javascript - JavaScript 中的互斥量——这看起来像是一个正确的实现吗？

Question

这不是一个完全严肃的问题，更像是一个淋浴的想法:JavaScript 的 await 关键字应该允许一些感觉非常像普通“并发语言”中的互斥体的东西。

function Mutex() {
    var self = this; // still unsure about how "this" is captured
    var mtx = new Promise(t => t()); // fulfilled promise ≡ unlocked mutex
    this.lock = async function() {
        await mtx;
        mtx = new Promise(t => {
            self.unlock = () => t();
        });
    }
}
// Lock
await mutex.lock();
// Unlock
mutex.unlock();

这是正确的实现吗(除了正确的错误处理)？而且……我可以使用 C++-RAII 风格的锁守卫吗？

Answer 1

您的实现允许尽可能多的消费者获得锁；每次调用 lock 都会等待一个 promise :

function Mutex() {
    var self = this; // still unsure about how "this" is captured
    var mtx = new Promise(t => t()); // fulfilled promise ≡ unlocked mutex
    this.lock = async function() {
        await mtx;
        mtx = new Promise(t => {
            self.unlock = () => t();
        });
    }
}

const mutex = new Mutex();

(async () => {
  await Promise.resolve();
  await mutex.lock();
  console.log("A got the lock");
})();
(async () => {
  await Promise.resolve();
  await mutex.lock();
  console.log("B got the lock");
})();

您需要实现一个 promise 队列，为每个锁定请求创建一个新 promise 。

旁注:

• new Promise(t => t()) 可以写得更简单和地道 Promise.resolve() :-)
• 如果您正在使用这样的箭头函数，则不需要 self；箭头函数关闭创建它们的this(就像关闭变量一样)
• unlock 可能是锁 promise 的解析值，因此只有获得锁的代码才能释放它

像这样:

function Mutex() {
    let current = Promise.resolve();
    this.lock = () => {
        let _resolve;
        const p = new Promise(resolve => {
            _resolve = () => resolve();
        });
        // Caller gets a promise that resolves when the current outstanding
        // lock resolves
        const rv = current.then(() => _resolve);
        // Don't allow the next request until the new promise is done
        current = p;
        // Return the new promise
        return rv;
    };
}

实例:

"use strict";
function Mutex() {
    let current = Promise.resolve();
    this.lock = () => {
        let _resolve;
        const p = new Promise(resolve => {
            _resolve = () => resolve();
        });
        // Caller gets a promise that resolves when the current outstanding
        // lock resolves
        const rv = current.then(() => _resolve);
        // Don't allow the next request until the new promise is done
        current = p;
        // Return the new promise
        return rv;
    };
}

const rand = max => Math.floor(Math.random() * max);

const delay = (ms, value) => new Promise(resolve => setTimeout(resolve, ms, value));

const mutex = new Mutex();

function go(name) {
    (async () => {
        console.log(name + " random initial delay");
        await delay(rand(50));
        console.log(name + " requesting lock");
        const unlock = await mutex.lock();
        console.log(name + " got lock");
        await delay(rand(1000));
        console.log(name + " releasing lock");
        unlock();
    })();
}
go("A");
go("B");
go("C");
go("D");

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  max-height: 100% !important;
}