Chapter 16 - Fearless Concurrency

Problems writing multithreaded code:

• Race conditions, where threads are accessing data or resources in an inconsistent order
• Deadlocks, where two threads are waiting for each other to finish using a resource the other thread has, preventing both threads from continuing

This model where a language calls the operating system APIs to create threads is sometimes called 1:1, meaning one operating system thread per one language thread.

Programming language-provided threads are known as green threads, and languages that use these green threads will execute them in the context of a different number of operating system threads. For this reason, the green-threaded model is called the M:N model: there are M green threads per N operating system threads, where M and N are not necessarily the same number.

Rust standard library only provides an implementation of 1:1 threading. But there are various libraries which provides M:N model.

Thread Primitives

• spawn
• join

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

fn main() {
    let handle = thread::spawn(|| {
        for i in 1..10 {
            println!("hi number {} from the spawned thread!", i);
            thread::sleep(Duration::from_millis(1));
        }
    });

    for i in 1..5 {
        println!("hi number {} from the main thread!", i);
        thread::sleep(Duration::from_millis(1));
    }

    handle.join().unwrap();
}

You will use the move keyword to make the closure take ownership of the values in threads:

use std::thread;

fn main() {
    let v = vec![1, 2, 3];

    let handle = thread::spawn(move || {
        println!("Here's a vector: {:?}", v);
    });

    handle.join().unwrap();
}

The above code won't work without using move as you can very well write invalid code like this:

use std::thread;

fn main() {
    let v = vec![1, 2, 3];

    let handle = thread::spawn(|| {
        println!("Here's a vector: {:?}", v);
    });

    drop(v); // oh no!

    handle.join().unwrap();
}

Message passing between Threads

One major tool Rust has for accomplishing message-sending concurrency is the channel.

A channel in programming has two halves: a transmitter and a receiver. One part of your code calls methods on the transmitter with the data you want to send, and another part checks the receiving end for arriving messages. A channel is said to be closed if either the transmitter or receiver half is dropped.

use std::thread;
use std::sync::mpsc;
use std::time::Duration;

fn main() {
    let (tx, rx) = mpsc::channel();

    thread::spawn(move || {
        let vals = vec![
            String::from("hi"),
            String::from("from"),
            String::from("the"),
            String::from("thread"),
        ];

        for val in vals {
            tx.send(val).unwrap();
            thread::sleep(Duration::from_secs(1));
        }
    });

    for received in rx {
        println!("Got: {}", received);
    }
}

• mpsc: multiple producer, single consumer
• tx: transmitter
• rx: receiver

Shared state Concurrency

Mutexes are one of the concurrency primitives for shared memory.

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.

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());
}

The result will be 10.

Sync and Send Traits

The Send marker trait indicates that ownership of the type implementing Send can be transferred between threads. Almost every Rust type is Send

The Sync marker trait indicates that it is safe for the type implementing Sync to be referenced from multiple threads. In other words, any type T is Sync if &T (a reference to T) is Send, meaning the reference can be sent safely to another thread.