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/**
* This file is part of Haketilo.
*
* Function: Implement a 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.
*/
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
*/
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