JavaScript 中的互斥锁 - 这看起来像一个正确的实现吗？

Question

不是一个完全严肃的问题，更像是一个淋浴的想法：JavaScript 的 await 关键字应该允许在您的平均“并发语言”中感觉非常像互斥锁的东西。

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();

这是一个正确的实现吗（除了正确的错误处理）？还有……我可以有 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 都会等待一个承诺：

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");
})();

您需要实现一个承诺队列，为每个锁定请求创建一个新的承诺。

旁注：

• new Promise(t => t()) 可以更简单地写成Promise.resolve() :-)
• 如果您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 可能是锁承诺的解析值，因此只有获得锁的代码才能释放它

类似这样的：

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;
    };
}

现场示例：

"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");

.as-console-wrapper {
  max-height: 100% !important;
}

Answer 2

这是一个正确的实现吗？

没有。如果两个任务（我不能说“线程”）在当前锁定时尝试执行mutex.lock()，它们将同时获得锁定。我怀疑这就是你想要的。

JS 中的互斥锁实际上只是一个布尔标志——检查它，在获得锁时设置它，在释放锁时清除它。在检查和获取之间没有对竞争条件的特殊处理，因为您可以在单线程 JS 中同步执行此操作，而无需任何其他线程干扰。

然而，您似乎正在寻找的是一个队列，即您可以安排自己获得锁并在前一个锁被释放时（通过承诺）得到通知的东西。

我会这样做

class Mutex {
    constructor() {
        this._lock = null;
    }
    isLocked() {
        return this._lock != null;
    }
    _acquire() {
        var release;
        const lock = this._lock = new Promise(resolve => {
            release = resolve;
        });
        return () => {
            if (this._lock == lock) this._lock = null;
            release();
        };
    }
    acquireSync() {
        if (this.isLocked()) throw new Error("still locked!");
        return this._acquire();
    }
    acquireQueued() {
        const q = Promise.resolve(this._lock).then(() => release);
        const release = this._acquire(); // reserves the lock already, but it doesn't count
        return q; // as acquired until the caller gets access to `release` through `q`
    }
}

演示：

class Mutex {
    constructor() {
        this._lock = Promise.resolve();
    }
    _acquire() {
        var release;
        const lock = this._lock = new Promise(resolve => {
            release = resolve;
        });
        return release;
    }
    acquireQueued() {
        const q = this._lock.then(() => release);
        const release = this._acquire();
        return q;
    }
}
const delay = t => new Promise(resolve => setTimeout(resolve, t));

const mutex = new Mutex();

async function go(name) {
    await delay(Math.random() * 500);
    console.log(name + " requests lock");
    const release = await mutex.acquireQueued();
    console.log(name + " acquires lock");
    await delay(Math.random() * 1000);
    release()
    console.log(name + " releases lock");
}
go("A");
go("B");
go("C");
go("D");

Answer 3

我建议使用像 async-mutex 这样的库：

const mutex = new Mutex();
// ...
const release = await mutex.acquire();
try {
    // ...
} finally {
    release();
}