create a data race in swift

To create a data race in Swift, multiple threads need to access and modify a shared variable concurrently without appropriate synchronization. Here is an example:

main.swift
var count = 0

DispatchQueue.concurrentPerform(iterations: 100) { _ in
    for _ in 1...1000 {
        count += 1
    }
}

print("Count: \(count)")
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In this example, we are using Grand Central Dispatch to run a closure concurrently 100 times. Each closure increments the count variable 1000 times. Since there is no synchronization mechanism, there is a high probability of encountering a data race, which can lead to unpredictable outcomes. Running this code multiple times can result in different output, like:

main.swift
Count: 100000
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or

main.swift
Count: 99962
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or any other value close to the expected count of 100000.

To fix this data race, we need to use synchronization mechanisms like locks or barriers to ensure that only one thread accesses the count variable at a time. Here is an example using a lock:

main.swift
var count = 0
let lock = NSLock()

DispatchQueue.concurrentPerform(iterations: 100) { _ in
    for _ in 1...1000 {
        lock.lock()
        count += 1
        lock.unlock()
    }
}

print("Count: \(count)")
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In this example, we are using an NSLock to synchronize access to the count variable. Each closure acquires the lock before modifying the variable and releases it after. This ensures that only one thread is accessing the variable at a time, making the code thread-safe and free from data races.