/**
 * This file is part of Haketilo.
 *
 * Function: Implement a lock (aka binary semaphore aka mutex).
 *
 * Copyright (C) 2021 Wojtek Kosior
 *
 * This program is free software: you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation, either version 3 of the License, or
 * (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * As additional permission under GNU GPL version 3 section 7, you
 * may distribute forms of that code without the copy of the GNU
 * GPL normally required by section 4, provided you include this
 * license notice and, in case of non-source distribution, a URL
 * through which recipients can access the Corresponding Source.
 * If you modify file(s) with this exception, you may extend this
 * exception to your version of the file(s), but you are not
 * obligated to do so. If you do not wish to do so, delete this
 * exception statement from your version.
 *
 * As a special exception to the GPL, any HTML file which merely
 * makes function calls to this code, and for that purpose
 * includes it by reference shall be deemed a separate work for
 * copyright law purposes. If you modify this code, you may extend
 * this exception to your version of the code, but you are not
 * obligated to do so. If you do not wish to do so, delete this
 * exception statement from your version.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program.  If not, see <https://www.gnu.org/licenses/>.
 *
 * I, Wojtek Kosior, thereby promise not to sue for violation of this file's
 * license. Although I request that you do not make use this code in a
 * proprietary program, I am not going to enforce this in court.
 */

/*
 * 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.
 */

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);
    }
}

/*
 * EXPORTS_START
 * EXPORT make_lock
 * EXPORT lock
 * EXPORT unlock
 * EXPORTS_END
 */