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/**
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* This file is part of Haketilo.
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*
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* Function: Implement a lock (aka binary semaphore aka mutex).
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*
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* Copyright (C) 2021 Wojtek Kosior
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*
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* This program is free software: you can redistribute it and/or modify
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* it under the terms of the GNU General Public License as published by
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* the Free Software Foundation, either version 3 of the License, or
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* (at your option) any later version.
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*
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* This program is distributed in the hope that it will be useful,
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* but WITHOUT ANY WARRANTY; without even the implied warranty of
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* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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* GNU General Public License for more details.
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*
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* As additional permission under GNU GPL version 3 section 7, you
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* may distribute forms of that code without the copy of the GNU
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* GPL normally required by section 4, provided you include this
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* license notice and, in case of non-source distribution, a URL
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* through which recipients can access the Corresponding Source.
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* If you modify file(s) with this exception, you may extend this
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* exception to your version of the file(s), but you are not
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* obligated to do so. If you do not wish to do so, delete this
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* exception statement from your version.
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*
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* As a special exception to the GPL, any HTML file which merely
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* makes function calls to this code, and for that purpose
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* includes it by reference shall be deemed a separate work for
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* copyright law purposes. If you modify this code, you may extend
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* this exception to your version of the code, but you are not
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* obligated to do so. If you do not wish to do so, delete this
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* exception statement from your version.
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*
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* You should have received a copy of the GNU General Public License
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* along with this program. If not, see <https://www.gnu.org/licenses/>.
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*
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* I, Wojtek Kosior, thereby promise not to sue for violation of this file's
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* license. Although I request that you do not make use this code in a
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* proprietary program, I am not going to enforce this in court.
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*/
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/*
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* Javascript runs single-threaded, with an event loop. Because of that,
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* explicit synchronization is often not needed. An exception is when we use
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* an API function that must wait. Ajax is an example. Callback passed to ajax
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* call doesn't get called immediately, but after some time. In the meantime
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* some other piece of code might get to execute and modify some variables.
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* Access to WebExtension local storage is another situation where this problem
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* can occur.
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*
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* This is a solution. A lock object, that can be used to delay execution of
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* some code until other code finishes its critical work. Locking is wrapped
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* in a promise.
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*/
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function make_lock() {
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return {free: true, queue: []};
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}
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function _lock(lock, cb) {
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if (lock.free) {
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lock.free = false;
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setTimeout(cb);
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} else {
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lock.queue.push(cb);
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}
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}
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function lock(lock) {
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return new Promise((resolve, reject) => _lock(lock, resolve));
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}
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function unlock(lock) {
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if (lock.free)
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throw new Exception("Attempting to release a free lock");
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if (lock.queue.length === 0) {
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lock.free = true;
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} else {
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let cb = lock.queue[0];
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lock.queue.splice(0, 1);
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setTimeout(cb);
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}
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}
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/*
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* EXPORTS_START
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* EXPORT make_lock
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* EXPORT lock
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* EXPORT unlock
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* EXPORTS_END
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*/
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