@sveltejs/kit/src/core/postbuild/queue.js

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/**
 * @typedef {{
 *   fn: () => Promise<any>,
 *   fulfil: (value: any) => void,
 *   reject: (error: Error) => void
 * }} Task
 */

/** @param {number} concurrency */
export function queue(concurrency) {
	/** @type {Task[]} */
	const tasks = [];

	let current = 0;

	// TODO: Whenever Node >21 is minimum supported version, we can use `Promise.withResolvers` to avoid this ceremony
	/** @type {(value?: any) => void} */
	let fulfil;

	/** @type {(error: Error) => void} */
	let reject;

	let closed = false;

	const done = new Promise((f, r) => {
		fulfil = f;
		reject = r;
	});

	done.catch(() => {
		// this is necessary in case a catch handler is never added
		// to the done promise by the user
	});

	function dequeue() {
		if (current < concurrency) {
			const task = tasks.shift();

			if (task) {
				current += 1;
				const promise = Promise.resolve(task.fn());

				void promise
					.then(task.fulfil, (err) => {
						task.reject(err);
						reject(err);
					})
					.then(() => {
						current -= 1;
						dequeue();
					});
			} else if (current === 0) {
				closed = true;
				fulfil();
			}
		}
	}

	return {
		/** @param {() => any} fn */
		add: (fn) => {
			if (closed) throw new Error('Cannot add tasks to a queue that has ended');

			const promise = new Promise((fulfil, reject) => {
				tasks.push({ fn, fulfil, reject });
			});

			dequeue();
			return promise;
		},

		done: () => {
			if (current === 0) {
				closed = true;
				fulfil();
			}

			return done;
		}
	};
}