asynchronous - 当 future 包装它被丢弃时，如何停止运行同步代码？

Question

我有调用同步代码的异步代码需要一段时间才能运行，所以我遵循了 What is the best approach to encapsulate blocking I/O in future-rs? 中概述的建议。 .但是，我的异步代码有一个超时，之后我不再对同步计算的结果感兴趣:

use std::{thread, time::Duration};
use tokio::{task, time}; // 0.2.10

// This takes 1 second
fn long_running_complicated_calculation() -> i32 {
    let mut sum = 0;
    for i in 0..10 {
        thread::sleep(Duration::from_millis(100));
        eprintln!("{}", i);
        sum += i;
        // Interruption point
    }
    sum
}

#[tokio::main]
async fn main() {
    let handle = task::spawn_blocking(long_running_complicated_calculation);
    let guarded = time::timeout(Duration::from_millis(250), handle);

    match guarded.await {
        Ok(s) => panic!("Sum was calculated: {:?}", s),
        Err(_) => eprintln!("Sum timed out (expected)"),
    }
}

运行此代码显示超时触发，但同步代码 还有继续运行:

0
1
Sum timed out (expected)
2
3
4
5
6
7
8
9

当 future 包装它被丢弃时，如何停止运行同步代码？

我不希望编译器能够神奇地停止我的同步代码。我用“中断点”注释了一行，我需要手动进行某种检查以提前退出我的函数，但我不知道如何轻松获得 spawn_blocking 的结果的通知(或 ThreadPool::spawn_with_handle ，对于纯基于 future 的代码)已被删除。

Answer 1

您可以传递一个原子 bool 值，然后使用它来将任务标记为需要取消。 (我不确定我是否为 Ordering/load 调用使用了合适的 store，这可能需要更多考虑)

这是您的代码的修改版本，输出

0
1
Sum timed out (expected)
2
Interrupted...

use std::sync::atomic::{AtomicBool, Ordering};
use std::sync::Arc;
use std::{thread, time::Duration};
use tokio::{task, time}; // 0.2.10

// This takes 1 second
fn long_running_complicated_calculation(flag: &AtomicBool) -> i32 {
    let mut sum = 0;
    for i in 0..10 {
        thread::sleep(Duration::from_millis(100));
        eprintln!("{}", i);
        sum += i;
        // Interruption point
        if !flag.load(Ordering::Relaxed) {
            eprintln!("Interrupted...");
            break;
        }
    }
    sum
}

#[tokio::main]
async fn main() {
    let some_bool = Arc::new(AtomicBool::new(true));

    let some_bool_clone = some_bool.clone();
    let handle =
        task::spawn_blocking(move || long_running_complicated_calculation(&some_bool_clone));
    let guarded = time::timeout(Duration::from_millis(250), handle);

    match guarded.await {
        Ok(s) => panic!("Sum was calculated: {:?}", s),
        Err(_) => {
            eprintln!("Sum timed out (expected)");
            some_bool.store(false, Ordering::Relaxed);
        }
    }
}

playground

在当前 Tokio 删除 future /句柄时自动发生这种情况是不可能的。 http://github.com/tokio-rs/tokio/issues/1830 中正在为此做一些工作。和 http://github.com/tokio-rs/tokio/issues/1879 .

但是，您可以通过将 future 包装在自定义类型中来获得类似的东西。

这是一个看起来与原始代码几乎相同的示例，但在模块中添加了一个简单的包装器类型。如果我实现 Future<T> 会更符合人体工程学在包装类型上，它只是转发到包裹的 handle ，但事实证明这很烦人。

mod blocking_cancelable_task {
    use std::sync::atomic::{AtomicBool, Ordering};
    use std::sync::Arc;
    use tokio::task;

    pub struct BlockingCancelableTask<T> {
        pub h: Option<tokio::task::JoinHandle<T>>,
        flag: Arc<AtomicBool>,
    }

    impl<T> Drop for BlockingCancelableTask<T> {
        fn drop(&mut self) {
            eprintln!("Dropping...");
            self.flag.store(false, Ordering::Relaxed);
        }
    }

    impl<T> BlockingCancelableTask<T>
    where
        T: Send + 'static,
    {
        pub fn new<F>(f: F) -> BlockingCancelableTask<T>
        where
            F: FnOnce(&AtomicBool) -> T + Send + 'static,
        {
            let flag = Arc::new(AtomicBool::new(true));
            let flag_clone = flag.clone();
            let h = task::spawn_blocking(move || f(&flag_clone));
            BlockingCancelableTask { h: Some(h), flag }
        }
    }

    pub fn spawn<F, T>(f: F) -> BlockingCancelableTask<T>
    where
        T: Send + 'static,
        F: FnOnce(&AtomicBool) -> T + Send + 'static,
    {
        BlockingCancelableTask::new(f)
    }
}

use std::sync::atomic::{AtomicBool, Ordering};
use std::{thread, time::Duration};
use tokio::time; // 0.2.10

// This takes 1 second
fn long_running_complicated_calculation(flag: &AtomicBool) -> i32 {
    let mut sum = 0;
    for i in 0..10 {
        thread::sleep(Duration::from_millis(100));
        eprintln!("{}", i);
        sum += i;
        // Interruption point
        if !flag.load(Ordering::Relaxed) {
            eprintln!("Interrupted...");
            break;
        }
    }
    sum
}

#[tokio::main]
async fn main() {
    let mut h = blocking_cancelable_task::spawn(long_running_complicated_calculation);
    let guarded = time::timeout(Duration::from_millis(250), h.h.take().unwrap());
    match guarded.await {
        Ok(s) => panic!("Sum was calculated: {:?}", s),
        Err(_) => {
            eprintln!("Sum timed out (expected)");
        }
    }
}

playground