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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;
})();