为什么这个等待线程完成的Rust代码无法正常工作？

john 发表于 Dev

约翰

我有一些多线程代码给我带来麻烦。这很简单，就像我可以复制它一样：

use std::thread;
use std::time;
use std::sync::{Arc, atomic::{Ordering, AtomicBool}};
use std::ops::Drop;

struct Container {
    // Join Handle for a thread
    th: Option<thread::JoinHandle<()>>,

    // Gets set to true when we want the thread to exit
    stop_thread: Arc<AtomicBool>,
}

impl Container {
    fn new() -> Self {
        // Create new instance
        let mut inst = Self {
            th: None,
            stop_thread: Arc::new(AtomicBool::new(false)),
        };

        let stop_thread = inst.stop_thread.clone();

        // Start a new thread that does some work
        let t = thread::spawn(move || {
            // Keep doing work until stop_thread gets set to true
            while !stop_thread.load(Ordering::SeqCst) {
                println!("Doing stuff...");

                thread::sleep(time::Duration::from_secs(1));
            }

            println!("Thread exited");
        });

        inst.th = Some(t);
        inst
    }
}

impl Drop for Container {
    fn drop(&mut self) {
        self.stop_thread.store(true, Ordering::SeqCst);

        if let Some(t) = self.th {
            t.join().unwrap();
        }
    }
}

fn main() {
    let c = Container::new();

    thread::sleep(time::Duration::from_secs(3));
    drop(c);
}

这个想法是当Container创建该结构的新实例时，将启动一个执行某些操作的后台线程。它一直运行直到实例被销毁为止，这时，我需要通知线程它需要退出。我还需要实际等待线程退出才能继续。

除了drop函数中的代码外，其他所有东西都运行良好。鲁斯不满意if let Some(t) = self.th。它说：

error[E0507]: cannot move out of `self.th.0` which is behind a mutable reference
  --> src/main.rs:45:26
   |
45 |         if let Some(t) = self.th {
   |                     -    ^^^^^^^ help: consider borrowing here: `&self.th`
   |                     |
   |                     data moved here
   |                     move occurs because `t` has type `JoinHandle<()>`, which does not implement the `Copy` trait

我为什么不能这样做？什么self.th.0啊

当我尝试接受Rust的建议而做时if let Some(t) = &self.th，它仍然无法编译：

error[E0507]: cannot move out of `*t` which is behind a shared reference
  --> src/main.rs:46:13
   |
46 |             t.join().unwrap();
   |             ^ move occurs because `*t` has type `JoinHandle<()>`, which does not implement the `Copy` trait

我究竟做错了什么？

马哈宁

如此答案中所指定（由Rabbid76链接），可以使用以下.take()函数来解决此问题：

impl Drop for Container {
    fn drop(&mut self) {
        self.stop_thread.store(true, Ordering::SeqCst);
        if let Some(t) = self.th.take() {
            t.join().unwrap();
        }
    }
}

尽管您可能想考虑在实现中等待另一个线程drop是否是一个好主意，如此处所述

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