iOS - How to understand and properly debug this type of crash : com.apple.root.background-qos?

Question

I know this question has been already asked several times, but either it stayed unanswered or the answer was totally not helping.

Here is my crash report :

This question and especially the comments helped me understand (kind of) my issue.

It seems from your crash log com.apple.root.background-qos, this is the thread request which might be over committed due to which it exist as it cannot take any more request.

If I understand it correctly, it may be possible that the thread handling requests might not be able to handle more requests ?

How do I debug and test this ? I am unable to reproduce the crash.

NB: In my launcher service I am doing 5 or 6 requests each one separated in a dispatchGroup.enter / .leave when finished, like this :

dispatchGroup.enter()
self.fetchObservationForecast(lat: lat, lon: lon) { result in
     switch result {
     case .success(let observationForecast) :
          print("Observation Forecast request succeeded")
          model["observationForecast"] = observationForecast
          break
     case .failure(let error) :
          print("Observation Forecast request failed : \(error)")
          model["observationForecast"] = error
          break
     }
     dispatchGroup.leave()
}

Am I looking at the wrong place ? How can I test and reproduce it in order to submit a fix ?

Any help would be appreciated.

Answer 1

The crash is coming from something in the closures in the LauncherService methods indicated. The trace indicates that you're crashing on setting a Dictionary value.

Mutable Swift Dictionaries are not thread-safe, and that fact along with the background.qos indictor make me think you're reading/writing a Dictionary on multiple threads.

It's probably something like the model["observationForecast"] = code below doing that.

dispatchGroup.enter()
self.fetchObservationForecast(lat: lat, lon: lon) { result in
     switch result {
     case .success(let observationForecast) :
          print("Observation Forecast request succeeded")
          model["observationForecast"] = observationForecast
          break
     case .failure(let error) :
          print("Observation Forecast request failed : \(error)")
          model["observationForecast"] = error
          break
     }
     dispatchGroup.leave()
}