Rust 中的 Tokio 线程同步机制

本文分享自天翼云开发者社区《Rust 中的 Tokio 线程同步机制》，作者：l****n

在并发编程中，线程同步是一个重要的概念，用于确保多个线程在访问共享资源时能够正确地协调。Tokio 是一个强大的异步运行时库，为 Rust 提供了多种线程同步机制。以下是一些常见的同步机制：

Mutex
RwLock
Barrier
Semaphore
Notify
oneshot 和 mpsc 通道
watch 通道
1. Mutex
Mutex（互斥锁）是最常见的同步原语之一，用于保护共享数据。它确保同一时间只有一个线程能够访问数据，从而避免竞争条件。

use tokio::sync::Mutex;
use std::sync::Arc;

#[tokio::main]
async fn main() {
    let data = Arc::new(Mutex::new(0));

    let mut handles = vec![];
    for _ in 0..10 {
        let data = data.clone();
        let handle = tokio::spawn(async move {
            let mut lock = data.lock().await;
            *lock += 1;
        });
        handles.push(handle);
    }

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

    println!("Result: {}", *data.lock().await);
}

2. RwLock

RwLock（读写锁）允许多线程同时读取数据，但只允许一个线程写入数据。它比 Mutex 更加灵活，因为在读取多于写入的场景下，它能提高性能。功能上，他是读写互斥、写写互斥、读读兼容。

use tokio::sync::RwLock;
use std::sync::Arc;

#[tokio::main]
async fn main() {
    let data = Arc::new(RwLock::new(0));

    let read_data = data.clone();
    let read_handle = tokio::spawn(async move {
        let lock = read_data.read().await;
        println!("Read: {}", *lock);
    });

    let write_data = data.clone();
    let write_handle = tokio::spawn(async move {
        let mut lock = write_data.write().await;
        *lock += 1;
        println!("Write: {}", *lock);
    });

    read_handle.await.unwrap();
    write_handle.await.unwrap();
}

3. Barrier

Barrier 是一种同步机制，允许多个线程在某个点上进行同步。当线程到达屏障时，它们会等待直到所有线程都到达，然后一起继续执行。

use tokio::sync::Barrier;
use std::sync::Arc;

#[tokio::main]
async fn main() {
    let barrier = Arc::new(Barrier::new(3));

    let mut handles = vec![];
    for i in 0..3 {
        let barrier = barrier.clone();
        let handle = tokio::spawn(async move {
            println!("Before wait: {}", i);
            barrier.wait().await;
            println!("After wait: {}", i);
        });
        handles.push(handle);
    }

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

4. Semaphore

Semaphore（信号量）是一种用于控制对资源访问的同步原语。它允许多个线程访问资源，但有一个最大并发数限制。

#[tokio::test]
async fn test_sem() {
    let semaphore = Arc::new(Semaphore::new(3));

    let mut handles = vec![];
    for i in 0..5 {
        let semaphore = semaphore.clone();
        let handle = tokio::spawn(async move {
            let permit = semaphore.acquire().await.unwrap();
            let now = Local::now();
            println!("Got permit: {} at {:?}", i, now);
            println!(
                "Semaphore available permits before sleep: {}",
                semaphore.available_permits()
            );
            sleep(Duration::from_secs(5)).await;
            drop(permit);
            println!(
                "Semaphore available permits after sleep: {}",
                semaphore.available_permits()
            );
        });
        handles.push(handle);
    }

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

最终的结果如下

Got permit: 0 at 2024-08-08T21:03:04.374666+08:00
Semaphore available permits before sleep: 2
Got permit: 1 at 2024-08-08T21:03:04.375527800+08:00
Semaphore available permits before sleep: 1
Got permit: 2 at 2024-08-08T21:03:04.375563+08:00
Semaphore available permits before sleep: 0
Semaphore available permits after sleep: 0
Semaphore available permits after sleep: 0
Semaphore available permits after sleep: 1
Got permit: 3 at 2024-08-08T21:03:09.376722800+08:00
Semaphore available permits before sleep: 1
Got permit: 4 at 2024-08-08T21:03:09.376779200+08:00
Semaphore available permits before sleep: 1
Semaphore available permits after sleep: 2
Semaphore available permits after sleep: 3

5. Notify

Notify 是一种用于线程间通知的简单机制。它允许一个线程通知其他线程某些事件的发生。

use tokio::sync::Notify;
use std::sync::Arc;

#[tokio::main]
async fn main() {
    let notify = Arc::new(Notify::new());
    let notify_clone = notify.clone();

    let handle = tokio::spawn(async move {
        notify_clone.notified().await;
        println!("Received notification");
    });

    notify.notify_one();
    handle.await.unwrap();
}

6. oneshot 和 mpsc 通道

oneshot 通道用于一次性发送消息，而 mpsc 通道则允许多个生产者发送消息到一个消费者。一般地onshot用于异常通知、启动分析等功能。mpsc用于实现异步消息同步``` oneshot

use tokio::sync::oneshot;

#[tokio::main]
async fn main() {
    let (tx, rx) = oneshot::channel();

    tokio::spawn(async move {
        tx.send("Hello, world!").unwrap();
    });

    let message = rx.await.unwrap();
    println!("Received: {}", message);
}

mpsc

use tokio::sync::mpsc;

#[tokio::main]
async fn main() {
    let (tx, mut rx) = mpsc::channel(32);

    tokio::spawn(async move {
        tx.send("Hello, world!").await.unwrap();
    });

    while let Some(message) = rx.recv().await {
        println!("Received: {}", message);
    }
}

7. watch 通道

watch 通道用于发送和接收共享状态的更新。它允许多个消费者监听状态的变化。

use tokio::sync::watch;

#[tokio::main]
async fn main() {
    let (tx, mut rx) = watch::channel("initial");

    tokio::spawn(async move {
        tx.send("updated").unwrap();
    });

    while rx.changed().await.is_ok() {
        println!("Received: {}", *rx.borrow());
    }
}

watch通道：

用于广播状态更新，一个生产者更新状态，多个消费者获取最新状态。适合配置变更、状态同步等场景。 mpsc通道：

用于传递消息队列，多个生产者发送消息，一个消费者逐条处理。适合任务队列、事件驱动等场景。