Iterating over Readable streams never resolves with fake timers

[mattdean-digicatapult]

Describe the bug

With sinon v19 and using fake timers Readable streams will never resolve. This causes tests that need to evaluate the contents of Readable streams to timeout. Now it might be I shouldn't expect this to work as I am doing async stuff, but this did work with sinon v18 so for now I'd at least consider this a bug.

To Reproduce
Steps to reproduce the behavior:

1. setup a fake timer using sinon.useFakeTimers or similar
2. try to read all the data from a Readable
3. The final chunk of the Readable will never resolve

I've created a reproduction repo of this https://github.com/mattdean-digicatapult/usefaketimers-bug-reproduction if you'd like to test it out. The test code is as follows:

import { Readable } from "node:stream";

import { expect } from "chai";
import { before, describe, it } from "mocha";

import sinon from "sinon";

describe("sinon bug reproduction", function () {
  let result;

  before(async function () {
    const clock = sinon.useFakeTimers({ now: 100 });
    const stream = Readable.from(Buffer.from("test"));
    const chunks = [];
    for await (const chunk of stream) {
      chunks.push(chunk);
    }
    result = Buffer.concat(chunks).toString("utf8");
    clock.restore();
  });

  it("should run successfully", function () {
    expect(result).to.equal("test");
  });
});

Expected behavior

The stream to finish reading as occurred in sinon v18

Screenshots

Context (please complete the following information):

• Sinon version : v19.0.0

• Runtime: nodejs 22

• Other relevant environmental info: macos

• Other libraries you are using: Mocha, Chai (see reproduction for versions)

• Example URL: https://github.com/mattdean-digicatapult/usefaketimers-bug-reproduction

Additional context

None

[fatso83]

I am guessing this is related to us now faking all timers by default in version 13 of Fake Timers, which is a breaking change (hence the new major version of Sinon): https://github.com/sinonjs/fake-timers/blob/main/CHANGELOG.md#1300--2024-08-25

Somehow the underlying code you are using might depend on nextTick() or queueMicroTask(). See what happens if you explicitly list the timers to fake toFake: [...] .

Ref sinonjs/fake-timers#323

[fatso83]

Great testcase. Made it very easy to reproduce and look into. Was exactly what I assumed. This fixes/avoids your issue:

-    const clock = sinon.useFakeTimers({ now: 100 });
+    const clock = sinon.useFakeTimers({ now: 100, toFake: ["setTimeout"] });

EDIT: while it avoids the issue, I find it much better to keep this as an async function and I suggest an alternative below.

[fatso83]

@SimenB This might be a side-effect you can be aware of in Jest. Not sure how we should document this.

[fatso83]

Just to add some info. It's the nextTick() timer that never resolves. Instead of explicitly saying you don't want to fake that, you could also of course use runToLastAsync() and use the fact that this is dealing with async work.

I pushed a "fix" to your example: fatso83/usefaketimers-bug-reproduction@54c812d

Hope that helps!

[fatso83]

I'll add this as a note to the migration guide: https://sinonjs.org/guides/migration-guide. Thanks!