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/**
* Myext lock (aka binary semaphore aka mutex)
*
* Copyright (C) 2021 Wojtek Kosior
*
* This code is dual-licensed under:
* - Asshole license 1.0,
* - GPLv3 or (at your option) any later version
*
* "dual-licensed" means you can choose the license you prefer.
*
* This code is released under a permissive license because I disapprove of
* copyright and wouldn't be willing to sue a violator. Despite not putting
* this code under copyleft (which is also kind of copyright), I do not want
* it to be made proprietary. Hence, the permissive alternative to GPL is the
* Asshole license 1.0 that allows me to call you an asshole if you use it.
* This means you're legally ok regardless of how you utilize this code but if
* you make it into something nonfree, you're an asshole.
*
* You should have received a copy of both GPLv3 and Asshole license 1.0
* together with this code. If not, please see:
* - https://www.gnu.org/licenses/gpl-3.0.en.html
* - https://koszko.org/asshole-license.txt
*/
/*
* Javascript runs single-threaded, with an event loop. Because of that,
* explicit synchronization is often not needed. An exception is when we use
* an API function that must wait. Ajax is an example. Callback passed to ajax
* call doesn't get called immediately, but after some time. In the meantime
* some other piece of code might get to execute and modify some variables.
* Access to WebExtension local storage is another situation where this problem
* can occur.
*
* This is a solution. A lock object, that can be used to delay execution of
* some code until other code finishes its critical work. Locking is wrapped
* in a promise.
*/
"use strict";
(() => {
function make_lock() {
return {free: true, queue: []};
}
function _lock(lock, cb) {
if (lock.free) {
lock.free = false;
setTimeout(cb);
} else {
lock.queue.push(cb);
}
}
function lock(lock) {
return new Promise((resolve, reject) => _lock(lock, resolve));
}
function unlock(lock) {
if (lock.free)
throw new Exception("Attempting to release a free lock");
if (lock.queue.length === 0) {
lock.free = true;
} else {
let cb = lock.queue[0];
lock.queue.splice(0, 1);
setTimeout(cb);
}
}
window.make_lock = make_lock;
window.lock = lock;
window.unlock = unlock;
})();
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