async-timeout-js/semaphore.js

function SemaphoreDisposed(message) { this.message = message || "Semaphore was disposed"; Object.freeze(this); }

function isPromise(obj) {
  return !!obj && (typeof obj === 'object' || typeof obj === 'function') && typeof obj.then === 'function';
}

/// An async semaphore with up to `cap` locks held at once.
/// 
/// To use as a mutex, make `cap` 1. 
///
/// # Throws
/// If `cap` is 0 or lower.
function Semaphore(cap)
{
	/*function Handle(next, id)
	{
		this.state = next;
		this.id = id || new_id();
	}

	Handle.create = function(resolve, reject, id) {
		let handle = new Handle(this, id);
		handle.state[handle.id] = (error) => error ? reject(error) : resolve();
		return handle;
	};*/

	if(!cap || cap <= 0) throw `Semaphore capacity cannot be '${cap}', must be a number greater than 0`;

	this.capacity = cap;
	this.length = 0;

	let self = this;

	// Waiters for the seamphore
	var next = {
		waiters: [],
		// wait for a slot to be available, then take it. If there are slots, the promise resolves immediately, if not, it is added to the release queue and will resolve when there is one available for it.
		acquire: () => {
			return new Promise((resolve, reject) => { // next.waiters.push(error => error ? reject(error) : resolve()))
				if(self.length < self.capacity) {
					// Slot available, return now.
					self.length += 1;
					resolve(); //TODO: Add IDs? pass a Handle to resolve?
				} else {
					// Slot unavailable, add to queue.
					next.waiters.push(resolve);//error => error ? reject(error) : resolve());
				}
			});
		}, 
		// release a raw lock on the semaphore. if there are pending `acquire()` promises, the length is not decremented, but the next queued up acquire is resolved insetad.
		release: () => { 
			if(next.waiters.length>0) // There are pending waiters
				next.waiters.shift()();  // No need to decrement, there is a pending waiter and that slot goes directly to them. (asyn)
			else self.length -= 1; // Deremenet the length (imm)
		}
		//TODO: ,dispose: (error = new SemaphoreDisposed()) => { } // reject all `waiters`, and cause all new `acquire()`s to immediately reject.
	};

	this.acquire_raw = async () => {
		await next.acquire();
	};

	this.release_raw = () => {
		next.release();
	};

	/// Acquire a semaphore lock (if one is not available, wait until one is) and then run `func()` with the lock held, afterwards, release the lock and return the result of `func()`.
	/// If `func` is an async function, it is awaited and the lock is released after it has resolved (or rejected), and the result of this function is the result of that promise.
	this.using = async (func) => {
		//if(!func || (typeof obj !== "function" && !isPromise(func))) throw `Parameter '{func}' is not a function or Promise.`;

		await self.acquire_raw();
		try {
			const rv = func();
			if(isPromise(rv)) return await rv;
			else		  return rv;
		} finally {
			self.release_raw();
		}
	};
	/// Returns a lambda that, when executed, runs `using(func)` and returns the awaitable promise.
	this.bind_using = (func) => { return () => self.using(func); };
}

/// Create a mutex (single cap semaphore)
Semaphore.mutex = () => new Semaphore(1);


const sem_example = async (locks) => {
	let semaphore = new Semaphore(locks || 1); //Semaphore.Mutex();

	const gen_runner = (name, num, time) => {
		if(!time || time < 0) return semaphore.bind_using(() => {
			for(let i=0;i<num;i++) console.log(`${name}: ${i}`);
			return num;
		}); 
		else return semaphore.bind_using(async () => {
			for(let i=0;i<num;i++) {
				await new Promise(resolve => setTimeout(resolve, time));
				console.log(`${name}: ${i}`);
			}
			return num;
		});
	};

	return await Promise.all([
		gen_runner("first", 10, 100)(),
		gen_runner("second", 15, 75)(),
		gen_runner("third", 500)(),
	]);
};