I've noticed that implementing NSURLSessionDataDelegate and starting a task will very occasionally throw an EXC_BAD_ACCESS. The actual calling method that gives the error seems to vary but always comes from CFNetwork. For the most part, the calling method comes from NSURLSession delegate_dataTask:didReceiveData:completionHandler. I've attached two crash logs with different callers below. I've also attached my implementation of NSURLSessionDataDelegate.
Unfortunately I cannot reliably reproduce the error, so I don't have an example script to share. Creating and starting Downloader objects will eventually create the error. It does seem to happen more often with larger files. Have I implemented something wrong here? Is there a good way to debug from this stacktrace?
I've tested on iOS10 and 10.1.1 with the same results.
Implementation:
class Downloader: NSObject, NSURLSessionDataDelegate { private let url: String var finished = false let finishCondition = NSCondition() init(url:String) { self.url = url super.init() } func start() { let config = NSURLSessionConfiguration.defaultSessionConfiguration() let session = NSURLSession(configuration: config, delegate: self, delegateQueue: nil) guard let u = NSURL(string: url) else { return } let request = NSMutableURLRequest(URL: u) let task = session.dataTaskWithRequest(request) task.resume() } func URLSession(session: NSURLSession, dataTask: NSURLSessionDataTask, didReceiveData data: NSData) { } func URLSession(session: NSURLSession, task: NSURLSessionTask, didCompleteWithError error: NSError?) { session.invalidateAndCancel() } func URLSession(session: NSURLSession, dataTask: NSURLSessionDataTask, didReceiveResponse response: NSURLResponse, completionHandler: (NSURLSessionResponseDisposition) -> Void) { completionHandler(NSURLSessionResponseDisposition.Allow) } func waitForFinish() { finishCondition.lock() while !finished { finishCondition.wait() } finishCondition.unlock() } func URLSession(session: NSURLSession, didBecomeInvalidWithError error: NSError?) { finishCondition.lock() finished = true finishCondition.broadcast() finishCondition.unlock() } } Crash log #1:
* thread #5: tid = 0x25923, 0x0000000100042e8c libBacktraceRecording.dylib`__gcd_queue_item_enqueue_hook_block_invoke, queue = 'com.apple.NSURLSession-work', stop reason = EXC_BAD_ACCESS (code=1, address=0xf8686a68b98c6ec8) * frame #0: 0x0000000100042e8c libBacktraceRecording.dylib`__gcd_queue_item_enqueue_hook_block_invoke frame #1: 0x000000010004241c libBacktraceRecording.dylib`gcd_queue_item_enqueue_hook + 232 frame #2: 0x000000010065dee8 libdispatch.dylib`_dispatch_introspection_queue_item_enqueue_hook + 40 frame #3: 0x000000010063cba4 libdispatch.dylib`_dispatch_queue_push + 196 frame #4: 0x000000018ba50500 Foundation`iop_promote_qos_outward + 112 frame #5: 0x000000018ba4e524 Foundation`-[NSOperation setQualityOfService:] + 168 frame #6: 0x000000018b9d7714 Foundation`-[NSOperationQueue addOperationWithBlock:] + 76 frame #7: 0x000000018b73f82c CFNetwork`-[NSURLSession delegate_dataTask:didReceiveData:completionHandler:] + 208 frame #8: 0x000000018b5a2c5c CFNetwork`-[__NSCFLocalSessionTask _task_onqueue_didReceiveDispatchData:completionHandler:] + 276 frame #9: 0x000000018b5a5474 CFNetwork`-[__NSCFLocalSessionTask connection:didReceiveData:completion:] + 164 frame #10: 0x000000018b647bf0 CFNetwork`__48-[__NSCFURLLocalSessionConnection _tick_running]_block_invoke + 120 frame #11: 0x000000018b647b60 CFNetwork`-[__NSCFURLLocalSessionConnection _tick_running] + 344 frame #12: 0x000000018b648c74 CFNetwork`-[__NSCFURLLocalSessionConnection _didReceiveData:] + 412 frame #13: 0x000000018b64af8c CFNetwork`SessionConnectionLoadable::_loaderClientEvent_DidReceiveData(__CFArray const*) + 52 frame #14: 0x000000018b6f823c CFNetwork`___ZN19URLConnectionLoader19protocolDidLoadDataEPK8__CFDatax_block_invoke_2 + 44 frame #15: 0x000000018b64b58c CFNetwork`___ZN25SessionConnectionLoadable21withLoaderClientAsyncEU13block_pointerFvP21LoaderClientInterfaceE_block_invoke + 32 frame #16: 0x000000010063125c libdispatch.dylib`_dispatch_call_block_and_release + 24 frame #17: 0x000000010063121c libdispatch.dylib`_dispatch_client_callout + 16 frame #18: 0x000000010063eb54 libdispatch.dylib`_dispatch_queue_serial_drain + 1136 frame #19: 0x0000000100634ce4 libdispatch.dylib`_dispatch_queue_invoke + 672 frame #20: 0x0000000100640e6c libdispatch.dylib`_dispatch_root_queue_drain + 584 frame #21: 0x0000000100640bb8 libdispatch.dylib`_dispatch_worker_thread3 + 140 frame #22: 0x000000018a01e2b8 libsystem_pthread.dylib`_pthread_wqthread + 1288 frame #23: 0x000000018a01dda4 libsystem_pthread.dylib`start_wqthread + 4 Crash log #2:
* thread #12: tid = 0x2521f, 0x000000010010ae8c libBacktraceRecording.dylib`__gcd_queue_item_enqueue_hook_block_invoke, queue = 'com.apple.CFNetwork.Connection', stop reason = EXC_BAD_ACCESS (code=1, address=0xd00f524835000200) * frame #0: 0x000000010010ae8c libBacktraceRecording.dylib`__gcd_queue_item_enqueue_hook_block_invoke frame #1: 0x000000010010a41c libBacktraceRecording.dylib`gcd_queue_item_enqueue_hook + 232 frame #2: 0x0000000100759ee8 libdispatch.dylib`_dispatch_introspection_queue_item_enqueue_hook + 40 frame #3: 0x0000000100738ba4 libdispatch.dylib`_dispatch_queue_push + 196 frame #4: 0x00000001975ccb3c libnetwork.dylib`nw_connection_read + 448 frame #5: 0x00000001975d938c libnetwork.dylib`tcp_connection_read + 168 frame #6: 0x000000018b719d54 CFNetwork`TCPIOConnection::read(unsigned long, unsigned long, void (dispatch_data_s*, CFStreamError) block_pointer) + 172 frame #7: 0x000000018b782af4 CFNetwork`HTTPEngine::_getBodyIntelligently(void (dispatch_data_s*, CFStreamError, bool) block_pointer) + 816 frame #8: 0x000000018b780d0c CFNetwork`HTTPEngine::_readBodyStartNextRead() + 76 frame #9: 0x000000018b783664 CFNetwork`___ZN10HTTPEngine21_getBodyIntelligentlyEU13block_pointerFvP15dispatch_data_s13CFStreamErrorbE_block_invoke.56 + 344 frame #10: 0x000000018b719f64 CFNetwork`___ZN15TCPIOConnection4readEmmU13block_pointerFvP15dispatch_data_s13CFStreamErrorE_block_invoke + 480 frame #11: 0x000000010072d25c libdispatch.dylib`_dispatch_call_block_and_release + 24 frame #12: 0x000000010072d21c libdispatch.dylib`_dispatch_client_callout + 16 frame #13: 0x000000010073ab54 libdispatch.dylib`_dispatch_queue_serial_drain + 1136 frame #14: 0x0000000100730ce4 libdispatch.dylib`_dispatch_queue_invoke + 672 frame #15: 0x000000010073ce6c libdispatch.dylib`_dispatch_root_queue_drain + 584 frame #16: 0x000000010073cbb8 libdispatch.dylib`_dispatch_worker_thread3 + 140 frame #17: 0x000000018a01e2b8 libsystem_pthread.dylib`_pthread_wqthread + 1288 frame #18: 0x000000018a01dda4 libsystem_pthread.dylib`start_wqthread + 4 UPDATE: I can now semi-reliably reproduce this error by running the loop pasted below in the iOS Simulator. This does not happen on iOS 9.3. If you run the code below, within a minute you should receive the error. Since it's very likely to occur in the simulator, compared with a device, I would assume it's a concurrency issue that becomes more likely with more processing power/cores. To reproduce error, run this:
var i = 0 while true { print("running: \(i)") // random url, larger files seem more likely to cause error let url = "http://qthttp.apple.com.edgesuite.net/1010qwoeiuryfg/3340/33409.ts" let c = Downloader(url: url) c.start() c.waitForFinish() i += 1 }