Add RateLimiter helper

Add helper class for implementing rate limited operations.

test/lib/RateLimiter.js

@@ -0,0 +1,64 @@
+"use strict";
+const assert = require("assert").strict;
+
+const { wait } = require("@clusterio/lib/helpers");
+const RateLimiter = require("@clusterio/lib/RateLimiter");
+
+
+describe("lib/RateLimiter", function() {
+	describe("class RateLimiter", function() {
+		let rateLimiter;
+		beforeEach(function() {
+			rateLimiter = new RateLimiter();
+		});
+		afterEach(function() {
+			rateLimiter.cancel();
+		});
+
+		it("should rate limit activations", function() {
+			assert(rateLimiter.activate(), "fist invocation was rate limited");
+			assert(!rateLimiter.activate(), "second invocation was not rate limited");
+		});
+
+		it("should invoke action immediately when not rate limited", function() {
+			let invoked = false;
+			rateLimiter.action = () => { invoked = true; };
+			rateLimiter.activate();
+			assert(invoked, "action was not invoked");
+		});
+
+		it("should invoke action at a later point in time if rate limited", async function() {
+			let invokedCount = 0;
+			rateLimiter.maxRate = 100;
+			rateLimiter.action = () => { invokedCount += 1; };
+			rateLimiter.activate();
+			rateLimiter.activate();
+			assert.equal(invokedCount, 1, "expected one invocation");
+			await wait(20);
+			assert.equal(invokedCount, 2, "expected two invocations");
+		});
+
+		it("should invoke action before timeout if maxRate changed", async function() {
+			let invokedCount = 0;
+			rateLimiter.action = () => { invokedCount += 1; };
+			rateLimiter.activate();
+			rateLimiter.activate();
+			assert.equal(invokedCount, 1, "expected one invocation");
+			await wait(20);
+			rateLimiter.maxRate = 100;
+			rateLimiter.activate();
+			assert.equal(invokedCount, 2, "expected two invocations");
+		});
+
+		it("should cancel action if canceled before timeout", async function() {
+			let invokedCount = 0;
+			rateLimiter.action = () => { invokedCount += 1; };
+			rateLimiter.activate();
+			rateLimiter.activate();
+			rateLimiter.cancel();
+			assert.equal(invokedCount, 1, "expected one invocation");
+			await wait(20);
+			assert.equal(invokedCount, 1, "expected one invocation");
+		});
+	});
+});