App hanging at __ulock_wait2 in NSLog

I've got an app which is hanging, I turned on Hang Detection and got an .ips file and the top of the hang stack has this:

Dispatch queue: com.apple.main-thread (0)
0 __ulock_wait2
7 NSLog
8 _MMKVLogWithLevel
<snip>

MMKV is a 3rd party library being used in the app. But this looks like MMKV is calling NSLog and then that is hanging? Is that correct, if so then how and why is NSLog hanging and how to solve this issue?

Here's an image of the full stack from XCode when the .ips file is imported:

And here is the heaviest stack info when the .ips is opened as a text file:

Heaviest stack for the main thread of the target process:
  411  start + 2240 (dyld + 23940) [0x1bcacad84]
  411  main + 96 (MyApp + 108384) [0x10232e760]
  411  UIApplicationMain + 340 (UIKitCore + 2270528) [0x19ad20540]
  411  -[UIApplication _run] + 888 (UIKitCore + 2273028) [0x19ad20f04]
  411  GSEventRunModal + 164 (GraphicsServices + 13536) [0x1dd8554e0]
  411  CFRunLoopRunSpecific + 608 (CoreFoundation + 211304) [0x1988a6968]
  397  __CFRunLoopRun + 1996 (CoreFoundation + 213528) [0x1988a7218]
  397  __CFRUNLOOP_IS_SERVICING_THE_MAIN_DISPATCH_QUEUE__ + 16 (CoreFoundation + 226588) [0x1988aa51c]
  397  _dispatch_main_queue_callback_4CF + 44 (libdispatch.dylib + 74168) [0x1a07831b8]
  396  _dispatch_main_queue_drain + 1060 (libdispatch.dylib + 75244) [0x1a07835ec]
  396  _dispatch_client_callout + 20 (libdispatch.dylib + 15828) [0x1a0774dd4]
  396  _dispatch_call_block_and_release + 32 (libdispatch.dylib + 8508) [0x1a077313c]
  396  __49-[RCTCxxBridge _prepareModulesWithDispatchGroup:]_block_invoke + 156 (MyApp + 11155392) [0x102db77c0]
  396  -[RCTModuleData instance] + 816 (MyApp + 11498212) [0x102e0b2e4]
  396  RCTUnsafeExecuteOnMainQueueSync + 52 (MyApp + 11906956) [0x102e6ef8c]
  396  __25-[RCTModuleData instance]_block_invoke + 44 (MyApp + 11499132) [0x102e0b67c]
  392  -[RCTModuleData setUpInstanceAndBridge:] + 1324 (MyApp + 11490784) [0x102e095e0]
  392  __115-[RCTModuleData initWithModuleClass:bridge:moduleRegistry:viewRegistry_DEPRECATED:bundleManager:callableJSModules:]_block_invoke + 36 (MyApp + 11488056) [0x102e08b38]
  392  -[MMKVNative init] + 100 (MyApp + 20934364) [0x10370aedc]
  392  RCTExecuteOnMainQueue + 52 (MyApp + 11906684) [0x102e6ee7c]
  392  __18-[MMKVNative init]_block_invoke + 232 (MyApp + 20934688) [0x10370b020]
  392  +[MMKV initializeMMKV:] + 48 (MyApp + 9265708) [0x102bea22c]
  392  +[MMKV initializeMMKV:logLevel:] + 80 (MyApp + 9265800) [0x102bea288]
  392  _MMKVLogWithLevel(mmkv::MMKVLogLevel, char const*, char const*, int, char const*, ...) + 348 (MyApp + 9518596) [0x102c27e04]
  392  NSLog + 56 (Foundation + 602868) [0x1977b42f4]
  392  _NSLogv + 164 (Foundation + 603072) [0x1977b43c0]
  392  _CFLogvEx3 + 252 (CoreFoundation + 652276) [0x1989123f4]
  392  _CFLogvEx2Predicate + 352 (CoreFoundation + 652792) [0x1989125f8]
  392  __CFLogCString + 84 (CoreFoundation + 652948) [0x198912694]
  392  _logToStderr + 144 (CoreFoundation + 653980) [0x198912a9c]
  392  __ulock_wait2 + 8 (libsystem_kernel.dylib + 59708) [0x1e1a7193c]
 *392  ??? (<7ABFF6F3-9E55-3D7B-8DB8-8CF19FB41EFE> + 5068548) [0xfffffff008229704]
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Piepants
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I have the same issue but with the "Logger" library (https://github.com/apple/swift-log).

Posted by
AndreaH
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More examples here: https://developer.apple.com/forums/thread/746567 - the app hangs in many different locations, but the .ips stack always features NSLog and __ulock_wait2.

Posted by
Piepants
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I find that if I launch a subprocess from another process, the NSLog in this subprocess may encounter blocking issues (__ulock_wait). However, when I close its parent process, causing its parent to become the system, it can then continue to execute normally.

    +                     2451 -[NSWorkspace isFilePackageAtPath:]  (in AppKit) + 212  [0x18e1e49a8]
    +                       2451 NSLog  (in Foundation) + 56  [0x18b042f84]
    +                         2451 _NSLogv  (in Foundation) + 164  [0x18b0fee98]
    +                           2451 _CFLogvEx3  (in CoreFoundation) + 252  [0x189f626e4]
    +                             2451 _CFLogvEx2Predicate  (in CoreFoundation) + 344  [0x189f62a7c]
    +                               2451 __CFLogCString  (in CoreFoundation) + 84  [0x189fe4974]
    +                                 2451 _logToStderr  (in CoreFoundation) + 164  [0x189f62d50]
    +                                   2451 writev  (in libsystem_kernel.dylib) + 8  [0x189e3472c]
Posted by
kklu
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