Crash when using Combine Publisher as AsyncSequence

[Bradleycorn]

I'm using version: 1.0.0-ALPHA-6

I have a Publisher, and I'm converting to an AsyncSequence like this:

let seq = $selectedRace
    .compactMap{$0} // don't emit nil values
    .removeDuplicates { prev, next in
        prev.raceId.description == next.raceId.description
    }
    .map { createPublisher(for: self.trackRepo.getRunners(id: $0.raceId)) }
    .switchToLatest()
    .values



for try await runners in seq {
        print("Got more runners")
        runnerList = runners
}

In the above, $selectedRace is the publisher for an @Published property in an @ObservableObject.
When the published property's value changes, I need to call my Repo with the new value (which returns a Flow) to get a stream of updated data.

When the above code runs, it works briefly (I get "Got more runners" a few times in the console output), but after a few seconds, it crashes. It seems to fail in NativeFlowSubscription, on line 49 of Publisher.swift (see screenshot below).

FWIW, in other areas of my app, I use an AsyncSequence with the same kotlin call (asyncSequence(for: self.trackRepo.getRunners(id: raceId)) and it works fine.

[rickclephas]

Hi, thanks for reporting this!
Could you possibly share a reproduction sample?
I am guessing $selectedRace plays a critical role in this issue.

[Bradleycorn]

@rickclephas It's part of a fairly big project. Let me see if I can put together a small demo to reproduce the issue. It might take me a few days.
In the meantime, here's a larger snippet of the code involved, in case that helps.

class ProgramViewModel: ObservableObject {
 
    @Published var selectedRace: Race? = nil
    
    @Published var runnerList: [Runner] = []

    let races: [Race]

    @MainActor // So that the uiState (which drives the views in the ui) is updated on the main thread.
    func loadProgram() async {
        Task { [self] in
             do {
                 let seq = $selectedRace
                    .compactMap{$0} // don't emit nil values
                    .removeDuplicates { prev, next in
                        prev.raceId.description == next.raceId.description
                    }
                    .map { createPublisher(for: self.trackRepo.getRunners(id: $0.raceId)) }
                    .switchToLatest()
                    .values
 
                for try await runners in seq {
                    print("Got more runners")
                    runnerList = runners
                }
            } catch {
                print("Error!!!!")
            }
        }
    }
}

In the View, selectedRace is used as a binding for a dropdown of Races. When a race is selected in the dropdown, the binding updates the value of selectedRace. Also, a .task modifier on the View calls the ProgramViewModel's loadProgram function.

struct ProgramView: View {
    @StateObject var viewModel = ProgramViewModel()
    
    var body: some View {
         VStack {
              Picker("Select a Race", selection: $viewModel.race) {
                  ForEach(viewModel.races, id: \.self) {
                          Text("Race \($0.raceNumber)")
                 }
             }
             
             List { ... this is a list that displays the runners in viewModel.runnerList }
         }
         .task { await viewModel.loadProgram() }
    }
}

When the view loads up, I pick a race from the dropdown, and everything works for a period of time. Usually a few seconds, sometimes a minute or so. And then I get the crash/error. I don't have to select a different race or anything.

[Bradleycorn]

@rickclephas it's also worth noting ... the Kotlin Flow (trackRepo.getRunners(id: RaceId)) is a Flow that will "never" finish/terminate until canceled. It's polling an API endpoint every X seconds to get updated data (and emit it to the Flow).

[rickclephas]

Hi @Bradleycorn thanks for the additional details. Unfortunately I wasn't able to reproduce the issue.
I am probably missing something in my attempt to reproduce this, so it would be great to have a reproducer for this.

[Bradleycorn]

@rickclephas Sorry, I got sidetracked on some things. I will try to get a demo project together asap.

[Bradleycorn]

@rickclephas I have created a demo project. You can find it here:
https://github.com/Bradleycorn/PublisherDemo

To reproduce the issue, Build and run the included iOS App (I've been using a Simulator on iOS 16.4).
When the app loads choose a value from the displayed Picker. You may have to wait a few seconds and/or select a few Picker values to get it to crash. But it should crash eventually. For me it usually crashes right away.

[rickclephas]

@Bradleycorn thanks again for reporting this and providing the demo project.
Turns out the issue was with the unimplemented demand logic.
.values will request a singel value at a time which wasn't being respected.
The demand/back pressure logic has been added, fixing this crash. I'll publish a new release somewhere this week.

[Bradleycorn]

Thanks @rickclephas !

[rickclephas]

FYI the fix has been released in v1.0.0-ALPHA-15.