We are getting a crash _dispatch_assert_queue_fail when the cancellationHandler on NSProgress is called. We do not see this with iOS 17.x, only on iOS 18. We are building in Swift 6 language mode and do not have any compiler warnings. We have a type whose init looks something like this: init( request: URLRequest, destinationURL: URL, session: URLSession ) { progress = Progress() progress.kind = .file progress.fileOperationKind = .downloading progress.fileURL = destinationURL progress.pausingHandler = { [weak self] in self?.setIsPaused(true) } progress.resumingHandler = { [weak self] in self?.setIsPaused(false) } progress.cancellationHandler = { [weak self] in self?.cancel() } When the progress is cancelled, and the cancellation handler is invoked. We get the crash. The crash is not reproducible 100% of the time, but it happens significantly often. Especially after cleaning and rebuilding and running our tests. * thread #4, queue = 'com.apple.root.default-qos', stop reason = EXC_BREAKPOINT (code=1, subcode=0x18017b0e8) * frame #0: 0x000000018017b0e8 libdispatch.dylib`_dispatch_assert_queue_fail + 116 frame #1: 0x000000018017b074 libdispatch.dylib`dispatch_assert_queue + 188 frame #2: 0x00000002444c63e0 libswift_Concurrency.dylib`swift_task_isCurrentExecutorImpl(swift::SerialExecutorRef) + 284 frame #3: 0x000000010b80bd84 MyTests`closure #3 in MyController.init() at MyController.swift:0 frame #4: 0x000000010b80bb04 MyTests`thunk for @escaping @callee_guaranteed @Sendable () -> () at <compiler-generated>:0 frame #5: 0x00000001810276b0 Foundation`__20-[NSProgress cancel]_block_invoke_3 + 28 frame #6: 0x00000001801774ec libdispatch.dylib`_dispatch_call_block_and_release + 24 frame #7: 0x0000000180178de0 libdispatch.dylib`_dispatch_client_callout + 16 frame #8: 0x000000018018b7dc libdispatch.dylib`_dispatch_root_queue_drain + 1072 frame #9: 0x000000018018bf60 libdispatch.dylib`_dispatch_worker_thread2 + 232 frame #10: 0x00000001012a77d8 libsystem_pthread.dylib`_pthread_wqthread + 224 Any thoughts on why this is crashing and what we can do to work-around it? I have not been able to extract our code into a simple reproducible case yet. And I mostly see it when running our code in a testing environment (XCTest). Although I have been able to reproduce it running an app a few times, it's just less common.

Hi there, I have some thread related questions regards to network framework completion callbacks. In short, how should I process cross thread data in the completion callbacks? Here are more details. I have a background serial dispatch queue (call it dispatch queue A) to sequentially process the nw_connection and any network io events. Meanwhile, user inputs are handled by serial dispatch queue ( dispatch queue B). How should I handle the cross thread user data in this case? (I write some simplified sample code below) struct { int client_status; char* message_to_sent; }user_data; nw_connection_t nw_connection; dispatch_queue_t dispatch_queue_A static void send_message(){ dispatch_data_t data = dispatch_data_create(message, len(message), dispath_event_loop->dispatch_queue, DISPATCH_DATA_DESTRUCTOR_DEFAULT); nw_connection_send( nw_connection, data, NW_CONNECTION_DEFAULT_MESSAGE_CONTEXT, false, ^(nw_error_t error) { user_data.client_status = SENT; mem_release(user_data.message_to_sent); }); }); } static void setup_connection(){ dispatch_queue_A= dispatch_queue_create("unique_id_a", DISPATCH_QUEUE_SERIAL); nw_connection = nw_connection_create(endpoint, params); nw_connection_set_state_changed_handler(){ if (state == nw_connection_state_ready) { user_data.client_status = CONNECTED } // ... other operations ... } nw_connection_start(nw_connection); nw_retain(nw_connection); } static void user_main(){ setup_connection() user_data.client_status = INIT; dispatch_queue_t dispatch_queue_B = dispatch_queue_create("unique_id_b", DISPATCH_QUEUE_SERIAL); // write socket dispatch_async(dispatch_queue_B, ^(){ if (user_data.client_status != CONNECTED ) return; user_data.message_to_sent = malloc(XX,XXX) // I would like to have all io events processed on dispatch queue A so that the io events would not interacted with the user events dispatch_async_f(dispatch_queue_A, send_message); // Disconnect block dispatch_async(dispatch_queue_B, ^(){ dispatch_async_f(dispatch_queue_A, ^(){ nw_connection_cancel(nw_connection) }); user_data.client_status = DISCONNECTING; }); // clean up connection and so on... } To be more specific, my questions would be: As I was using serial dispatch queue, I didn't protect the user_data here. However, which thread would the send_completion_handler get called? Would it be a data race condition where the Disconnect block and send_completion_handler both access user_data? If I protect the user_data with lock, it might block the thread. How does the dispatch queue make sure it would NOT put a related execution block onto the "blocked thread"?

Swift Concurrency Resources: DevForums tags: Concurrency The Swift Programming Language > Concurrency documentation Migrating to Swift 6 documentation WWDC 2022 Session 110351 Eliminate data races using Swift Concurrency — This ‘sailing on the sea of concurrency’ talk is a great introduction to the fundamentals. WWDC 2021 Session 10134 Explore structured concurrency in Swift — The table that starts rolling out at around 25:45 is really helpful. Swift Async Algorithms package Swift Concurrency Proposal Index DevForum post Why is flow control important? DevForums post Matt Massicotte’s blog Dispatch Resources: DevForums tags: Dispatch Dispatch documentation — Note that the Swift API and C API, while generally aligned, are different in many details. Make sure you select the right language at the top of the page. Dispatch man pages — While the standard Dispatch documentation is good, you can still find some great tidbits in the man pages. See Reading UNIX Manual Pages. Start by reading dispatch in section 3. WWDC 2015 Session 718 Building Responsive and Efficient Apps with GCD [1] WWDC 2017 Session 706 Modernizing Grand Central Dispatch Usage [1] Avoid Dispatch Global Concurrent Queues DevForums post Share and Enjoy — Quinn “The Eskimo!” @ Developer Technical Support @ Apple let myEmail = "eskimo" + "1" + "@" + "apple.com" [1] These videos may or may not be available from Apple. If not, the URL should help you locate other sources of this info.

Hi, I work on a game for iOS and the framerate decreases progressively when the debugger is attached. Running it for 2mins, it went from 30 to 1 FPS while rendering a simple static scene. I narrowed it down to a call to dispatch_async_f which takes longer to execute over time. clock_t t1 = clock(); dispatch_async_f(queue, context, function); clock_t t2 = clock(); double duration = (double)(t2 -t1)/(double)CLOCKS_PER_SEC; Dodumentation says dispatch_async_f is supposed to return immediatly. So what could explain duration to increases in debug? Am i measuring this incorrectly? The game is written in mixed C++ and ObjC. It uses Metal as graphic API and GCD for dispatching jobs. I have Xcode 13.4.1 and test on an iPhone 13 Pro with iOS 15.7. Thanks.