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I develop an application which communicates with an external device over TCP. Occassionally the app crashes when user kills the app (by swipping it up). I have no clue why it crashes and to me it seems to be an OS bug. In the callback aplicationWillTerminate TCP connection is closed. The method looks as follows:-(void)stop { @synchronized(self) { [self closeClientConnection]; if([FileUtility isFileDescriptorValid: serverSocketDescriptor] && close(serverSocketDescriptor) == ERROR) { [logger error:@"Cannot close server socket connection" filename:@__FILE__ line:__LINE__ function:[[NSString alloc]initWithUTF8String:__FUNCTION__]]; } else { [logger info:@"Server socket closed" filename:@__FILE__ line:__LINE__ function:[[NSString alloc]initWithUTF8String:__FUNCTION__]]; } [serverThread cancel]; } }Stack trace can be seen below: 0x104334d44 CLSHandler + 4329868612# Organization: none # Platform: ios # Version: 1.0 (20) # Date: 2019-11-06T18:26:00Z # OS Version: 13.2.0 (17B84) # Device: iPhone 7 # RAM Free: 5.4% # Disk Free: 14% #0. Crashed: com.apple.main-thread 0 libsystem_kernel.dylib 0x197aeee8c close + 8 1 MyAppAppStore 0x1042f4960 -[TCPServer stop] + 258 (TCPServer.m:258) 2 MyAppAppStore 0x10431a8bc @objc AppDelegate.applicationWillTerminate(_:) (<compiler-generated>) 3 UIKitCore 0x19bd5349c -[UIApplication _deactivateForReason:notify:] + 760 4 UIKitCore 0x19b52b1fc -[_UISceneLifecycleMultiplexer _performBlock:withApplicationOfDeactivationReasons:fromReasons:] + 264 5 UIKitCore 0x19b52b5f0 -[_UISceneLifecycleMultiplexer _evalTransitionToSettings:fromSettings:forceExit:withTransitionStore:] + 736 6 UIKitCore 0x19b52aeac -[_UISceneLifecycleMultiplexer uiScene:transitionedFromState:withTransitionContext:] + 336 7 UIKitCore 0x19b52f3c8 __186-[_UIWindowSceneFBSSceneTransitionContextDrivenLifecycleSettingsDiffAction _performActionsForUIScene:withUpdatedFBSScene:settingsDiff:fromSettings:transitionContext:lifecycleActionType:]_block_invoke_2 + 188 8 UIKitCore 0x19b8f26a4 +[BSAnimationSettings(UIKit) tryAnimatingWithSettings:actions:completion:] + 824 9 UIKitCore 0x19b9d95b0 _UISceneSettingsDiffActionPerformChangesWithTransitionContext + 244 10 UIKitCore 0x19b52f100 __186-[_UIWindowSceneFBSSceneTransitionContextDrivenLifecycleSettingsDiffAction _performActionsForUIScene:withUpdatedFBSScene:settingsDiff:fromSettings:transitionContext:lifecycleActionType:]_block_invoke + 136 11 UIKitCore 0x19b9d94ac _UISceneSettingsDiffActionPerformActionsWithDelayForTransitionContext + 100 12 UIKitCore 0x19b52ef6c -[_UIWindowSceneFBSSceneTransitionContextDrivenLifecycleSettingsDiffAction _performActionsForUIScene:withUpdatedFBSScene:settingsDiff:fromSettings:transitionContext:lifecycleActionType:] + 376 13 UIKitCore 0x19b3a0ba4 __64-[UIScene scene:didUpdateWithDiff:transitionContext:completion:]_block_invoke + 636 14 UIKitCore 0x19b39f70c -[UIScene _emitSceneSettingsUpdateResponseForCompletion:afterSceneUpdateWork:] + 248 15 UIKitCore 0x19b3a08dc -[UIScene scene:didUpdateWithDiff:transitionContext:completion:] + 220 16 UIKitCore 0x19b91350c -[UIApplicationSceneClientAgent scene:handleEvent:withCompletion:] + 464 17 FrontBoardServices 0x19cdfb2f8 -[FBSSceneImpl updater:didUpdateSettings:withDiff:transitionContext:completion:] + 544 18 FrontBoardServices 0x19ce1fdd8 __88-[FBSWorkspaceScenesClient sceneID:updateWithSettingsDiff:transitionContext:completion:]_block_invoke_2 + 120 19 FrontBoardServices 0x19ce04fb4 -[FBSWorkspace _calloutQueue_executeCalloutFromSource:withBlock:] + 232 20 FrontBoardServices 0x19ce1fd0c __88-[FBSWorkspaceScenesClient sceneID:updateWithSettingsDiff:transitionContext:completion:]_block_invoke + 184 21 libdispatch.dylib 0x1979c3184 _dispatch_client_callout + 16 22 libdispatch.dylib 0x19796bfd8 _dispatch_block_invoke_direct$VARIANT$mp + 224 23 FrontBoardServices 0x19ce444c8 __FBSSERIALQUEUE_IS_CALLING_OUT_TO_A_BLOCK__ + 40 24 FrontBoardServices 0x19ce44194 -[FBSSerialQueue _queue_performNextIfPossible] + 404 25 FrontBoardServices 0x19ce446bc -[FBSSerialQueue _performNextFromRunLoopSource] + 28 26 CoreFoundation 0x197c737c4 __CFRUNLOOP_IS_CALLING_OUT_TO_A_SOURCE0_PERFORM_FUNCTION__ + 24 27 CoreFoundation 0x197c7371c __CFRunLoopDoSource0 + 80 28 CoreFoundation 0x197c72eb4 __CFRunLoopDoSources0 + 180 29 CoreFoundation 0x197c6e000 __CFRunLoopRun + 1080 30 CoreFoundation 0x197c6d8a0 CFRunLoopRunSpecific + 464 31 GraphicsServices 0x1a1bc5328 GSEventRunModal + 104 32 UIKitCore 0x19bd5e768 UIApplicationMain + 1936 33 MyAppAppStore 0x1042f7b0c main + 15 (AppDelegate.swift:15) 34 libdyld.dylib 0x197af8360 start + 4 -- #0. Crashed: com.apple.main-thread 0 libsystem_kernel.dylib 0x197aeee8c close + 8 1 MyAppAppStore 0x1042f4960 -[TCPServer stop] + 258 (TCPServer.m:258) 2 MyAppAppStore 0x10431a8bc @objc AppDelegate.applicationWillTerminate(_:) (<compiler-generated>) 3 UIKitCore 0x19bd5349c -[UIApplication _deactivateForReason:notify:] + 760 4 UIKitCore 0x19b52b1fc -[_UISceneLifecycleMultiplexer _performBlock:withApplicationOfDeactivationReasons:fromReasons:] + 264 5 UIKitCore 0x19b52b5f0 -[_UISceneLifecycleMultiplexer _evalTransitionToSettings:fromSettings:forceExit:withTransitionStore:] + 736 6 UIKitCore 0x19b52aeac -[_UISceneLifecycleMultiplexer uiScene:transitionedFromState:withTransitionContext:] + 336 7 UIKitCore 0x19b52f3c8 __186-[_UIWindowSceneFBSSceneTransitionContextDrivenLifecycleSettingsDiffAction _performActionsForUIScene:withUpdatedFBSScene:settingsDiff:fromSettings:transitionContext:lifecycleActionType:]_block_invoke_2 + 188 8 UIKitCore 0x19b8f26a4 +[BSAnimationSettings(UIKit) tryAnimatingWithSettings:actions:completion:] + 824 9 UIKitCore 0x19b9d95b0 _UISceneSettingsDiffActionPerformChangesWithTransitionContext + 244 10 UIKitCore 0x19b52f100 __186-[_UIWindowSceneFBSSceneTransitionContextDrivenLifecycleSettingsDiffAction _performActionsForUIScene:withUpdatedFBSScene:settingsDiff:fromSettings:transitionContext:lifecycleActionType:]_block_invoke + 136 11 UIKitCore 0x19b9d94ac _UISceneSettingsDiffActionPerformActionsWithDelayForTransitionContext + 100 12 UIKitCore 0x19b52ef6c -[_UIWindowSceneFBSSceneTransitionContextDrivenLifecycleSettingsDiffAction _performActionsForUIScene:withUpdatedFBSScene:settingsDiff:fromSettings:transitionContext:lifecycleActionType:] + 376 13 UIKitCore 0x19b3a0ba4 __64-[UIScene scene:didUpdateWithDiff:transitionContext:completion:]_block_invoke + 636 14 UIKitCore 0x19b39f70c -[UIScene _emitSceneSettingsUpdateResponseForCompletion:afterSceneUpdateWork:] + 248 15 UIKitCore 0x19b3a08dc -[UIScene scene:didUpdateWithDiff:transitionContext:completion:] + 220 16 UIKitCore 0x19b91350c -[UIApplicationSceneClientAgent scene:handleEvent:withCompletion:] + 464 17 FrontBoardServices 0x19cdfb2f8 -[FBSSceneImpl updater:didUpdateSettings:withDiff:transitionContext:completion:] + 544 18 FrontBoardServices 0x19ce1fdd8 __88-[FBSWorkspaceScenesClient sceneID:updateWithSettingsDiff:transitionContext:completion:]_block_invoke_2 + 120 19 FrontBoardServices 0x19ce04fb4 -[FBSWorkspace _calloutQueue_executeCalloutFromSource:withBlock:] + 232 20 FrontBoardServices 0x19ce1fd0c __88-[FBSWorkspaceScenesClient sceneID:updateWithSettingsDiff:transitionContext:completion:]_block_invoke + 184 21 libdispatch.dylib 0x1979c3184 _dispatch_client_callout + 16 22 libdispatch.dylib 0x19796bfd8 _dispatch_block_invoke_direct$VARIANT$mp + 224 23 FrontBoardServices 0x19ce444c8 __FBSSERIALQUEUE_IS_CALLING_OUT_TO_A_BLOCK__ + 40 24 FrontBoardServices 0x19ce44194 -[FBSSerialQueue _queue_performNextIfPossible] + 404 25 FrontBoardServices 0x19ce446bc -[FBSSerialQueue _performNextFromRunLoopSource] + 28 26 CoreFoundation 0x197c737c4 __CFRUNLOOP_IS_CALLING_OUT_TO_A_SOURCE0_PERFORM_FUNCTION__ + 24 27 CoreFoundation 0x197c7371c __CFRunLoopDoSource0 + 80 28 CoreFoundation 0x197c72eb4 __CFRunLoopDoSources0 + 180 29 CoreFoundation 0x197c6e000 __CFRunLoopRun + 1080 30 CoreFoundation 0x197c6d8a0 CFRunLoopRunSpecific + 464 31 GraphicsServices 0x1a1bc5328 GSEventRunModal + 104 32 UIKitCore 0x19bd5e768 UIApplicationMain + 1936 33 MyAppAppStore 0x1042f7b0c main + 15 (AppDelegate.swift:15) 34 libdyld.dylib 0x197af8360 start + 4 #1. Thread 0 libsystem_kernel.dylib 0x197aeea7c __workq_kernreturn + 8 1 libsystem_pthread.dylib 0x197a12fd4 _pthread_wqthread + 352 2 libsystem_pthread.dylib 0x197a15ad4 start_wqthread + 8 #2. Thread 0 libsystem_kernel.dylib 0x197aeea7c __workq_kernreturn + 8 1 libsystem_pthread.dylib 0x197a12fd4 _pthread_wqthread + 352 2 libsystem_pthread.dylib 0x197a15ad4 start_wqthread + 8 #3. com.apple.uikit.eventfetch-thread 0 libsystem_kernel.dylib 0x197acc634 mach_msg_trap + 8 1 libsystem_kernel.dylib 0x197acbaa0 mach_msg + 72