生锈 "future cannot be sent between threads safely"

Question

我正在调用异步实现的方法：

let mut safebrowsing: MutexGuard<Safebrowsing> = self.safebrowsing.lock().unwrap();
safebrowsing.is_safe(input: &message.content).await;

is_safe-方法：

pub async fn is_safe(&mut self, input: &str) {
    let links = self.finder.links(input);

    for link in links {
        match reqwest::get("url").await {
            Ok(response) => {
                println!(
                    "{}",
                    response.text().await.expect("response has no content")
                );
            }
            Err(_) => {
                println!("Unable to get safebrowsing-response")
            }
        }
    }
}

但不幸的是，通过异步调用 is_safe-方法，编译器告诉我线程无法安全发送。错误是关于：

future cannot be sent between threads safely
within `impl std::future::Future`, the trait `std::marker::Send` is not implemented for `std::sync::MutexGuard<'_, Safebrowsing>`
required for the cast to the object type `dyn std::future::Future<Output = ()> + std::marker::Send`

handler.rs(31, 9): future is not `Send` as this value is used across an await
           ^-- safebrowsing.is_safe(input: &message.content).await;
---

future cannot be sent between threads safely
the trait `std::marker::Send` is not implemented for `(dyn for<'r> Fn(&'r [u8]) -> Option<usize> + 'static)`
required for the cast to the object type `dyn std::future::Future<Output = ()> + std::marker::Send`

safebrowsing.rs(22, 19): future is not `Send` as this value is used across an await
               ^-- match reqwest::get("url").await

我已经尝试将 Send-Trait 实现到我的 Safebrowsing-Struct，但这也不起作用。我需要做些什么才能使其正常工作吗？因为我不知道为什么会出现这种情况

Answer 1

这个错误的关键在于MutexGuard<T> is not Send。这意味着您正在尝试在互斥量被锁定时执行 await，这通常是一个坏主意，如果您考虑一下：await 原则上可能会无限期地等待，但是通过等待只要持有互斥锁，任何其他试图锁定互斥锁的线程都会无限期地阻塞（当然，除非你设置了超时）。

因此，根据经验，您永远不应该在锁定互斥锁的情况下睡觉。例如，您的代码可以重写为（完全未经测试）：

pub async fn is_safe(this: &Mutex<Safebrowsing>, input: &str) {
    //lock, find, unlock
    let links = this.lock().unwrap().finder.links(input);
    //now we can await safely
    for link in links {
        match reqwest::get("url").await {
            Ok(response) => {
                println!(
                    "{}",
                    response.text().await.expect("response has no content")
                );
            }
            Err(_) => {
                println!("Unable to get safebrowsing-response")
            }
        }
    }
}

如果您稍后需要在函数中锁定 Mutex，请注意争用！它可能已被其他线程修改，也许 input 不再是一个东西。

生锈 "future cannot be sent between threads safely"

Rust "future cannot be sent between threads safely"

multithreading

rust

mutex