Add function to manually check rate limit (#346)

[Charioteer]

Hey fastify contributors,

Following on from issue #346 and my comment, I finally had some spare time to implement a draft version of my idea as suggested by @mcollina. I realized that any store (child) instance receives all the options provided to fastify.rateLimit when it is constructed, which is why I had to adjust my initial idea and come up with a slightly different solution.

I did not provide any documentation or additional tests yet (all existing tests run green tho), because I wanted to ask for your feedback first.

Please let me know what you think and as soon as everything looks good to you, I will finalize my PR with proper docs and tests.

Thanks and cheers
Patrick

How to Use

import Fastify from "fastify";
import { fastifyRateLimit } from "@fastify/rate-limit";

const fastify = Fastify();

await fastify.register(fastifyRateLimit, {
  global: false,
  max: 10,
  timeWindow: "10 seconds",
});

const checkRateLimit = fastify.createRateLimit(); // this will use the global options provided to fastifyRateLimit

fastify.get("/", async (request, reply) => {
  const limit = await checkRateLimit(request);

  if(!limit.isAllowed && limit.isExceeded) {
    return reply.code(429).send("Limit exceeded");
  }

  return reply.send("Hello world");
});


const checkCustomRateLimit = fastify.createRateLimit({ max: 100 }); // max provided to override global options

fastify.get("/custom", async (request, reply) => {
  const limit = await checkCustomRateLimit(request);

  if(!limit.isAllowed && limit.isExceeded) {
    return reply.code(429).send("Limit exceeded");
  }

  return reply.send("Hello world");
});

Checklist

• run npm run test and npm run benchmark
• tests and/or benchmarks are included
• documentation is changed or added
• commit message and code follows the Developer's Certification of Origin
  and the Code of conduct

Copilot reviewed 2 out of 2 changed files in this pull request and generated no suggestions.

Comments skipped due to low confidence (3)

index.js:285

• The variable timeWindowString is only assigned a value if params.timeWindow is a number. Ensure it is always defined or handle the undefined case properly.

timeWindowString = ms.format(params.timeWindow, true)

index.js:193

• [nitpick] The function name createLimiterArgs is ambiguous. Consider renaming it to generateLimiterArguments for better clarity.

function createLimiterArgs (pluginComponent, globalParams, options) {

index.js:216

• Ensure that the new behavior introduced by the applyRateLimit function is covered by tests.

async function applyRateLimit (pluginComponent, params, req) {

[mcollina]

Looks good! Finish it up (be careful that CI is failing).