Issue using TestScheduler in XCTestCase

[edhirama]

Describe the bug
I'm getting intermittent timeouts in my unit tests when using await scheduler.advance() or await scheduler.run() when running in the CI (Bitrise).

To Reproduce
I've created a simple project to see if I could reproduce the delay using the library, and I could successfully see it hanging in the advance() command.
https://github.com/edhirama/CombineSchedulersPOC

// And/or enter code that reproduces the behavior here.
final class CombineSchedulersPOCTests: XCTestCase {

    func test() async {
        let startTime = CFAbsoluteTimeGetCurrent()
        var testSubject = PassthroughSubject<Bool, Error>()
        let test = DispatchQueue.test

        print("##### INITIALIZED #####")
        print(CFAbsoluteTimeGetCurrent() - startTime)

        testSubject.send(true)
        print("##### sent true #####")
        print(CFAbsoluteTimeGetCurrent() - startTime)

        testSubject.send(completion: .finished)
        print("##### sent finished #####")
        print(CFAbsoluteTimeGetCurrent() - startTime)

        await test.advance()
        print("##### advance #####")
        print(CFAbsoluteTimeGetCurrent() - startTime)
    }

}

Expected behavior
It should not hang in the await test.advance() call.

Screenshots

Environment

• Xcode 16.2
• Swift 5
• OS (if applicable): iOS 18.0

Additional context
I don't know if it's an issue when using only the async/await version of the advance function or if it's a misuse. I'd love to get more clarification on how to fix this, since it looks like in Xcode 16 it got worse (I'd see it hanging in Xcode 15, but now CI failures are happening with a higher frequency).

[mbrandonw]

Hi @edhirama, this is just due to the fact that async/await is still not very testable in Swift, and so we are forced to resort to tricks like this megaYield to make sure work has time to execute:

combine-schedulers/Sources/CombineSchedulers/TestScheduler.swift

Line 136 in 5928286

await Task.megaYield()

If you want to invoke the non-async version of advance you can always wrap it in a synchronous closure and invoke it:

func test() async {
  let startTime = CFAbsoluteTimeGetCurrent()
  let test = DispatchQueue.test
  _ = { test.advance() }()
  print("##### advance #####")
  print(CFAbsoluteTimeGetCurrent() - startTime)
}

Since this isn't an issue with the library I am going to convert it to a discussion. Please feel free to continue the conversation over there!