threadsanitizer.swift

func update(_ arg: inout String) {
    arg = "UPDATED \(arg)"
}

let mainQueue = DispatchQueue.init(label: "mainQueue", qos: .userInitiated, attributes: [], autoreleaseFrequency: .workItem, target: nil)
let workerQueue =  DispatchQueue.init(label: "workerQueue", qos: .userInitiated, attributes: [.concurrent], autoreleaseFrequency: .workItem, target: nil)

let systemQueue =  DispatchQueue.init(label: "systemQueue", qos: .userInitiated, attributes: [], autoreleaseFrequency: .workItem, target: nil)
print("Sample Started")

mainQueue.async {
    let group = DispatchGroup()
    var dict = Dictionary<Int, String>()
    let end = 1000000
    dict.reserveCapacity(end)
    let range = (0..<end)
    range.forEach { dict[$0] = $0.description }
    group.enter()
    
    systemQueue.async {
        let cop = dict
        range.forEach { id in
            group.enter()
            workerQueue.async { [x = cop[id]!] in
                var x = x
                update(&x)
                mainQueue.async {
                    dict[id] = x
                    group.leave()
                }
            }
        }
        
        group.leave()
    }
    
    group.notify(queue: DispatchQueue.main, execute: {
        let finished = dict.values.filter { $0.hasPrefix("UPDATED ") }
        print(finished.count)
    })
}