Swift Async/Await testing crashes in unit test

Question

there is a LocationRepo class that gets a CoreDataWorker injected. This class has a function fetchAllLocations that exectues a get on the worker to retrieve data.

So far, so good - the worker's get function and the worker itself are under test and working without problems.

However, when calling the fetchAllLocations function from a test, then the test hangs first, before crashing with this message:

Message from debugger: Terminated due to signal 9
Program ended with exit code: 9

The SUT:

public class LocationRepo: LocationRepository {
    
    let worker: CoreDataWorkerProtocol
    private var currentLocation: Location?
    
    public init(worker: CoreDataWorkerProtocol = CoreDataWorker()) {

        self.worker = worker
    }
    
    public func fetchAllLocations() async -> Result<[Location], Error> {
        
        debugPrint(self.worker)
        
        let result: Result<[Location], Error> = await worker.get(
            with: nil,
            sortDescriptors: nil,
            fetchLimit: nil
        )
        
        switch result {
            case .success(let locations):
                return .success(locations)
                
            case .failure(let error):
                debugPrint(error)
                return .failure(error)
        }
    }
...
}

The unit test:

class LocationRepoTests: XCTestCase {

    func testLocationRepo_fetchAll() async throws {
        
        Resolver.reset()
        Resolver.registerPersistence(persistenceServiceEnvironment: .previews)

        let persistenceService: PersistenceService = Resolver.resolve()
        let worker = CoreDataWorker(persistenceService: persistenceService)
        
        
        let repo = LocationRepo(worker: worker)

        let result = await repo.fetchAllLocations()

        switch result {
            case .success(let list):
                XCTAssertTrue(list.isEmpty)

            default:
                XCTFail("Unexpected result")
        }
    }
}

Calling the content of the fetchAllLocations function from the test works fine.

So: Why does that wrapping function fetchAllLocations lead to crash? Is it a bug in Xcode, or am I doing something wrong?

Answer 1

I noticed a continuously increasing memory usage during the "hanging" phase of the crash. This lead me on the right track: There was a protocol extension that was triggered by the LocationRepo's "get" call, that caused a recursion. However, what is strange, is that this situation did not happen when the call itself was put directly into the unit test. So, I fixed the problem, but I don't understand why it occurs in one situation, and not in another...