Demonstrates how a Swift value constant can mutate when using Copy-on-Write (CoW) and multi-threading.

LetTheCoWGo.swift

import Foundation

struct NuclearPowerStationOperator {
    
    class Storage {
        var turnOffCores: Bool = false
        func copy() -> Storage {
            let new = Storage()
            new.turnOffCores = turnOffCores
            return new
        }
    }
    
    private var storage: Storage = Storage()
    
    var turnOffCores: Bool {
        get {
            return storage.turnOffCores
        }
        set {
            if isKnownUniquelyReferenced(&storage) {
                Thread.sleep(forTimeInterval: 1.0) // Sleep to simulate race condition
                storage.turnOffCores = newValue
            } else {
                storage = storage.copy()
                storage.turnOffCores = newValue
            }
        }
    }
    
    var description: String {
        return "\(turnOffCores ? "We are in danger" : "We are safe")"
    }
}

// Create a mutable value
var crazyOperator = NuclearPowerStationOperator()

DispatchQueue.global(qos: .background).async {
    Thread.sleep(forTimeInterval: 0.5) // Sleep a little to give main thread time to start setting property
    let saneOperator = crazyOperator // Create a constant copy of the operator from main thread
    print(saneOperator.description) // Print our intial property value
    Thread.sleep(forTimeInterval: 2.0) // Simulate race by waiting for setter on main thread to finish
    print(saneOperator.description) // Test property (it will be different)
}

// Update the value. Note that the setter simulates a race condition by being very slow
crazyOperator.turnOffCores = true

A similar implementation in Rust would be the following:

use std::thread;
use std::time::Duration;

struct NuclearPowerStationOperator {
    turn_off_core: bool,
}

fn main() {
    let mut crazy_operator = NuclearPowerStationOperator {
        turn_off_core: false,
    };

    let handle = thread::spawn(move || {
        thread::sleep(Duration::from_millis(500));

        let sane_operator = crazy_operator;
        println!("Core is {}", sane_operator.turn_off_core);

        thread::sleep(Duration::from_millis(2000));

        println!("Core is {}", sane_operator.turn_off_core);
    });

    thread::sleep(Duration::from_millis(1000));
    crazy_operator.turn_off_core = true;

    handle.join().unwrap();
}

If I run cargo build, this is the output:

~/Development/rust/shared_state HEAD cargo build
   Compiling shared_state v0.1.0 (/Users/jessearmand/Development/rust/shared_state)
warning[E0382]: assign to part of moved value: `crazy_operator`
  --> src/main.rs:25:5
   |
13 |     let handle = thread::spawn(move || {
   |                                ------- value moved into closure here
...
16 |         let sane_operator = crazy_operator;
   |                             -------------- variable moved due to use in closure
...
25 |     crazy_operator.turn_off_core = true;
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ value partially assigned here after move
   |
   = note: move occurs because `crazy_operator` has type `NuclearPowerStationOperator`, which does not implement the `Copy` trait
   = warning: This error has been downgraded to a warning for backwards compatibility with previous releases.
           It represents potential unsoundness in your code.
           This warning will become a hard error in the future.

    Finished dev [unoptimized + debuginfo] target(s) in 2.33s

Then if I run the executable with cargo run

~/Development/rust/shared_state HEAD cargo run
    Finished dev [unoptimized + debuginfo] target(s) in 0.54s
     Running `target/debug/shared_state`
Core is false
Core is false

The difference is with Rust, a data type needs to implement Copy trait explicitly to have copy-on-write. By default the crazy_operator is moved to the thread closure. I haven't tried to implement Copy trait for this. Also there's a similar concept: Clone trait which is done on the heap allocated data, while Copy is done on a stack allocated data.

Regardless, the build warning shows you that the assignment there should not happen.

Nice illustration of an anti-pattern for implementing COW types. At a high level, it seems the best fix would be to ensure that the check isUniquelyReferenced(&storage) and the updation: storage.turnOffCores = newValue happens atomically. That is, no context switch should be possible between the two.

@ravikandhadai A context switch is not relevant. The relevant part is that if you share a reference to a value type, and concurrently read/write to that reference from multiple threads, your program is buggy. That's the end of the discussion.

Specifically, the behaviour here has racing reads and writes from multiple threads without synchronisation. As @helje5 points out, that's not safe to do with any value, ever.

You can see this clearly by translating @drewmccormack's simplified example on Int:

var i: Int = 0
DispatchQueue.global(qos: .background).async {
    let o = i
    print("\(o) original value copy", o)
    usleep(10)
    print("\(o) final value of copy", o)
}
i += 1

into C:

int i = 0;

void test(void) {
    dispatch_async(dispatch_get_global_queue(QOS_CLASS_BACKGROUND, 0), ^{
        int o = i;
        printf("Here's the value: %d\n", o);
    });
    i = 1;
}

test();

This code is clearly not right. There is a race on the read from i and the write to i. On some architectures, this may happen to work some of the time, but it is fundamentally not thread safe. You are never allowed to race unsynchronised reads and writes from multiple threads. The fact that CoW values make this harder does not change the fact that it is simply never safe to do this.

You have to add crazyOperator to a capture list of the closure to get the behaviour you wanted. Without it, closure will have a reference to your variable, it will not copy it on creation.

Change

DispatchQueue.global(qos: .background).async {

into

DispatchQueue.global(qos: .background).async { [crazyOperator] in

To see the difference between a closure with and without a capture list, have a look at the following code:

import Foundation

var int: Int? = 100
var array: [Int]? = [200]

DispatchQueue.global(qos: .background).async {
    let copy_int = int
    let copy_array = array
    print("Without capture list int: \(String(describing: copy_int)), array: \(String(describing: copy_array))")
}

DispatchQueue.global(qos: .background).async { [int, array] in
    let copy_int = int
    let copy_array = array
    print("With capture list int: \(String(describing: copy_int)), array: \(String(describing: copy_array))")
}

int = nil
array = nil
Thread.sleep(forTimeInterval: 1)

You have to add crazyOperator to a capture list of the closure to get the behaviour you wanted. Without it, closure will have a reference to your variable, it will not copy it on creation.

Change

DispatchQueue.global(qos: .background).async {

into

DispatchQueue.global(qos: .background).async { [crazyOperator] in

To see the difference between a closure with and without a capture list, have a look at the following code:

import Foundation

var int: Int? = 100
var array: [Int]? = [200]

DispatchQueue.global(qos: .background).async {
    let copy_int = int
    let copy_array = array
    print("Without capture list int: \(String(describing: copy_int)), array: \(String(describing: copy_array))")
}

DispatchQueue.global(qos: .background).async { [int, array] in
    let copy_int = int
    let copy_array = array
    print("With capture list int: \(String(describing: copy_int)), array: \(String(describing: copy_array))")
}

int = nil
array = nil
Thread.sleep(forTimeInterval: 1)

In this demo, you're not actually modifying the variable in multiple threads, the copy of the variable is on the same thread, which is completely different from the topic under discussion.