Avoid Data Race condition in swift

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I am getting race conditions in my code when I run TSan tool. As same code has been accessed from different queues and threads at the same time that's why I can not use Serial queues or barrier as Queue will block only single queues accessing the shared resource not the other queues.

I used objc_sync_enter(object) | objc_sync_exit(object) and locks NSLock() or NSRecursiveLock() to protect shared resource but these are also not working.

While when I use @synchronized() keyword in Objective C to protect shared resource, it's working fine as expected and I am not getting race conditions in particular block of code.

So, what is an alternative to protect data in Swift as we can not use @synchronized() keyword in Swift language.

PFA screenshot for reference - Data Race

swift concurrency ios
1 Answers

I don't understand "I can not use Serial queues or barrier as Queue will block only single queues accessing the shared resource not the other queues." Using a queue is the standard solution to this problem.

class MultiAccess {
    private var _property: String = ""
    private let queue = DispatchQueue(label: "MultiAccess")
    var property: String {
        get {
            var result: String!
            queue.sync {
                result = self._property
            }
            return result
        }
        set {
            queue.async {
                self._property = newValue
            }
        }
    }
}

With this construction, access to property is atomic and thread-safe without the caller having to do anything special. Note that this intentionally uses a single queue for the class, not a queue per-property. As a rule, you want a relatively small number of queues doing a lot of work, not a large number of queues doing a little work. If you find that you're accessing a mutable object from lots of different threads, you need to rethink your system (probably reducing the number of threads). There's no simple pattern that will make that work efficiently and safely without you having to think about your specific use case carefully.

But this construction is useful for problems where system frameworks may call you back on random threads with minimal contention. It is simple to implement and fairly easy to use correctly. If you have a more complex problem, you will have to design a solution for that problem.

Edit: I haven't thought about this answer in a long time, but Brennan's comments brought it back to my attention. Because of the bug I had in the original code, my original answer was ok, but if you fixed the bug it was bad. (If you want to see my original code that used a barrier, look in the edit history, I don't want to put it here because people will copy it.) I've changed it to use a standard serial queue rather than a concurrent queue.

Don't generate concurrent queues without careful thought about how threads will be generated. If you are going to have many simultaneous accesses, you're going to create a lot of threads, which is bad. If you're not going to have many simultaneous accesses, then you don't need a concurrent queue. GCD talks make promises about managing threads that it doesn't actually live up to. You definitely can get thread explosion (as Brennan mentions.)

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