Shared-State Concurrency - The Rust Programming Language

## Shared-State Concurrency

Message passing is a fine way of handling concurrency, but it’s not the only one. Consider this part of the slogan from the Go language documentation again: “do not communicate by sharing memory.”

What would communicating by sharing memory look like? In addition, why would message-passing enthusiasts not use it and do the opposite instead?

In a way, channels in any programming language are similar to single ownership, because once you transfer a value down a channel, you should no longer use that value. Shared memory concurrency is like multiple ownership: multiple threads can access the same memory location at the same time. As you saw in Chapter 15, where smart pointers made multiple ownership possible, multiple ownership can add complexity because these different owners need managing. Rust’s type system and ownership rules greatly assist in getting this management correct. For an example, let’s look at mutexes, one of the more common concurrency primitives for shared memory.

### Using Mutexes to Allow Access to Data from One Thread at a Time

Mutex is an abbreviation for mutual exclusion, as in, a mutex allows only one thread to access some data at any given time. To access the data in a mutex, a thread must first signal that it wants access by asking to acquire the mutex’s lock. The lock is a data structure that is part of the mutex that keeps track of who currently has exclusive access to the data. Therefore, the mutex is described as guarding the data it holds via the locking system.

Mutexes have a reputation for being difficult to use because you have to remember two rules:

- You must attempt to acquire the lock before using the data.
- When you’re done with the data that the mutex guards, you must unlock the data so other threads can acquire the lock.

For a real-world metaphor for a mutex, imagine a panel discussion at a conference with only one microphone. Before a panelist can speak, they have to ask or signal that they want to use the microphone. When they get the microphone, they can talk for as long as they want to and then hand the microphone to the next panelist who requests to speak. If a panelist forgets to hand the microphone off when they’re finished with it, no one else is able to speak. If management of the shared microphone goes wrong, the panel won’t work as planned!

Management of mutexes can be incredibly tricky to get right, which is why so many people are enthusiastic about channels. However, thanks to Rust’s type system and ownership rules, you can’t get locking and unlocking wrong.

#### The API of `Mutex<T>`

As an example of how to use a mutex, let’s start by using a mutex in a single-threaded context, as shown in Listing 16-12:

Filename: src/main.rs

    use std::sync::Mutex;

    fn main() {
        let m = Mutex::new(5);

        {
            let mut num = m.lock().unwrap();
            *num = 6;
        }

        println!("m = {:?}", m);
    }

Listing 16-12: Exploring the API of `Mutex<T>` in a single-threaded context for simplicity

As with many types, we create a `Mutex<T>` using the associated function `new`. To access the data inside the mutex, we use the `lock` method to acquire the lock. This call will block the current thread so it can’t do any work until it’s our turn to have the lock.

The call to `lock` would fail if another thread holding the lock panicked. In that case, no one would ever be able to get the lock, so we’ve chosen to `unwrap` and have this thread panic if we’re in that situation.

After we’ve acquired the lock, we can treat the return value, named `num` in this case, as a mutable reference to the data inside. The type system ensures that we acquire a lock before using the value in `m`: `Mutex<i32>` is not an `i32`, so we must acquire the lock to be able to use the `i32` value. We can’t forget; the type system won’t let us access the inner `i32` otherwise.

As you might suspect, `Mutex<T>` is a smart pointer. More accurately, the call to `lock` returns a smart pointer called `MutexGuard`, wrapped in a `LockResult` that we handled with the call to `unwrap`. The `MutexGuard` smart pointer implements `Deref` to point at our inner data; the smart pointer also has a `Drop` implementation that releases the lock automatically when a `MutexGuard` goes out of scope, which happens at the end of the inner scope in Listing 16-12. As a result, we don’t risk forgetting to release the lock and blocking the mutex from being used by other threads because the lock release happens automatically.

After dropping the lock, we can print the mutex value and see that we were able to change the inner `i32` to 6.

#### Sharing a `Mutex<T>` Between Multiple Threads

Now, let’s try to share a value between multiple threads using `Mutex<T>`. We’ll spin up 10 threads and have them each increment a counter value by 1, so the counter goes from 0 to 10. Note that the next few examples will have compiler errors, and we’ll use those errors to learn more about using `Mutex<T>` and how Rust helps us use it correctly. Listing 16-13 has our starting example:

Filename: src/main.rs

    use std::sync::Mutex;
    use std::thread;

    fn main() {
        let counter = Mutex::new(0);
        let mut handles = vec![];

        for _ in 0..10 {
            let handle = thread::spawn(move || {
                let mut num = counter.lock().unwrap();

                *num += 1;
            });
            handles.push(handle);
        }

        for handle in handles {
            handle.join().unwrap();
        }

        println!("Result: {}", *counter.lock().unwrap());
    }

Listing 16-13: Ten threads each increment a counter guarded by a `Mutex<T>`

We create a `counter` variable to hold an `i32` inside a `Mutex<T>`, as we did in Listing 16-12. Next, we create 10 threads by iterating over a range of numbers. We use `thread::spawn` and give all the threads the same closure, one that moves the counter into the thread, acquires a lock on the `Mutex<T>` by calling the `lock` method, and then adds 1 to the value in the mutex. When a thread finishes running its closure, `num` will go out of scope and release the lock so another thread can acquire it.

In the main thread, we collect all the join handles. Then, as we did in Listing 16-2, we call `join` on each handle to make sure all the threads finish. At that point, the main thread will acquire the lock and print the result of this program.

We hinted that this example wouldn’t compile. Now let’s find out why!

    error[E0382]: capture of moved value: `counter`
      --> src/main.rs:10:27
       |
    9  |         let handle = thread::spawn(move || {
       |                                    ------- value moved (into closure) here
    10 |             let mut num = counter.lock().unwrap();
       |                           ^^^^^^^ value captured here after move
       |
       = note: move occurs because `counter` has type `std::sync::Mutex<i32>`,
       which does not implement the `Copy` trait

    error[E0382]: use of moved value: `counter`
      --> src/main.rs:21:29
       |
    9  |         let handle = thread::spawn(move || {
       |                                    ------- value moved (into closure) here
    ...
    21 |     println!("Result: {}", *counter.lock().unwrap());
       |                             ^^^^^^^ value used here after move
       |
       = note: move occurs because `counter` has type `std::sync::Mutex<i32>`,
       which does not implement the `Copy` trait

    error: aborting due to 2 previous errors

The error message states that the `counter` value is moved into the closure and then captured when we call `lock`. That description sounds like what we wanted, but it’s not allowed!

Let’s figure this out by simplifying the program. Instead of making 10 threads in a `for` loop, let’s just make two threads without a loop and see what happens. Replace the first `for` loop in Listing 16-13 with this code instead:

    use std::sync::Mutex;
    use std::thread;

    fn main() {
        let counter = Mutex::new(0);
        let mut handles = vec![];

        let handle = thread::spawn(move || {
            let mut num = counter.lock().unwrap();

            *num += 1;
        });
        handles.push(handle);

        let handle2 = thread::spawn(move || {
            let mut num2 = counter.lock().unwrap();

            *num2 += 1;
        });
        handles.push(handle2);

        for handle in handles {
            handle.join().unwrap();
        }

        println!("Result: {}", *counter.lock().unwrap());
    }

We make two threads and change the variable names used with the second thread to `handle2` and `num2`. When we run the code this time, compiling gives us the following:

    error[E0382]: capture of moved value: `counter`
      --> src/main.rs:16:24
       |
    8  |     let handle = thread::spawn(move || {
       |                                ------- value moved (into closure) here
    ...
    16 |         let mut num2 = counter.lock().unwrap();
       |                        ^^^^^^^ value captured here after move
       |
       = note: move occurs because `counter` has type `std::sync::Mutex<i32>`,
       which does not implement the `Copy` trait

    error[E0382]: use of moved value: `counter`
      --> src/main.rs:26:29
       |
    8  |     let handle = thread::spawn(move || {
       |                                ------- value moved (into closure) here
    ...
    26 |     println!("Result: {}", *counter.lock().unwrap());
       |                             ^^^^^^^ value used here after move
       |
       = note: move occurs because `counter` has type `std::sync::Mutex<i32>`,
       which does not implement the `Copy` trait

    error: aborting due to 2 previous errors

Aha! The first error message indicates that `counter` is moved into the closure for the thread associated with `handle`. That move is preventing us from capturing `counter` when we try to call `lock` on it and store the result in `num2` in the second thread! So Rust is telling us that we can’t move ownership of `counter` into multiple threads. This was hard to see earlier because our threads were in a loop, and Rust can’t point to different threads in different iterations of the loop. Let’s fix the compiler error with a multiple-ownership method we discussed in Chapter 15.

#### Multiple Ownership with Multiple Threads

In Chapter 15, we gave a value multiple owners by using the smart pointer `Rc<T>` to create a reference counted value. Let’s do the same here and see what happens. We’ll wrap the `Mutex<T>` in `Rc<T>` in Listing 16-14 and clone the `Rc<T>` before moving ownership to the thread. Now that we’ve seen the errors, we’ll also switch back to using the `for` loop, and we’ll keep the `move` keyword with the closure.

Filename: src/main.rs

    use std::rc::Rc;
    use std::sync::Mutex;
    use std::thread;

    fn main() {
        let counter = Rc::new(Mutex::new(0));
        let mut handles = vec![];

        for _ in 0..10 {
            let counter = Rc::clone(&counter);
            let handle = thread::spawn(move || {
                let mut num = counter.lock().unwrap();

                *num += 1;
            });
            handles.push(handle);
        }

        for handle in handles {
            handle.join().unwrap();
        }

        println!("Result: {}", *counter.lock().unwrap());
    }

Listing 16-14: Attempting to use `Rc<T>` to allow multiple threads to own the `Mutex<T>`

Once again, we compile and get... different errors! The compiler is teaching us a lot.

    error[E0277]: the trait bound `std::rc::Rc<std::sync::Mutex<i32>>:
    std::marker::Send` is not satisfied in `[closure@src/main.rs:11:36:
    15:10 counter:std::rc::Rc<std::sync::Mutex<i32>>]`
      --> src/main.rs:11:22
       |
    11 |         let handle = thread::spawn(move || {
       |                      ^^^^^^^^^^^^^ `std::rc::Rc<std::sync::Mutex<i32>>`
    cannot be sent between threads safely
       |
       = help: within `[closure@src/main.rs:11:36: 15:10
    counter:std::rc::Rc<std::sync::Mutex<i32>>]`, the trait `std::marker::Send` is
    not implemented for `std::rc::Rc<std::sync::Mutex<i32>>`
       = note: required because it appears within the type
    `[closure@src/main.rs:11:36: 15:10 counter:std::rc::Rc<std::sync::Mutex<i32>>]`
       = note: required by `std::thread::spawn`

Wow, that error message is very wordy! Here are some important parts to focus on: the first inline error says ``std::rc::Rc<std::sync::Mutex<i32>>` cannot be sent between threads safely`. The reason for this is in the next important part to focus on, the error message. The distilled error message says `the trait bound `Send` is not satisfied`. We’ll talk about `Send` in the next section: it’s one of the traits that ensures the types we use with threads are meant for use in concurrent situations.

Unfortunately, `Rc<T>` is not safe to share across threads. When `Rc<T>` manages the reference count, it adds to the count for each call to `clone` and subtracts from the count when each clone is dropped. But it doesn’t use any concurrency primitives to make sure that changes to the count can’t be interrupted by another thread. This could lead to wrong counts—subtle bugs that could in turn lead to memory leaks or a value being dropped before we’re done with it. What we need is a type exactly like `Rc<T>` but one that makes changes to the reference count in a thread-safe way.

#### Atomic Reference Counting with `Arc<T>`

Fortunately, `Arc<T>` is a type like `Rc<T>` that is safe to use in concurrent situations. The a stands for atomic, meaning it’s an atomically reference counted type. Atomics are an additional kind of concurrency primitive that we won’t cover in detail here: see the standard library documentation for `std::sync::atomic` for more details. At this point, you just need to know that atomics work like primitive types but are safe to share across threads.

You might then wonder why all primitive types aren’t atomic and why standard library types aren’t implemented to use `Arc<T>` by default. The reason is that thread safety comes with a performance penalty that you only want to pay when you really need to. If you’re just performing operations on values within a single thread, your code can run faster if it doesn’t have to enforce the guarantees atomics provide.

Let’s return to our example: `Arc<T>` and `Rc<T>` have the same API, so we fix our program by changing the `use` line, the call to `new`, and the call to `clone`. The code in Listing 16-15 will finally compile and run:

Filename: src/main.rs

    use std::sync::{Mutex, Arc};
    use std::thread;

    fn main() {
        let counter = Arc::new(Mutex::new(0));
        let mut handles = vec![];

        for _ in 0..10 {
            let counter = Arc::clone(&counter);
            let handle = thread::spawn(move || {
                let mut num = counter.lock().unwrap();

                *num += 1;
            });
            handles.push(handle);
        }

        for handle in handles {
            handle.join().unwrap();
        }

        println!("Result: {}", *counter.lock().unwrap());
    }

Listing 16-15: Using an `Arc<T>` to wrap the `Mutex<T>` to be able to share ownership across multiple threads

This code will print the following:

    Result: 10

We did it! We counted from 0 to 10, which may not seem very impressive, but it did teach us a lot about `Mutex<T>` and thread safety. You could also use this program’s structure to do more complicated operations than just incrementing a counter. Using this strategy, you can divide a calculation into independent parts, split those parts across threads, and then use a `Mutex<T>` to have each thread update the final result with its part.

### Similarities Between `RefCell<T>`/`Rc<T>` and `Mutex<T>`/`Arc<T>`

You might have noticed that `counter` is immutable but we could get a mutable reference to the value inside it; this means `Mutex<T>` provides interior mutability, as the `Cell` family does. In the same way we used `RefCell<T>` in Chapter 15 to allow us to mutate contents inside an `Rc<T>`, we use `Mutex<T>` to mutate contents inside an `Arc<T>`.

Another detail to note is that Rust can’t protect you from all kinds of logic errors when you use `Mutex<T>`. Recall in Chapter 15 that using `Rc<T>` came with the risk of creating reference cycles, where two `Rc<T>` values refer to each other, causing memory leaks. Similarly, `Mutex<T>` comes with the risk of creating deadlocks. These occur when an operation needs to lock two resources and two threads have each acquired one of the locks, causing them to wait for each other forever. If you’re interested in deadlocks, try creating a Rust program that has a deadlock; then research deadlock mitigation strategies for mutexes in any language and have a go at implementing them in Rust. The standard library API documentation for `Mutex<T>` and `MutexGuard` offers useful information.

We’ll round out this chapter by talking about the `Send` and `Sync` traits and how we can use them with custom types.