MockAgent causes memory leak when used with fuzzing

[willfarrell]

Bug Description

When using MockAgent alongside fuzz testing it was found that a memory leak started happening.

Reproducible By

https://github.com/middyjs/middy/blob/620fdf7c6275708e587e8b223ae6c3d4848267e3/packages/s3-object-response/index.fuzz.js

Expected Behavior

Expected a memory leak not to happen.

Reverted back to using:

global.fetch = (url, request) => {
	return Promise.resolve(
		new Response("", {
			status: 200,
			statusText: "OK",
			headers: new Headers({
				"Content-Type": "application/json; charset=UTF-8",
			}),
		}),
	);
};

Logs & Screenshots

https://github.com/middyjs/middy/actions/runs/20599570479/job/59161706234
https://github.com/middyjs/middy/actions/runs/20599603819/job/59172523477
https://github.com/middyjs/middy/actions/runs/20610082297/job/59192953759

Environment

Node v24.12.0 running on GitHub Actions

Full setup: https://github.com/middyjs/middy/blob/develop/.github/workflows/test-dast.yml

Additional context

[metcoder95]

Thanks for the report! Can you provide an isolated Minimum Reproducible Example?

[willfarrell]

node --test --max-old-space-size=8 undici.test.js

import { test } from "node:test";
import { MockAgent, setGlobalDispatcher } from "undici";
import fc from "fast-check"; // this is just used to trigger it n-times

const agent = new MockAgent();
setGlobalDispatcher(agent);
agent
  .get("http://localhost")
  .intercept({
    path: "/",
    method: "GET",
  })
  .reply(200, "{}", {
    headers: {
      "Content-Type": "application/json; charset=UTF-8",
    },
  })
  .persist();

test("fuzz `fetch`", async () => {
  await fc.assert(
    fc.asyncProperty(fc.string(), async (event) => {
      await fetch("http://localhost");
    }),
    {
      numRuns: 1_000_000,
      verbose: 2,

      examples: [],
    },
  );
});