webpack/lib/util/Semaphore.js

Packs ECMAScript/CommonJs/AMD modules for the browser. Allows you to split your codebase into multiple bundles, which can be loaded on demand. Supports loaders to preprocess files, i.e. json, jsx, es7, css, less, ... and your custom stuff.

/*
	MIT License http://www.opensource.org/licenses/mit-license.php
	Author Tobias Koppers @sokra
*/

"use strict";

/**
 * Simple counting semaphore used to limit how many asynchronous tasks may run
 * concurrently.
 */
class Semaphore {
	/**
	 * Initializes the semaphore with the number of permits that may be held at
	 * the same time.
	 * @param {number} available the amount available number of "tasks"
	 * in the Semaphore
	 */
	constructor(available) {
		this.available = available;
		/** @type {(() => void)[]} */
		this.waiters = [];
		/** @private */
		this._continue = this._continue.bind(this);
	}

	/**
	 * Acquires a permit for the callback immediately when one is available or
	 * queues the callback until another task releases its permit.
	 * @param {() => void} callback function block to capture and run
	 * @returns {void}
	 */
	acquire(callback) {
		if (this.available > 0) {
			this.available--;
			callback();
		} else {
			this.waiters.push(callback);
		}
	}

	/**
	 * Releases a permit and schedules the next waiting callback, if any.
	 */
	release() {
		this.available++;
		if (this.waiters.length > 0) {
			process.nextTick(this._continue);
		}
	}

	/**
	 * Drains the next waiting callback after a permit becomes available.
	 */
	_continue() {
		if (this.available > 0 && this.waiters.length > 0) {
			this.available--;
			const callback = /** @type {(() => void)} */ (this.waiters.pop());
			callback();
		}
	}
}

module.exports = Semaphore;