Data race demo

Counter.swift

import Foundation
import Synchronization

final class Counter: Sendable {
    private let mutex = Mutex(0)
    
    var count: Int {
        get {
            return mutex.withLock { count in
                return count
            }
        }
    }
    
    func setCount(_ newCount: Int) {
        mutex.withLock { count in
            count = newCount
        }
    }
    
    func increment() {
        setCount(count + 1)
    }
}

let counter = Counter()

await withTaskGroup { group in
    for _ in 0..<1000 {
        group.addTask {
            counter.increment()
        }
    }
    
    await group.waitForAll()
}

print(counter.count)

It's safe like this:

final class Counter: Sendable {
    private let mutex = Mutex(0)
    
    var count: Int {
        get {
            return mutex.withLock { count in
                return count
            }
        }
    }
    
    func updateCount(_ update: (inout Int) -> Void) {
        mutex.withLock { count in
            update(&count)
        }
    }

    func increment() {
        updateCount { $0 += 1 }
    }
}