如何从 Arc<Mutex<T>> 获取 T 的所有权？

Question

我想 return 来自受 Mutex 保护的函数的值，但无法理解如何正确执行。此代码不起作用：

use std::sync::{Arc, Mutex};

fn func() -> Result<(), String> {
    let result_my = Arc::new(Mutex::new(Ok(())));
    let result_his = result_my.clone();

    let t = std::thread::spawn(move || {
        let mut result = result_his.lock().unwrap();
        *result = Err("something failed".to_string());
    });

    t.join().expect("Unable to join thread");

    let guard = result_my.lock().unwrap();
    *guard
}

fn main() {
    println!("func() -> {:?}", func());
}

Playground

编译器抱怨：

error[E0507]: cannot move out of borrowed content
  --> src/main.rs:16:5
   |
16 |     *guard
   |     ^^^^^^ cannot move out of borrowed content

Answer 1

您可以使用 mem::replace 通过将其替换为新值来转移可变引用的所有权。（返回并移动旧值）

use std::sync::{Arc, Mutex};
use std::mem;

fn func() -> Result<(), String> {
    let result_my = Arc::new(Mutex::new(Ok(())));
    let result_his = result_my.clone();

    let t = std::thread::spawn(move || {
        let mut result = result_his.lock().unwrap();
        *result = Err("something failed".to_string());
    });

    t.join();

    let mut guard = result_my.lock().unwrap();
    mem::replace(&mut guard, Ok(()))
}

fn main() {
    println!("func() -> {:?}", func());
}

Answer 2

到目前为止我找到的最好的解决方案是将结果包装成一个Option然后取出来：

fn func() -> Result<(), String> {
    let result_my = Arc::new(Mutex::new(Some(Ok(()))));
    let result_his = result_my.clone();

    let t = std::thread::spawn(move || {
        let mut result = result_his.lock().unwrap();
        *result = Some(Err("something failed".to_string()));
    });

    t.join().expect("Unable to join thread");

    let mut guard = result_my.lock().unwrap();
    guard.take().unwrap()
}

似乎比好，因为不需要构造一个空的T来交换，而且它可以防止多次提取。

Answer 3

在 Rust 1.15 中，您可以使用 Arc::try_unwrap and Mutex::into_inner:

use std::sync::{Arc, Mutex};

fn func() -> Result<(), String> {
    let result_my = Arc::new(Mutex::new(Ok(())));
    let result_thread = result_my.clone();

    let t = std::thread::spawn(move || {
        let mut result = result_thread.lock().unwrap();
        *result = Err("something failed".to_string());
    });

    t.join().expect("Unable to join threads");

    let lock = Arc::try_unwrap(result_my).expect("Lock still has multiple owners");
    lock.into_inner().expect("Mutex cannot be locked")
}

fn main() {
    println!("func() -> {:?}", func());
}

RwLock::into_inner 自 Rust 1.6 以来也存在。

如何从 Arc<Mutex<T>> 获取 T 的所有权？

How to take ownership of T from Arc<Mutex<T>>?

rust