SBTProxyURLProtocol calls completionHandler in the main thread

[Fernando]

Hi, I've noticed that SBTProxyURLProtocol.startLoading returns the response for the subbed request in the main thread:

dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(stubbingResponseTime * NSEC_PER_SEC)), dispatch_get_main_queue(), ^{
    ....
}

Can we use dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0) or any other queue that's not the main thread instead (or at least allow customization)? There are scenarios in my suite where I mock HW by using mocks within the main target whose values are customized from a UI-Test. For this, I use dummy http requests. The mocked functions can be async or sync. In order for the sync methods to work I need to block the main thread. However the test hangs indefinitely because the main thread is blocked and SBTProxyURLProtocol tries to return the data into the main queue.

I don't think the change I suggest breaks anything and I think is more accurate as you'd expect the response to come from a background thread instead of the main thread.

[Tomas Camin]

Modifying that queue would likely lead to thread-safety issues, so I would recommend against such changes. Instead of blocking the main thread with a semaphore, consider using the run loop (specifically, [NSRunLoop.mainRunLoop runUntilDate:]). While this approach isn't ideal, it could serve as a viable workaround.

[Fernando]

Modifying that queue would likely lead to thread-safety issues
startLoading is already called in a different thread than it's then dispatching the response (in the main queue now) so I don't understand why changing to a different queue would introduce any issue. At the very least I think it should use dispatch_get_current_queue.

consider using the run loop
how would you do that? I do something like this:

func foo<Output>(
        for call: @escaping (@escaping (Output) -> Void) -> Any
    ) -> Output {
        let completionQueue = DispatchQueue(label: "com.test.UITest", qos: .background)

        var output: Output?
        completionQueue.async {
            call { opResult in
                output = opResult
            }
        }

        RunLoop.main.run(until: Date(timeIntervalSinceNow: 10))

        guard let output else {
            fatalError("Shouldn't time out")
        }

        return output
    }

But same difference, UI hangs for 10 seconds until it fails. Main thread is blocked so SBTProxyURLProtocol doesn't get called

[Tomas Camin]

You need to invoke the RunLoop.main.run repeatedly until you detect that your operation is completed. There are a few examples in the tunnel codebase to take a look at.

[Fernando]

@tcamin from what I see, it's really easy to enter a deadlock (see here), which I think it's my case and it's not something very difficult to achieve.

I still don't understand why SBTProxyURLProtocol.startLoading has to be dispatched in the main queue. I've tried replacing the source code myself and it works just fine if a different queue is used instead