How to avoid deadlock calling custom DispatchQueue sync from main thread?

Question

I am trying to safely read/write data from multiple threads, like described here: Create thread safe array in Swift

Here's a snippet how I read data

    private let annotationsQueue = DispatchQueue(label: "myCustoLabel", attributes: .concurrent)
    private var unsafeAnnotations = [MapAnnotation]()
    private var annotations: [MapAnnotation] {
        var annotationsCopy: [MapAnnotation]!
        annotationsQueue.sync {
            annotationsCopy = self.unsafeAnnotations
        }
        return annotationsCopy
    }

My issue is that annotations is sometimes being called from DispatchQueue.main.async, which causes dead lock.

Here's are screenshot of stack trace, when I reach dead lock

My question is how should I handle such situation? Should I somehow force my annotationsQueue run on background thread? Or I should write my code, so annotations are never called from DispatchQueue.main.async?

here you read some annotation data. right? `annotationsQueue.sync` — , May 03 '21 at 06:06
for some reason low priority task it will not work so please try getter setter method like this https://stackoverflow.com/questions/35084754/objc-sync-enter-objc-sync-exit-not-working-with-dispatch-queue-priority-low — , May 03 '21 at 06:48
@JatinRB you linked a question, not answer. And it seems my implementation is the same as in some answers — Xernox, May 03 '21 at 07:18
@matt won't I have issues, when one thread will write and other will ready if I wont use sync? — Xernox, May 03 '21 at 07:18
No, async on a serial queue is a form of locking. But you won’t be able to make this a computed property. — matt, May 03 '21 at 07:41

score 0 · Answer 1 · answered May 05 '21 at 18:15

As you are creating concurrent queue can use dispatch barrier for read/write from multiple threads.

 private let annotationsQueue = DispatchQueue(label: "myCustoLabel", attributes: .concurrent)
   

private var unsafeAnnotations = [MapAnnotation]()

private var annotations: [MapAnnotation]? {
    var annotationsCopy: [MapAnnotation]!
    get {
      annotationsQueue.sync {
        return annotationsCopy
    }
    return nil

    }
    set {
      annotationsQueue.async(flag: .barrier) {
           annotationsCopy = self.unsafeAnnotations
       }
    }   

}

How to avoid deadlock calling custom DispatchQueue sync from main thread?

1 Answers1