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