I have a publisher, pipelinePublisher, which runs a combine pipeline of various operations, some of which send a state update to a statePublisher passed in as an argument. pipelinePublisher gets removed on completion of its Combine pipeline:
func myFunction(_ request: MyRequest) -> PassthroughSubject<State, Never> {
let statePublisher = PassthroughSubject<State, Never>()
let presentationSubject = CurrentValueSubject<MyRequest, Error>(request)
var pipelinePublisher: AnyCancellable!
pipelinePublisher = presentationSubject
.eraseToAnyPublisher()
.checkSomething(returningStateTo: statePublisher)
// a few more operators here...
.sink(
receiveCompletion: { [weak self] _ in
self?.cancellables.remove(pipelinePublisher) // Crash happens here
},
receiveValue: { _ in }
)
pipelinePublisher.store(in: &cancellables)
return statePublisher
.receive(on: RunLoop.main)
.eraseToAnyPublisher()
}
However, very occasionally when I call the function multiple times in very quick succession, the app crashes on the line self?.cancellables.remove(pipelinePublisher).
This usually brings up one of two possible stack traces. This first is this:
2022-12-21 15:24:51.926131+0000 MyApp[23082:12933690] -[_NSCoreDataTaggedObjectID member:]: unrecognized selector sent to instance 0x8000000000000000
2022-12-21 15:24:51.931941+0000 MyApp[23082:12933690] *** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '-[_NSCoreDataTaggedObjectID member:]: unrecognized selector sent to instance 0x8000000000000000'
*** First throw call stack:
(
0 CoreFoundation 0x000000018040e7c8 __exceptionPreprocess + 172
1 libobjc.A.dylib 0x0000000180051144 objc_exception_throw + 56
2 CoreFoundation 0x000000018041d47c +[NSObject(NSObject) instanceMethodSignatureForSelector:] + 0
3 CoreFoundation 0x00000001804126c8 ___forwarding___ + 1308
4 CoreFoundation 0x0000000180414b4c _CF_forwarding_prep_0 + 92
5 libswiftCore.dylib 0x000000018be6ee68 $sSh8_VariantV6removeyxSgxF + 160
6 MyApp 0x00000001026c6080 $s12MyApp0A0C17myFunctiony7Combine12AnyPublisherVyAA12StateOs5NeverOGAA19MyRequestVFyAE11SubscribersO10CompletionOy_s5Error_pGcfU_ + 440
7 Combine 0x000000019baa2a70 $s7Combine11SubscribersO4SinkC7receive10completionyAC10CompletionOy_q_G_tF + 364
8 Combine 0x000000019baa2f28 $s7Combine11SubscribersO4SinkCy_xq_GAA10SubscriberA2aGP7receive10completionyAC10CompletionOy_7FailureQzG_tFTW + 20
9 Combine 0x000000019bb541cc $s7Combine10PublishersO7FlatMapV5Outer33_E91C3F00A6DFAAFEA2009FAF507AE039LLC7receive10completionyAA11SubscribersO10CompletionOy_7FailureQzG_tF + 1516
10 Combine 0x000000019bb55328 $s7Combine10PublishersO7FlatMapV5Outer33_E91C3F00A6DFAAFEA2009FAF507AE039LLCy_xq__qd__GAA10SubscriberA2aJP7receive10completionyAA11SubscribersO10CompletionOy_7FailureQzG_tFTW + 20
11 Combine 0x000000019bb53474 $s7Combine10PublishersO7FlatMapV5Outer33_E91C3F00A6DFAAFEA2009FAF507AE039LLC12receiveInner10completion_yAA11SubscribersO10CompletionOy_7FailureQzG_SitF + 1668
12 Combine 0x000000019bb52de4 $s7Combine10PublishersO7FlatMapV5Outer33_E91C3F00A6DFAAFEA2009FAF507AE039LLC4SideV7receive10completionyAA11SubscribersO10CompletionOy_7FailureQzG_tF + 20
13 Combine 0x000000019bac45ec $s7Combine6FutureC7Conduit33_3AE68DE9BADC00342FC052FEBC7D3BA6LLC7fulfillyys6ResultOyxq_GF + 1056
14 Combine 0x000000019bac4960 $s7Combine6FutureC7Conduit33_3AE68DE9BADC00342FC052FEBC7D3BA6LLC6finish10completionyAA11SubscribersO10CompletionOy_q_G_tF + 336
15 Combine 0x000000019bac2de4 $s7Combine6FutureC7promise33_3AE68DE9BADC00342FC052FEBC7D3BA6LLyys6ResultOyxq_GFyAA11ConduitBaseCyxq_GXEfU0_ + 156
16 Combine 0x000000019bac6b28 $s7Combine6FutureC7promise33_3AE68DE9BADC00342FC052FEBC7D3BA6LLyys6ResultOyxq_GFyAA11ConduitBaseCyxq_GXEfU0_TA + 16
17 Combine 0x000000019bae5140 $s7Combine11ConduitListO7forEachyyyAA0B4BaseCyxq_GKXEKF + 212
18 Combine 0x000000019bac2bfc $s7Combine6FutureC7promise33_3AE68DE9BADC00342FC052FEBC7D3BA6LLyys6ResultOyxq_GF + 716
19 Combine 0x000000019bac6b08 $s7Combine6FutureCyACyxq_Gyys6ResultOyxq_GcccfcyAGcfU_TA + 20
20 MyApp 0x0000000102541dd8 $s7Combine6FutureC12MyApps5Error_pRs_rlE9operationACyxsAE_pGxyYaKc_tcfcyys6ResultOyxsAE_pGccfU_yyYaYbcfU_TY2_ + 212
21 MyApp 0x0000000102542705 $s7Combine6FutureC12MyApps5Error_pRs_rlE9operationACyxsAE_pGxyYaKc_tcfcyys6ResultOyxsAE_pGccfU_yyYaYbcfU_TATQ0_ + 1
22 MyApp 0x000000010242f1a1 $sxIeghHr_xs5Error_pIegHrzo_s8SendableRzs5NeverORs_r0_lTRTQ0_ + 1
23 MyApp 0x000000010242f749 $sxIeghHr_xs5Error_pIegHrzo_s8SendableRzs5NeverORs_r0_lTRTA.24TQ0_ + 1
24 libswift_Concurrency.dylib 0x00000001b03bedcd _ZL23completeTaskWithClosurePN5swift12AsyncContextEPNS_10SwiftErrorE + 1
)
libc++abi: terminating with uncaught exception of type NSException
The second is in the same place but with the error: *** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '-[__NSCFNumber member:]: unrecognized selector sent to instance 0x8000000000000000'
What is causing this? I've tried making pipelinePublisher optional and having a check that it exists before it gets removed but it does actually exist and still crashes. I can't figure this out, please help!
EDIT
I am calling myFunction(_:) by using the method foo which gets a publisher.
static func publisher(forParam: String) -> AnyPublisher<State, Never> {
return Future {
// Do some stuff here
return objects
}
.flatMap { objects in
let request = MyRequest(objects)
return shared.myFunction(request)
}
.eraseToAnyPublisher()
}
static func foo(
param: String,
handler: ((State) -> Void)? = nil
) {
var cancellable: AnyCancellable!
cancellable = publisher(forParam: param)
.sink(
receiveCompletion: { _ in
self.shared.fooItems.cancellables.remove(cancellable) // sometimes crashes here too with the exact same crash!
}, receiveValue: { state in
handler?(state)
}
)
cancellable.store(in: &shared.fooItems.cancellables)
}
A few things to note:
Foo has to use completion block as part of an API.publisher(forParam:) and therefore myFunction(_:) are only called from foo. However foo is called from many places.pipelinePublisher cancels by an error that’s thrown at the same time as sending a state and a completion through to foo.As you can see above, foo also sometimes gets the same error when removing the cancellable.
With help from people in the comments of my question, it looks like I had a race condition possibly caused by threads interacting with each other. I was removing from a Set, but Sets aren't thread safe. I solved this by changing the code to use Subscribers.Sink:
func myFunction(_ request: MyRequest) -> PassthroughSubject<State, Never> {
let statePublisher = PassthroughSubject<State, Never>()
let presentationSubject = CurrentValueSubject<MyRequest, Error>(request)
var pipelinePublisher: AnyCancellable!
pipelinePublisher = presentationSubject
.eraseToAnyPublisher()
.checkSomething(returningStateTo: statePublisher)
// a few more operators here...
.subscribe(Subscribers.Sink(
receiveCompletion: { _ in },
receiveValue: { _ in }
))
pipelinePublisher.store(in: &cancellables)
return statePublisher
.receive(on: RunLoop.main)
.eraseToAnyPublisher()
}
This fixed the problem. Combine handles the clean up of the subscription and the subscriber when the pipeline returns.
This was taken from the answer given here: With Combine, how to deallocate the Subscription after a network request