Crash: Simultaneous accesses, but modification requires exclusive access

[kevinrenskers]

ReactiveKit 3.16.3 (via Cocoapods).
Xcode 11.3.1
iOS 13.3

Simultaneous accesses to 0x6000024dd210, but modification requires exclusive access.
Previous access (a modification) started at ReactiveKit`AtomicObserver.dispose() + 135 (0x1097499c7).
Current access (a read) started at:
0    libswiftCore.dylib                 0x000000010b9a1590 swift_beginAccess + 568
1    ReactiveKit                        0x0000000109749110 AtomicObserver.on(_:) + 249
2    ReactiveKit                        0x0000000109749c60 protocol witness for ObserverProtocol.on(_:) in conformance AtomicObserver<A, B> + 16
3    ReactiveKit                        0x0000000109749f20 ObserverProtocol.receive(completion:) + 594
4    ReactiveKit                        0x0000000109777120 closure #1 in closure #1 in SignalProtocol<>.castError<A>() + 750
5    ReactiveKit                        0x0000000109749110 AtomicObserver.on(_:) + 1044
6    ReactiveKit                        0x0000000109749c60 protocol witness for ObserverProtocol.on(_:) in conformance AtomicObserver<A, B> + 16
7    ReactiveKit                        0x0000000109749f20 ObserverProtocol.receive(completion:) + 594
8    ReactiveKit                        0x000000010978de40 closure #1 in closure #1 in SignalProtocol<>.ignoreNils() + 1704
9    ReactiveKit                        0x0000000109749110 AtomicObserver.on(_:) + 1044
10   ReactiveKit                        0x0000000109749c60 protocol witness for ObserverProtocol.on(_:) in conformance AtomicObserver<A, B> + 16
11   ReactiveKit                        0x0000000109749f20 ObserverProtocol.receive(completion:) + 594
12   ReactiveKit                        0x0000000109785600 closure #1 in closure #1 in SignalProtocol.map<A>(_:) + 1470
13   ReactiveKit                        0x0000000109749110 AtomicObserver.on(_:) + 1044
14   ReactiveKit                        0x0000000109749c60 protocol witness for ObserverProtocol.on(_:) in conformance AtomicObserver<A, B> + 16
15   ReactiveKit                        0x0000000109749f20 ObserverProtocol.receive(completion:) + 594
16   ReactiveKit                        0x000000010978de40 closure #1 in closure #1 in SignalProtocol<>.ignoreNils() + 1704
17   ReactiveKit                        0x0000000109749110 AtomicObserver.on(_:) + 1044
18   ReactiveKit                        0x0000000109749c60 protocol witness for ObserverProtocol.on(_:) in conformance AtomicObserver<A, B> + 16
19   ReactiveKit                        0x0000000109749f20 ObserverProtocol.receive(completion:) + 594
20   ReactiveKit                        0x0000000109795940 closure #1 in closure #1 in SignalProtocol.scan<A>(_:_:) + 1906
21   ReactiveKit                        0x0000000109796fb0 partial apply for closure #1 in closure #1 in SignalProtocol.scan<A>(_:_:) + 66
22   ReactiveKit                        0x0000000109749110 AtomicObserver.on(_:) + 1044
23   ReactiveKit                        0x0000000109749c60 protocol witness for ObserverProtocol.on(_:) in conformance AtomicObserver<A, B> + 16
24   ReactiveKit                        0x0000000109749f20 ObserverProtocol.receive(completion:) + 594
25   ReactiveKit                        0x0000000109795940 closure #1 in closure #1 in SignalProtocol.scan<A>(_:_:) + 1906
26   ReactiveKit                        0x0000000109796fb0 partial apply for closure #1 in closure #1 in SignalProtocol.scan<A>(_:_:) + 66
27   ReactiveKit                        0x0000000109748bb0 thunk for @escaping @callee_guaranteed (@in_guaranteed Signal<A, B>.Event) -> () + 12
28   ReactiveKit                        0x00000001097af3a0 partial apply for thunk for @escaping @callee_guaranteed (@in_guaranteed Signal<A, B>.Event) -> () + 54
29   ReactiveKit                        0x00000001097a4df0 thunk for @escaping @callee_guaranteed (@in_guaranteed Signal<A, B>.Event) -> (@out ()) + 12
30   ReactiveKit                        0x00000001097a48c0 Subject.on(_:) + 1234
31   ReactiveKit                        0x00000001097a61e0 PassthroughSubject.on(_:) + 113
32   ReactiveKit                        0x00000001097a5ab0 protocol witness for ObserverProtocol.on(_:) in conformance Subject<A, B> + 24
33   ReactiveKit                        0x00000001097a3830 SubjectProtocol.send(completion:) + 623
34   ReactiveKit                        0x000000010974c240 Property.deinit + 126
35   ReactiveKit                        0x000000010974c310 Property.__deallocating_deinit + 39
36   libswiftCore.dylib                 0x000000010b9a2f00 _swift_release_dealloc + 16
37   LastFm                             0x0000000107e20020 ReportsViewModel.deinit + 33
38   LastFm                             0x0000000107e20050 ReportsViewModel.__deallocating_deinit + 25
39   libswiftCore.dylib                 0x000000010b9a2f00 _swift_release_dealloc + 16
40   LastFm                             0x0000000107e30070 outlined assign with take of ReportPeriod + 469
41   libswiftCore.dylib                 0x000000010b9a2f00 _swift_release_dealloc + 16
42   LastFm                             0x0000000107e1dd40 thunk for @escaping @callee_guaranteed (@in_guaranteed ReportPeriod) -> (@owned Signal<ReportsViewModel.Status, Never>) + 69
43   libswiftCore.dylib                 0x000000010b9a2f00 _swift_release_dealloc + 16
44   ReactiveKit                        0x000000010978ce00 outlined destroy of Subscribers.Completion<A1.Error> + 138
45   libswiftCore.dylib                 0x000000010b9a2f00 _swift_release_dealloc + 16
46   ReactiveKit                        0x0000000109748f60 outlined consume of @escaping @callee_guaranteed (@in_guaranteed Signal<A, B>.Event) -> ()?<A, B> + 27
47   ReactiveKit                        0x0000000109749940 AtomicObserver.dispose() + 168
48   ReactiveKit                        0x0000000109749c80 protocol witness for Disposable.dispose() in conformance AtomicObserver<A, B> + 16
49   ReactiveKit                        0x0000000109749b00 closure #1 in AtomicObserver.dispose() + 74
50   libswiftCore.dylib                 0x000000010b8208b0 Sequence.forEach(_:) + 452
51   ReactiveKit                        0x0000000109749940 AtomicObserver.dispose() + 377
52   ReactiveKit                        0x0000000109749c80 protocol witness for Disposable.dispose() in conformance AtomicObserver<A, B> + 16
53   ReactiveKit                        0x0000000109731900 closure #1 in CompositeDisposable.dispose() + 74
54   libswiftCore.dylib                 0x000000010b8208b0 Sequence.forEach(_:) + 452
55   ReactiveKit                        0x00000001097316f0 CompositeDisposable.dispose() + 399
56   ReactiveKit                        0x0000000109731a50 protocol witness for Disposable.dispose() in conformance CompositeDisposable + 16
57   ReactiveKit                        0x0000000109749b00 closure #1 in AtomicObserver.dispose() + 74
58   libswiftCore.dylib                 0x000000010b8208b0 Sequence.forEach(_:) + 452
59   ReactiveKit                        0x0000000109749940 AtomicObserver.dispose() + 377
60   ReactiveKit                        0x0000000109749c80 protocol witness for Disposable.dispose() in conformance AtomicObserver<A, B> + 16
61   ReactiveKit                        0x0000000109728840 closure #1 in closure #1 in ConnectableSignalProtocol.refCount() + 715
62   ReactiveKit                        0x000000010972fee0 BlockDisposable.dispose() + 360
63   ReactiveKit                        0x0000000109730140 protocol witness for Disposable.dispose() in conformance BlockDisposable + 16
64   ReactiveKit                        0x0000000109749b00 closure #1 in AtomicObserver.dispose() + 74
65   libswiftCore.dylib                 0x000000010b8208b0 Sequence.forEach(_:) + 452
66   ReactiveKit                        0x0000000109749940 AtomicObserver.dispose() + 377
67   ReactiveKit                        0x0000000109749c80 protocol witness for Disposable.dispose() in conformance AtomicObserver<A, B> + 16
68   ReactiveKit                        0x0000000109731900 closure #1 in CompositeDisposable.dispose() + 74
69   libswiftCore.dylib                 0x000000010b8208b0 Sequence.forEach(_:) + 452
70   ReactiveKit                        0x00000001097316f0 CompositeDisposable.dispose() + 399
71   ReactiveKit                        0x0000000109731a50 protocol witness for Disposable.dispose() in conformance CompositeDisposable + 16
72   ReactiveKit                        0x0000000109749720 closure #1 in AtomicObserver.on(_:) + 74
73   libswiftCore.dylib                 0x000000010b8208b0 Sequence.forEach(_:) + 452
74   ReactiveKit                        0x0000000109749110 AtomicObserver.on(_:) + 867
75   ReactiveKit                        0x0000000109749c60 protocol witness for ObserverProtocol.on(_:) in conformance AtomicObserver<A, B> + 16
76   ReactiveKit                        0x0000000109749f20 ObserverProtocol.receive(completion:) + 594
77   ReactiveKit                        0x0000000109782780 closure #1 in closure #1 in SignalProtocol.prefix<A>(untilOutputFrom:) + 371
78   ReactiveKit                        0x0000000109749110 AtomicObserver.on(_:) + 1044
79   ReactiveKit                        0x0000000109749c60 protocol witness for ObserverProtocol.on(_:) in conformance AtomicObserver<A, B> + 16
80   ReactiveKit                        0x000000010974adf0 partial apply + 83
81   ReactiveKit                        0x0000000109748bb0 thunk for @escaping @callee_guaranteed (@in_guaranteed Signal<A, B>.Event) -> () + 12
82   ReactiveKit                        0x00000001097af3a0 partial apply for thunk for @escaping @callee_guaranteed (@in_guaranteed Signal<A, B>.Event) -> () + 54
83   ReactiveKit                        0x00000001097a4df0 thunk for @escaping @callee_guaranteed (@in_guaranteed Signal<A, B>.Event) -> (@out ()) + 12
84   ReactiveKit                        0x00000001097a48c0 Subject.on(_:) + 1234
85   ReactiveKit                        0x00000001097a7bd0 ReplayOneSubject.on(_:) + 1026
86   ReactiveKit                        0x00000001097a5ab0 protocol witness for ObserverProtocol.on(_:) in conformance Subject<A, B> + 24
87   ReactiveKit                        0x00000001097a3830 SubjectProtocol.send(completion:) + 623
88   ReactiveKit                        0x00000001097332d0 DisposeBag.deinit + 222
89   ReactiveKit                        0x00000001097334a0 DisposeBag.__deallocating_deinit + 25
90   libswiftCore.dylib                 0x000000010b9a2f00 _swift_release_dealloc + 16
91   libobjc.A.dylib                    0x00007fff5140d3d1 _object_remove_assocations + 239
92   libobjc.A.dylib                    0x00007fff5140a473 objc_destructInstance + 79
93   libobjc.A.dylib                    0x00007fff5141093a -[NSObject dealloc] + 21
94   UIKitCore                          0x00007fff480bf631 -[UIResponder dealloc] + 145
95   UIKit                              0x000000010c490925 -[UIResponderAccessibility dealloc] + 55
96   UIKitCore                          0x00007fff47a0f887 -[UIViewController dealloc] + 1392
97   libobjc.A.dylib                    0x00007fff514126c8 AutoreleasePoolPage::releaseUntil(objc_object**) + 147
98   libobjc.A.dylib                    0x00007fff514125b3 objc_autoreleasePoolPop + 199
99   UIKitCore                          0x00007fff47a0f887 -[UIViewController dealloc] + 1359
100  UIKitCore                          0x00007fff47940244 -[UITabBarController dealloc] + 689
101  libobjc.A.dylib                    0x00007fff514126c8 AutoreleasePoolPage::releaseUntil(objc_object**) + 147
102  libobjc.A.dylib                    0x00007fff514125b3 objc_autoreleasePoolPop + 199
103  CoreFoundation                     0x00007fff23c92380 _CFAutoreleasePoolPop + 22
104  UIKitCore                          0x00007fff4808c43a _prepareForCAFlush + 129
105  UIKitCore                          0x00007fff480bc354 _afterCACommitHandler + 140
106  CoreFoundation                     0x00007fff23bd3850 __CFRUNLOOP_IS_CALLING_OUT_TO_AN_OBSERVER_CALLBACK_FUNCTION__ + 23
107  CoreFoundation                     0x00007fff23bce150 __CFRunLoopDoObservers + 430
108  CoreFoundation                     0x00007fff23bce390 __CFRunLoopRun + 1514
109  CoreFoundation                     0x00007fff23bcdeb0 CFRunLoopRunSpecific + 438
110  GraphicsServices                   0x00007fff384c0b6f GSEventRunModal + 65
111  UIKitCore                          0x00007fff480926f8 UIApplicationMain + 1621
112  LastFm                             0x0000000107e93e20 main + 75
113  libdyld.dylib                      0x00007fff5227ec24 start + 1
(lldb) 

[kevinrenskers]

Updated to 3.17.0 (it's not on CocoaPods so switched it to SPM), still the same problem.

[TeunVR]

Same problem here :(
I have a bit complex combineLatest of some properties.
The object containing the properties is deallocated when the disposeBag is emptied that contains the disposable for the combineLatest-subscription.

If i remove the subject.send(completion: .finished) from the Property.deinit the crash goes away.
Might be something in my own code, i'll try to make a demo-project.

[lmcd]

I'm seeing a similar issue in Atomic.swift, not sure if related:

As far as I tell, everything is happening on the main thread. I'm not setting up any observers elsewhere

[TeunVR]

Ignore my previous reaction. Was my own fault.
I was indirectly capturing the property being observed inside the onNext.
Works ok for me now.

[srdanrasic]

My assumption is that these are valid problems caused by the way ReactiveKit is used, caused by reasons like @TeunVR had. For example mutating a property that trigged the event from the observer or binding of that property is not a valid usage and is expected to throw a simultaneous accesses error.

[lmcd]

Yeah turns out merely referencing the property inside an observer is enough to crash once it's disposed. Fixed, thanks for clarifiying.

[kevinrenskers]

Not sure where my problem could come from. My VC is something like this:

class ReportsContainerViewController: UIViewController {
  private let viewModel = ReportsViewModel()

  private func setupBindings() {
    viewModel.statusSignal.bind(to: self) { vc, status in
      // Do things
    }
  }
}

Now, the crash happens when the user logs out: that results in the root window's rootViewController getting set to a login VC. That in turn causes the ReportsContainerViewController to be released, and that releases the ViewModel.

[kevinrenskers]

Okay, I found the problem.

This is my ViewModel:

class ReportsViewModel {
  enum Status {
    case loading
    case loaded(data: ReportData)
    case error(error: ReportsViewModelError)
  }

  public var period = Property<ReportPeriod>(ReportPeriod(rawValue: 0))
  public var statusSignal: Signal<Status, Never> {
    return period.removeDuplicates().flatMapLatest(getData)
  }

  private let urlCache: URLCache
  private let sessionManager: SessionManager

  init() {
    urlCache = URLCache(memoryCapacity: 20 * 1024 * 1024, diskCapacity: 200 * 1024 * 1024, diskPath: nil)

    let configuration = URLSessionConfiguration.default
    configuration.requestCachePolicy = .returnCacheDataElseLoad
    configuration.urlCache = urlCache
    configuration.timeoutIntervalForRequest = 15
    sessionManager = Alamofire.SessionManager(configuration: configuration)
  }

  private func getData(period: ReportPeriod) -> Signal<Status, Never> {
    // ...
  }
}

If I change the statusSignal to no longer be a computed property but initialized in the init, no more crash.

class ReportsViewModel {
  enum Status {
    case loading
    case loaded(data: ReportData)
    case error(error: ReportsViewModelError)
  }

  public var period = Property<ReportPeriod>(ReportPeriod(rawValue: 0))
  public var statusSignal: Signal<Status, Never>!

  private let urlCache: URLCache
  private let sessionManager: SessionManager

  init() {
    urlCache = URLCache(memoryCapacity: 20 * 1024 * 1024, diskCapacity: 200 * 1024 * 1024, diskPath: nil)

    let configuration = URLSessionConfiguration.default
    configuration.requestCachePolicy = .returnCacheDataElseLoad
    configuration.urlCache = urlCache
    configuration.timeoutIntervalForRequest = 15
    sessionManager = Alamofire.SessionManager(configuration: configuration)

    statusSignal = period.removeDuplicates().flatMapLatest(getData)
  }

  private func getData(period: ReportPeriod) -> Signal<Status, Never> {
    // ...
  }
}

The reason why I did it like the first way first, was because I was getting 'self' used in method call 'getData' before all stored properties are initialized errors from the compiler, when statusSignal was not an implicitly unwrapped variable, which I wanted to avoid. Oh well :)