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/**
 * Myext lock (aka binary semaphore aka mutex)
 *
 * Copyright (C) 2021 Wojtek Kosior
 * Redistribution terms are gathered in the `copyright' file.
 */

/*
 * 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;
})();