this crash is being caused by that Reachability code, and have I erred in using -fobjc-arc with Reachability.m?
Exception Type: EXC_BAD_ACCESS (SIGSEGV)
Exception Subtype: KERN_INVALID_ADDRESS at 0x2000000c
Triggered by Thread: 14
Thread 0:
0 libsystem_kernel.dylib 0x39cafa84 mach_msg_trap + 20
1 libsystem_kernel.dylib 0x39caf87c mach_msg + 36
2 CoreFoundation 0x2f3e255c __CFRunLoopServiceMachPort + 152
3 CoreFoundation 0x2f3e0cc2 __CFRunLoopRun + 858
4 CoreFoundation 0x2f34b53c CFRunLoopRunSpecific + 520
5 CoreFoundation 0x2f34b31e CFRunLoopRunInMode + 102
6 GraphicsServices 0x340822e6 GSEventRunModal + 134
7 UIKit 0x31c021e0 UIApplicationMain + 1132
8 MyApp 0x00053ad4 main (main.m:33)
9 libdyld.dylib 0x39c0bab4 start + 0
Thread 1:
0 libsystem_kernel.dylib 0x39caf838 kevent64 + 24
1 libdispatch.dylib 0x39bfe0d0 _dispatch_mgr_invoke + 228
2 libdispatch.dylib 0x39bf863e _dispatch_mgr_thread + 34
Thread 2 name: com.apple.NSURLConnectionLoader
Thread 2:
0 libsystem_kernel.dylib 0x39cb0910 close + 8
1 CoreFoundation 0x2f370ef6 CFSocketInvalidate + 434
2 CFNetwork 0x2f0122a2 Schedulables::_SchedulablesInvalidateApplierFunction(void const*, void*) + 14
3 CoreFoundation 0x2f34af6e CFArrayApplyFunction + 34
4 CFNetwork 0x2f011a94 SocketStream::close(void const*) + 280
5 CFNetwork 0x2f01194e CoreStreamBase::_streamInterface_Close() + 46
6 CFNetwork 0x2f030286 HTTPReadFilter::_streamImpl_Close() + 66
7 CFNetwork 0x2f01194e CoreStreamBase::_streamInterface_Close() + 46
8 CFNetwork 0x2f0301c6 NetConnection::shutdownConnectionStreams() + 98
9 CFNetwork 0x2f0309c8 NetConnection::closeStreamsIfPossibleOrSignalThatThatNeedsToBeDonePrettyPlease() + 56
10 CFNetwork 0x2f030d12 HTTPConnectionCacheEntry::removeUnauthConnection(NetConnection*) + 182
11 CoreFoundation 0x2f34af6e CFArrayApplyFunction + 34
12 CFNetwork 0x2f075678 HTTPConnectionCacheEntry::purgeIdleConnections(double, double) + 256
13 CFNetwork 0x2f030ad4 HTTPConnectionCache::performIdleSweep() + 156
14 CFNetwork 0x2f073b42 HTTPConnectionCache::timeoutIdleCellConnections() + 18
15 CFNetwork 0x2f0b3394 ___ZNK17CoreSchedulingSet13_performAsyncEPKcU13block_pointerFvvE_block_invoke + 16
16 CoreFoundation 0x2f34af6e CFArrayApplyFunction + 34
17 CFNetwork 0x2f019f10 RunloopBlockContext::perform() + 160
18 CFNetwork 0x2f019de2 MultiplexerSource::perform() + 218
19 CFNetwork 0x2f019c70 MultiplexerSource::_perform(void*) + 44
20 CoreFoundation 0x2f3e2f24 __CFRUNLOOP_IS_CALLING_OUT_TO_A_SOURCE0_PERFORM_FUNCTION__ + 12
21 CoreFoundation 0x2f3e23ea __CFRunLoopDoSources0 + 202
22 CoreFoundation 0x2f3e0bda __CFRunLoopRun + 626
23 CoreFoundation 0x2f34b53c CFRunLoopRunSpecific + 520
24 CoreFoundation 0x2f34b31e CFRunLoopRunInMode + 102
25 Foundation 0x2fd8664c +[NSURLConnection(Loader) _resourceLoadLoop:] + 316
26 Foundation 0x2fdfbdc2 __NSThread__main__ + 1058
27 libsystem_pthread.dylib 0x39d28c5a _pthread_body + 138
28 libsystem_pthread.dylib 0x39d28bca _pthread_start + 98
29 libsystem_pthread.dylib 0x39d26ccc thread_start + 4
Thread 3 name: WebThread
Thread 3:
0 CoreGraphics 0x2f49163c CGRectIsEmpty + 0
1 WebCore 0x37291700 -[WAKView setNeedsDisplayInRect:] + 76
2 WebKit 0x37cad22a -[WebHTMLView setNeedsDisplayInRect:] + 214
3 WebCore 0x372915cc WebCore::ScrollView::platformRepaintContentRectangle(WebCore::IntRect const&, bool) + 148
4 WebCore 0x37291412 WebCore::ScrollView::repaintContentRectangle(WebCore::IntRect const&, bool) + 98
5 WebCore 0x37291272 WebCore::FrameView::doDeferredRepaints() + 90
6 WebCore 0x372dc96c WebCore::FrameView::layout(bool) + 1748
7 WebCore 0x3721fb94 WebCore::ThreadTimers::sharedTimerFiredInternal() + 132
8 WebCore 0x3721fae6 WebCore::timerFired(__CFRunLoopTimer*, void*) + 22
9 CoreFoundation 0x2f3e2e84 __CFRUNLOOP_IS_CALLING_OUT_TO_A_TIMER_CALLBACK_FUNCTION__ + 12
10 CoreFoundation 0x2f3e2a9e __CFRunLoopDoTimer + 790
11 CoreFoundation 0x2f3e0e26 __CFRunLoopRun + 1214
12 CoreFoundation 0x2f34b53c CFRunLoopRunSpecific + 520
13 CoreFoundation 0x2f34b31e CFRunLoopRunInMode + 102
14 WebCore 0x372ae7d8 RunWebThread(void*) + 416
15 libsystem_pthread.dylib 0x39d28c5a _pthread_body + 138
16 libsystem_pthread.dylib 0x39d28bca _pthread_start + 98
17 libsystem_pthread.dylib 0x39d26ccc thread_start + 4
Thread 4:
0 libsystem_kernel.dylib 0x39cafa84 mach_msg_trap + 20
1 libsystem_kernel.dylib 0x39caf87c mach_msg + 36
2 CoreFoundation 0x2f3e255c __CFRunLoopServiceMachPort + 152
3 CoreFoundation 0x2f3e0c7c __CFRunLoopRun + 788
4 CoreFoundation 0x2f34b53c CFRunLoopRunSpecific + 520
5 CoreFoundation 0x2f34b31e CFRunLoopRunInMode + 102
6 libAVFAudio.dylib 0x2e3295ae GenericRunLoopThread::Entry(void*) + 126
7 libAVFAudio.dylib 0x2e31dbf4 CAPThread::Entry(CAPThread*) + 176
8 libsystem_pthread.dylib 0x39d28c5a _pthread_body + 138
9 libsystem_pthread.dylib 0x39d28bca _pthread_start + 98
10 libsystem_pthread.dylib 0x39d26ccc thread_start + 4
Thread 5 name: JavaScriptCore::BlockFree
Thread 5:
0 libsystem_kernel.dylib 0x39cc1f38 __psynch_cvwait + 24
1 libsystem_pthread.dylib 0x39d28262 _pthread_cond_wait + 538
2 libsystem_pthread.dylib 0x39d2903c pthread_cond_wait + 36
3 JavaScriptCore 0x3036f408 JSC::BlockAllocator::blockFreeingThreadMain() + 204
4 JavaScriptCore 0x3036ca70 WTF::wtfThreadEntryPoint(void*) + 12
5 libsystem_pthread.dylib 0x39d28c5a _pthread_body + 138
6 libsystem_pthread.dylib 0x39d28bca _pthread_start + 98
7 libsystem_pthread.dylib 0x39d26ccc thread_start + 4
Thread 6 name: JavaScriptCore::Marking
Thread 6:
0 libsystem_kernel.dylib 0x39cc1f38 __psynch_cvwait + 24
1 libsystem_pthread.dylib 0x39d28262 _pthread_cond_wait + 538
2 libsystem_pthread.dylib 0x39d2903c pthread_cond_wait + 36
3 JavaScriptCore 0x3050daf2 JSC::GCThread::waitForNextPhase() + 74
4 JavaScriptCore 0x3050db4c JSC::GCThread::gcThreadMain() + 48
5 JavaScriptCore 0x3036ca70 WTF::wtfThreadEntryPoint(void*) + 12
6 libsystem_pthread.dylib 0x39d28c5a _pthread_body + 138
7 libsystem_pthread.dylib 0x39d28bca _pthread_start + 98
8 libsystem_pthread.dylib 0x39d26ccc thread_start + 4
Thread 7 name: com.apple.CFSocket.private
Thread 7:
0 libsystem_kernel.dylib 0x39cc2440 select$DARWIN_EXTSN + 20
1 CoreFoundation 0x2f3e645e __CFSocketManager + 482
2 libsystem_pthread.dylib 0x39d28c5a _pthread_body + 138
3 libsystem_pthread.dylib 0x39d28bca _pthread_start + 98
4 libsystem_pthread.dylib 0x39d26ccc thread_start + 4
Thread 8 name: AFNetworking
Thread 8:
0 libsystem_kernel.dylib 0x39cafa84 mach_msg_trap + 20
1 libsystem_kernel.dylib 0x39caf87c mach_msg + 36
2 CoreFoundation 0x2f3e255c __CFRunLoopServiceMachPort + 152
3 CoreFoundation 0x2f3e0c7c __CFRunLoopRun + 788
4 CoreFoundation 0x2f34b53c CFRunLoopRunSpecific + 520
5 CoreFoundation 0x2f34b31e CFRunLoopRunInMode + 102
6 Foundation 0x2fd39822 -[NSRunLoop(NSRunLoop) runMode:beforeDate:] + 250
7 Foundation 0x2fd8a664 -[NSRunLoop(NSRunLoop) run] + 76
8 MyApp 0x002c33d0 +[AFURLConnectionOperation networkRequestThreadEntryPoint:] (AFURLConnectionOperation.m:184)
9 Foundation 0x2fdfbdc2 __NSThread__main__ + 1058
10 libsystem_pthread.dylib 0x39d28c5a _pthread_body + 138
11 libsystem_pthread.dylib 0x39d28bca _pthread_start + 98
12 libsystem_pthread.dylib 0x39d26ccc thread_start + 4
Thread 9 name: WebCore: CFNetwork Loader
Thread 9:
0 libsystem_kernel.dylib 0x39cafa84 mach_msg_trap + 20
1 libsystem_kernel.dylib 0x39caf87c mach_msg + 36
2 CoreFoundation 0x2f3e255c __CFRunLoopServiceMachPort + 152
3 CoreFoundation 0x2f3e0c7c __CFRunLoopRun + 788
4 CoreFoundation 0x2f34b53c CFRunLoopRunSpecific + 520
5 CoreFoundation 0x2f34b31e CFRunLoopRunInMode + 102
6 WebCore 0x372f7872 WebCore::runLoaderThread(void*) + 250
7 JavaScriptCore 0x3036ca70 WTF::wtfThreadEntryPoint(void*) + 12
8 libsystem_pthread.dylib 0x39d28c5a _pthread_body + 138
9 libsystem_pthread.dylib 0x39d28bca _pthread_start + 98
10 libsystem_pthread.dylib 0x39d26ccc thread_start + 4
Thread 10:
0 libsystem_kernel.dylib 0x39cafad4 semaphore_wait_trap + 8
1 libdispatch.dylib 0x39bfcdec _dispatch_semaphore_wait_slow + 172
2 libxpc.dylib 0x39d370d6 xpc_connection_send_message_with_reply_sync + 150
3 SystemConfiguration 0x31b77362 _reach_server_target_remove + 90
4 SystemConfiguration 0x31b772d2 __SCNetworkReachabilityServer_targetRemove + 38
5 SystemConfiguration 0x31b5e318 __SCNetworkReachabilityDeallocate + 92
6 CoreFoundation 0x2f347efc CFRelease + 464
7 libdispatch.dylib 0x39bfc7e0 _dispatch_root_queue_drain + 220
8 libdispatch.dylib 0x39bfc9cc _dispatch_worker_thread2 + 52
9 libsystem_pthread.dylib 0x39d26dfc _pthread_wqthread + 296
10 libsystem_pthread.dylib 0x39d26cc0 start_wqthread + 4
Thread 11:
0 libsystem_kernel.dylib 0x39cc2c7c __workq_kernreturn + 8
1 libsystem_pthread.dylib 0x39d26e06 _pthread_wqthread + 306
2 libsystem_pthread.dylib 0x39d26cc0 start_wqthread + 4
Thread 12:
0 libsystem_kernel.dylib 0x39cc2c7c __workq_kernreturn + 8
1 libsystem_pthread.dylib 0x39d26e06 _pthread_wqthread + 306
2 libsystem_pthread.dylib 0x39d26cc0 start_wqthread + 4
Thread 13:
0 libsystem_kernel.dylib 0x39cafa84 mach_msg_trap + 20
1 libsystem_kernel.dylib 0x39caf87c mach_msg + 36
2 CoreFoundation 0x2f3e255c __CFRunLoopServiceMachPort + 152
3 CoreFoundation 0x2f3e0c7c __CFRunLoopRun + 788
4 CoreFoundation 0x2f34b53c CFRunLoopRunSpecific + 520
5 CoreFoundation 0x2f38f1a6 CFRunLoopRun + 94
6 CoreMotion 0x2fa03394 ___lldb_unnamed_function1407$$CoreMotion + 724
7 libsystem_pthread.dylib 0x39d28c5a _pthread_body + 138
8 libsystem_pthread.dylib 0x39d28bca _pthread_start + 98
9 libsystem_pthread.dylib 0x39d26ccc thread_start + 4
Thread 14 Crashed:
0 libobjc.A.dylib 0x3970db66 objc_msgSend + 6
1 CoreFoundation 0x2f347f58 CFRelease + 556
2 libdispatch.dylib 0x39bf7100 _dispatch_call_block_and_release + 8
3 libdispatch.dylib 0x39bfbe72 _dispatch_queue_drain + 370
4 libdispatch.dylib 0x39bf8f96 _dispatch_queue_invoke + 38
5 libdispatch.dylib 0x39bfc74e _dispatch_root_queue_drain + 74
6 libdispatch.dylib 0x39bfc9cc _dispatch_worker_thread2 + 52
7 libsystem_pthread.dylib 0x39d26dfc _pthread_wqthread + 296
8 libsystem_pthread.dylib 0x39d26cc0 start_wqthread + 4

Answer 1

This crash could be caused by just about any piece of code, including your own.
The stack trace indicates that a block has previously retained a pointer that is now being released with CFRelease. If the pointer was not properly retained in the first place - or was in the process of being dealloc-ed when it was originally retained, you could see this stack trace.
If you are accessing the pointer on one thread, but releasing it another, then you may well have the kind of race condition that can cause this kind of crash.