Need bug fix on crash: thunk for @escaping @callee_guaranteed () -> () + 28 (<compiler-generated>:0)

This bug suggested a thread in a DispatchGroup is leaving early or not leaving. Hoping some senior iOS Swift devs can give thoughts on this

My way to take care of this is to review each DispatchGroup and check to see if the error can be narrowed: - This has been a bug for a few years - the problem is the stack trace does not locate where the crash is happening I suppose the error is here:

    Legacy Code:
    let group = DispatchGroup()
    group.enter() 
    DispatchQueue.walletQueue.async {
    if self.walletManager.signTransaction(tx, pin: pin) {
        self.publish(completion: completion)
        success = true    
    } 
    group.leave()
    }
    let result = group.wait(timeout: .now() + 30.0)
    if result == .timedOut {
    ...


    vs.

    Proposed Code:

    let group = DispatchGroup()
    group.enter() 
    DispatchQueue.walletQueue.async {
        if self.walletManager.signTransaction(tx, pin: pin) {
            self.publish(completion: completion)
            success = true 
            group.leave()
        } else {
            didSignTransaction = false 
            group.leave()
        } 
    }
    let result = group.wait(timeout: .now() + 30.0)
    if result == .timedOut {
    ... 

```

    Date/Time:           2020-01-11 14:25:01.5722 +0100
    Launch Time:         2020-01-11 14:12:54.0779 +0100
    OS Version:          iPhone OS 13.3 (17C54)
    Release Type:        User
    Baseband Version:    5.30.01
    Report Version:      104

    Exception Type:  EXC_BREAKPOINT (SIGTRAP)
    Exception Codes: 0x0000000000000001, 0x00000001949ae500
    Termination Signal: Trace/BPT trap: 5
    Termination Reason: Namespace SIGNAL, Code 0x5
    Terminating Process: exc handler [2913]
    Triggered by Thread:  0

    Thread 0 name:
    Thread 0 Crashed:
    0   libdispatch.dylib               0x00000001949ae500 dispatch_group_leave$VARIANT$armv81.cold.1 + 36
    1   libdispatch.dylib               0x00000001949796a8 dispatch_group_leave$VARIANT$armv81 + 112 (semaphore.c:303)
    2   Litewallet                      0x0000000104a72e3c thunk for @escaping @callee_guaranteed () -> () + 28 (<compiler-generated>:0)
    3   libdispatch.dylib               0x00000001949a6610 _dispatch_call_block_and_release + 24 (init.c:1408)
    4   libdispatch.dylib               0x00000001949a7184 _dispatch_client_callout + 16 (object.m:495)
    5   libdispatch.dylib               0x000000019498a34c _dispatch_main_queue_callback_4CF$VARIANT$armv81 + 996 (inline_internal.h:2484)
    6   CoreFoundation                  0x0000000194c585e4 __CFRUNLOOP_IS_SERVICING_THE_MAIN_DISPATCH_QUEUE__ + 12 (CFRunLoop.c:1749)
    7   CoreFoundation                  0x0000000194c535d8 __CFRunLoopRun + 2004 (CFRunLoop.c:3069)
    8   CoreFoundation                  0x0000000194c52adc CFRunLoopRunSpecific + 464 (CFRunLoop.c:3192)
    9   GraphicsServices                0x000000019ebd8328 GSEventRunModal + 104 (GSEvent.c:2246)
    10  UIKitCore                       0x0000000198d4dae0 UIApplicationMain + 1936 (UIApplication.m:4773)
    11  Litewallet                      0x0000000104a5e308 main + 68 (BuyTableViewController.swift:31)
    12  libdyld.dylib                   0x0000000194adc360 start + 4

    Thread 1 name:
    Thread 1:
    0   libsystem_kernel.dylib          0x0000000194ab0634 mach_msg_trap + 8
    1   libsystem_kernel.dylib          0x0000000194aafaa0 mach_msg + 72 (mach_msg.c:103)
    2   CoreFoundation                  0x0000000194c58288 __CFRunLoopServiceMachPort + 216 (CFRunLoop.c:2575)
    3   CoreFoundation                  0x0000000194c533a8 __CFRunLoopRun + 1444 (CFRunLoop.c:2931)
    4   CoreFoundation                  0x0000000194c52adc CFRunLoopRunSpecific + 464 (CFRunLoop.c:3192)
    5   Foundation                      0x0000000194f927f4 -[NSRunLoop(NSRunLoop) runMode:beforeDate:] + 228 (NSRunLoop.m:374)
    6   Foundation                      0x0000000194f926d4 -[NSRunLoop(NSRunLoop) runUntilDate:] + 88 (NSRunLoop.m:421)
    7   UIKitCore                       0x0000000198de6204 -[UIEventFetcher threadMain] + 152 (UIEventFetcher.m:637)
    8   Foundation                      0x00000001950c30a4 __NSThread__start__ + 848 (NSThread.m:724)
    9   libsystem_pthread.dylib         0x00000001949f5d8c _pthread_start + 156 (pthread.c:896)
    10  libsystem_pthread.dylib         0x00000001949f976c thread_start + 8

    Thread 2:
    0   libsystem_pthread.dylib         0x00000001949f9758 start_wqthread + 0

    Thread 3:
    0   libsystem_pthread.dylib         0x00000001949f9758 start_wqthread + 0

    Thread 4:
    0   libsystem_pthread.dylib         0x00000001949f9758 start_wqthread + 0

    Thread 5:
    0   libsystem_pthread.dylib         0x00000001949f9758 start_wqthread + 0

    Thread 6 name:
    Thread 6:
    0   libsystem_kernel.dylib          0x0000000194ab0634 mach_msg_trap + 8
    1   libsystem_kernel.dylib          0x0000000194aafaa0 mach_msg + 72 (mach_msg.c:103)
    2   CoreFoundation                  0x0000000194c58288 __CFRunLoopServiceMachPort + 216 (CFRunLoop.c:2575)
    3   CoreFoundation                  0x0000000194c533a8 __CFRunLoopRun + 1444 (CFRunLoop.c:2931)
    4   CoreFoundation                  0x0000000194c52adc CFRunLoopRunSpecific + 464 (CFRunLoop.c:3192)
    5   CFNetwork                       0x0000000197f094e8 -[__CoreSchedulingSetRunnable runForever] + 192 (CoreSchedulingSet.mm:1372)
    6   Foundation                      0x00000001950c30a4 __NSThread__start__ + 848 (NSThread.m:724)
    7   libsystem_pthread.dylib         0x00000001949f5d8c _pthread_start + 156 (pthread.c:896)
    8   libsystem_pthread.dylib         0x00000001949f976c thread_start + 8

    Thread 7:
    0   libsystem_pthread.dylib         0x00000001949f9758 start_wqthread + 0

    Thread 0 crashed with ARM Thread State (64-bit):
        x0: 0x0000000000000000   x1: 0x0000000000000002   x2: 0x0000000000000002   x3: 0x00000001949c26e8
        x4: 0x0000001400000002   x5: 0x0000001600000002   x6: 0x0000000000000003   x7: 0x0000000000000680
        x8: 0x0000000000000000   x9: 0x0000000000000004  x10: 0x0000000000000000  x11: 0x0000000200000002
       x12: 0x0000000000000100  x13: 0x0000000093a0792e  x14: 0x0000000093c08000  x15: 0x0000000000000010
       x16: 0x0000000194979638  x17: 0x0000000013c00000  x18: 0x0000000000000000  x19: 0x0000000104ac651c
       x20: 0x0000000281b70140  x21: 0x0000000280e8f740  x22: 0x0000000280e803c0  x23: 0x0000000000000000
       x24: 0x0000000002ffffff  x25: 0x0000000280e80600  x26: 0x00000001050718e0  x27: 0x00000000000020ff
       x28: 0x0000000000000114   fp: 0x000000016b3a6ac0   lr: 0x00000001949796a8
        sp: 0x000000016b3a6ac0   pc: 0x00000001949ae500 cpsr: 0x00000000
       esr: 0xf2000001  Address size fault
```

Hard to pinpoint the exact cause of the crash without looking deeper into the source, but the above stack trace is referencing BuyTableViewController.swift:31.

I would use breakpoints to pinpoint the cause, starting with line 31 in BuyTableViewController.swift as the source of the crash.