Moya: Crash when using Combine + stubs

[jshier]

Using the new Combine integration in 15.0.0-alpha.1, there is a crash in requestPublisher when using local stubs using Xcode 11.6, iOS 13.6 simulator.

``* thread #1, queue = 'com.apple.main-thread', stop reason = EXC_BAD_INSTRUCTION (code=EXC_I386_INVOP, subcode=0x0) frame #0: 0x00007fff235a9aca CombineCombine.Publishers.Catch.(Inner in _4B811D2092663748FE3021A2BDB676AF).receivePre(A.Output) -> Combine.Subscribers.Demand + 186
frame #1: 0x00007fff235a99ff CombineCombine.Publishers.Catch.(Inner in _4B811D2092663748FE3021A2BDB676AF).UncaughtS.receive(A.Output) -> Combine.Subscribers.Demand + 15 frame #2: 0x00007fff235ab567 Combinemerged protocol witness for Combine.Subscriber.receive(A.Input) -> Combine.Subscribers.Demand in conformance Combine.Publishers.Catch.(Inner in _4B811D2092663748FE3021A2BDB676AF).UncaughtS : Combine.Subscriber in Combine + 39
frame #3: 0x00007fff23510703 CombineCombine.AnySubscriberBox.receive(A.Input) -> Combine.Subscribers.Demand + 35 frame #4: 0x00007fff23511982 CombineCombine.AnySubscriber.receive(A) -> Combine.Subscribers.Demand + 18

• frame #5: 0x000000010620d1ea Moya`closure #1 in closure #1 in MoyaProvider.requestPublisher(result=success, subscriber=Combine.AnySubscriber @ 0x00007b1400036bb0) at MoyaProvider+Combine.swift:21:40

I'm not familiar enough with Combine to speculate about the cause here. According to the frame, `subscriber` has the value:

(AnySubscriber) subscriber = {
box = 0x00007b0800066880 {}
descriptionThunk = 0x00007fff23511e20 Combinepartial apply forwarder for closure #1 () -> Swift.String in Combine.AnySubscriber.init<A where A == A1.Input, B == A1.Failure, A1: Combine.Subscriber>(A1) -> Combine.AnySubscriber<A, B> customMirrorThunk = 0x00007fff23511d20 Combinepartial apply forwarder for closure #3 () -> Swift.Mirror in Combine.AnySubscriber.init(A1) -> Combine.AnySubscriber
playgroundDescriptionThunk = 0x00007fff23511de0 Combine`partial apply forwarder for closure #4 () -> Any in Combine.AnySubscriber.init(A1) -> Combine.AnySubscriber
combineIdentifier = {}
}

My testing code looks something like this:

```swift
func testThatValueCanBeFetchedAndObservedThroughValueProperty() {
    // Given
    let api = API(endpointClosure: API.all,
                 stubClosure: MoyaProvider.immediatelyStub)
    let expect = expectation(description: "value should be fetched")
    var value: Value?

    // When
    store {
       api.$value
            .compactMap { $0 }
            .sink { value = $0; expect.fulfill() }
    }
    api.refresh()

    waitForExpectations(timeout: timeout)

    // Then
    XCTAssertNotNil(value)
}

Where API has @Published properties which receive the output of the Moya network calls and refresh() calls a common function.

Using the app in the simulator works just fine, so this issue seems to be limited to testing.

[jshier]

Also getting the same crash in the .failure case, but getting more info in that case:

Thread 1: Fatal error: Unexpected state: received completion but do not have subscription

We're investigating whether this may be sim related.

[jshier]

Further investigation shows that this crash is likely due to a race between the stubbed response and the demand from the subscription being fully processed. Since MoyaPublisher.Subscription calls callback immediately, the request is finished before the subscription is fully received.

[jshier]

Yeah, it's definitely the race, as replacing immediatelyStub with delayedStub(0.1) fixes the issue. If I have time I'll look into a proper fix PR, but I need to finish this work first.

[jshier]

delayedStub(0) works as well, so really we just need the run loop to progress so the subscription is properly processed. Even though MoyaPublisher doesn't care about demand, it should wait until a demand is issued before executing the callback closure.