Multithreaded rendering only crashes on iOS 13

In my game engine, I have an option to enable the multithreaded rendering. In iOS 12 (and below) my app did not crash.

I have not changed anything in my code, but it crashes on iOS 13.

2019-10-02 11:36:07.931 SimpleGame[293:11150] ---> (Inner Exception #0) Foundation.MonoTouchException: Objective-C exception thrown.  Name: NSInternalInconsistencyException Reason: threading violation: expected the main thread

Oct  2 09:57:48 GL04207-MT SimpleGame[923] <Notice>:    0   CoreFoundation                      0x00000001aace1c44 97285ACB-7B21-393A-ABF6-03F1DBB5D2A2 + 1256516
Oct  2 09:57:48 GL04207-MT SimpleGame[923] <Notice>:    1   libobjc.A.dylib                     0x00000001aa9fc0c8 objc_exception_throw + 60
Oct  2 09:57:48 GL04207-MT SimpleGame[923] <Notice>:    2   CoreFoundation                      0x00000001aabde75c 97285ACB-7B21-393A-ABF6-03F1DBB5D2A2 + 194396
Oct  2 09:57:48 GL04207-MT SimpleGame[923] <Notice>:    3   Foundation                          0x00000001ab01f960 672CF0CB-4951-3B91-89DF-55E953AEA00F + 571744
Oct  2 09:57:48 GL04207-MT SimpleGame[923] <Notice>:    4   FrontBoardServices                  0x00000001afe9791c 124B166D-3905-3F51-A46A-646AE3424213 + 366876
Oct  2 09:57:48 GL04207-MT SimpleGame[923] <Notice>:    5   FrontBoardServices                  0x00000001afe4a288 124B166D-3905-3F51-A46A-646AE3424213 + 49800
Oct  2 09:57:48 GL04207-MT SimpleGame[923] <Notice>:    6   UIKitCore                           0x00000001ae51a87c 55D60569-DBB7-3BC9-8A97-DCAC95C64D4B + 1915004
Oct  2 09:57:48 GL04207-MT SimpleGame[923] <Notice>:    7   UIKitCore                           0x00000001ae517554 55D60569-DBB7-3BC9-8A97-DCAC95C64D4B + 1901908
Oct  2 09:57:48 GL04207-MT SimpleGame[923] <Notice>:    8   UIKitCore                           0x00000001ae5177b4 55D60569-DBB7-3BC9-8A97-DCAC95C64D4B + 1902516
Oct  2 09:57:48 GL04207-MT SimpleGame[923] <Notice>:    9   UIKitCore                           0x00000001aeface14 55D60569-DBB7-3BC9-8A97-DCAC95C64D4B + 13000212
Oct  2 09:57:48 GL04207-MT SimpleGame[923] <Notice>:    10  UIKitCore                           0x00000001aedaf02c 55D60569-DBB7-3BC9-8A97-DCAC95C64D4B + 10911788
Oct  2 09:57:48 GL04207-MT SimpleGame[923] <Notice>:    11  UIKitCore                           0x00000001aedaff00 55D60569-DBB7-3BC9-8A97-DCAC95C64D4B + 10915584
Oct  2 09:57:48 GL04207-MT SimpleGame[923] <Notice>:    12  
Oct  2 09:57:48 GL04207-MT SimpleGame[923] <Notice>:    22  SimpleGame                   0x000000010487c2a4 World_FlushRenderWorkQueue_private + 2577804
Oct  2 09:57:48 GL04207-MT SimpleGame[923] <Notice>:    23  SimpleGame                   0x00000001048b45e8 World_FlushRenderWorkQueue_private + 2808016
Oct  2 09:57:48 GL04207-MT SimpleGame[923] <Notice>:    24  SimpleGame                   0x00000001048b54c0 World_FlushRenderWorkQueue_private + 2811816
Oct  2 09:57:48 GL04207-MT SimpleGame[923] <Notice>:    25  SimpleGame                   0x00000001051d7198 World_FlushRenderWorkQueue_private + 12387456
Oct  2 09:57:48 GL04207-MT SimpleGame[923] <Notice>:    26  SimpleGame                   0x000000010503c058 World_FlushRenderWorkQueue_private + 10703680
Oct  2 09:57:48 GL04207-MT SimpleGame[923] <Notice>:    27  SimpleGame                   0x000000010503b774 World_FlushRenderWorkQueue_private + 10701404
Oct  2 09:57:48 GL04207-MT SimpleGame[923] <Notice>:    28  SimpleGame                   0x00000001051d5400 World_FlushRenderWorkQueue_private + 12379880
Oct  2 09:57:48 GL04207-MT SimpleGame[923] <Notice>:    29 SimpleGame                   0x000000010487d904 World_FlushRenderWorkQueue_private + 2583532
Oct  2 09:57:48 GL04207-MT SimpleGame[923] <Notice>:    30  SimpleGame                   0x000000010487c2a4 World_FlushRenderWorkQueue_private + 2577804
Oct  2 09:57:48 GL04207-MT SimpleGame[923] <Notice>:    31  SimpleGame                   0x000000010487c038 World_FlushRenderWorkQueue_private + 2577184
Oct  2 09:57:48 GL04207-MT SimpleGame[923] <Notice>:    32  SimpleGame                   0x000000010487da00 World_FlushRenderWorkQueue_private + 2583784
Oct  2 09:57:48 GL04207-MT SimpleGame[923] <Notice>:    33  SimpleGame                   0x00000001047081a8 World_FlushRenderWorkQueue_private + 1053840
Oct  2 09:57:48 GL04207-MT SimpleGame[923] <Notice>:    34  SimpleGame                   0x0000000105bb1434 World_FlushRenderWorkQueue_private + 22718236
Oct  2 09:57:48 GL04207-MT SimpleGame[923] <Notice>:    35  SimpleGame                   0x0000000105c18dd4 World_FlushRenderWorkQueue_private + 23142588
Oct  2 09:57:48 GL04207-MT SimpleGame[923] <Notice>:    36  PSimpleGame                   0x0000000105c44080 World_FlushRenderWorkQueue_private + 23319400
Oct  2 09:57:48 GL04207-MT SimpleGame[923] <Notice>:    37  libsystem_pthread.dylib             0x00000001aa9ecd5c _pthread_start + 128
Oct  2 09:57:48 GL04207-MT SimpleGame[923] <Notice>:    38  libsystem_pthread.dylib             0x00000001aa9f4c84 thread_start + 8

When I disable the multithreaded rendering option, it did not crash. There must be a change in the threading system in iOS 13, but could not find anything from their documetation.

As Gene said, the issue is that previous versions of iOS wouldn’t stop when you incorrectly interacted with the UI from a background thread. iOS 13 will.

To help identifying the issue, go to “Editor” » “Scheme” » “Edit Scheme...” and then check “Main Thread Checker”. If you tap on the right arrow next to this, it will even create a breakpoint for issues, where it will stop execution at the line at which you are trying to update the UI from a background thread: