crash iOS 14 - Unbalanced call to dispatch_group_leave()

I encountered the same problem.This crash only apears above iOS14,including iOS15.There is no such case in versions below iOS 14 too And there is not frame about the function of my app on the top of the crashed stack trace.So I can hardly find why this crash happends.

Crashed: com.apple.main-thread 0 libdispatch.dylib 0x19b7b87b8

dispatch_group_leave.cold.1 + 36 1 libdispatch.dylib 0x19b784638 _dispatch_group_wake + 138

2 libdispatch.dylib 0x19b781a54 _dispatch_call_block_and_release + 32

3 libdispatch.dylib 0x19b7837ec _dispatch_client_callout + 20

4 libdispatch.dylib 0x19b791c40 _dispatch_main_queue_callback_4CF + 884

5 CoreFoundation 0x19bb101f8 CFRUNLOOP_IS_SERVICING_THE_MAIN_DISPATCH_QUEUE + 16

6 CoreFoundation 0x19bb0a0d0 __CFRunLoopRun + 2524

7 CoreFoundation 0x19bb091c0 CFRunLoopRunSpecific + 600

8 GraphicsServices 0x1b30f1734 GSEventRunModal + 164

9 UIKitCore 0x19e5777e4 -[UIApplication _run] + 1072

10 UIKitCore 0x19e57d054 UIApplicationMain + 168

11 MyApp 0x1022eae98 main + 22 (main.m:22)

12 libdyld.dylib 0x19b7c5cf8 start + 4

The immediate cause of this crash is clear: Some code within your process is using a Dispatch group in an unbalanced way. For example, this code will trigger the crash:

let g = DispatchGroup()
g.enter()
g.leave()
g.leave()

As to why it’s happening in your app, it’s hard to say. Can you post a full Apple crash report? Use the text attachment feature (click the paperclip icon and then choose Add File) to avoid clogging up the timeline.

Share and Enjoy
—
Quinn “The Eskimo!” @ Developer Technical Support @ Apple
let myEmail = "eskimo" + "1" + "@" + "apple.com"

I have checked all call to dispatch_group_leave in my project

All the source code, right? This problem could be caused by a framework for which you don’t have source, either an Apple one built in to the OS or a third-party one that you bundle in your app.

Please, can I do anything else to help you locate the crash ?

Nothing that immediately springs to mind. An Apple crash report might offer some further insight into this, but even that’s uncertain.

Share and Enjoy
—
Quinn “The Eskimo!” @ Developer Technical Support @ Apple
let myEmail = "eskimo" + "1" + "@" + "apple.com"

I also encountered a very similar issue. And now, it's fixed. Hope my findings can be helpful.

By adding event tracks, I finally found the completion handlers of these functions are eventually called more than once in several async tasks!

1. 
- (void)application:(UIApplication *)application performFetchWithCompletionHandler:(void (^)(UIBackgroundFetchResult result))completionHandler;

2. 
- (void)application:(UIApplication *)application didReceiveRemoteNotification:(NSDictionary *)userInfo fetchCompletionHandler:(void (^)(UIBackgroundFetchResult result))completionHandler;

BTW, to help me investigate this issue, I wrapped the completion handler and passed the wrapping handler to the downstream. So, when the downside calls the wrapping completion handler, I can count the times of calling it. Then, if the wrapping completion handler is called before, it should return without calling the wrapped completion handler again.