Incorrect coverage reported with Node 20.10.0

[Snafuh]

Under Node >=20.10.0 Jest reports incorrect line coverage with the v8 coverage collector.

The issue was initially raised here jestjs/jest#14766 and here jestjs/jest#14764

The issue was further forwarded to the Node team here nodejs/node#51251 but it seems like Node does report the correct coverage.
There was a small change in the format of the V8 Coverage JSON, which might lead to the incorrect results in jest. Jest seems to use this package for v8 coverage, so I think the issue might originate here.

The difference in the coverage report for one of the reproductions was outline in this answer
nodejs/node#51251 (comment)

[AriPerkkio]

This sounds a bit similar as https://bugs.chromium.org/p/v8/issues/detail?id=10029. Though it doesn't make sense why this comes up only when Node version is upgraded. Maybe branches are counted differently based on the order the files are executed?

Can you post minimal reproduction case without Jest or any dependencies? Include the source file and the code the test files are executing.

Ideally script below should be able to reproduce this issue:

import { writeFileSync } from "node:fs";
import inspector from "node:inspector";

// Import the source file here
import * as sourceFile from "./source-file"

const session = new inspector.Session();
const collectedCoverage = [];

session.connect();
session.post("Profiler.enable");
session.post("Runtime.enable");
session.post("Profiler.startPreciseCoverage", {
  callCount: true,
  detailed: true,
});

// Run the methods the tests are executing here
sourceFile.someFunction(); // Comment out this line to see how coverage changes
await collectCoverage();

sourceFile.anotherFunction(); // Comment out this line to see how coverage changes
await collectCoverage();

// Report coverage
writeFileSync(
  "./coverage.json",
  JSON.stringify(collectedCoverage, null, 2),
  "utf8"
);

async function collectCoverage() {
  await new Promise((resolve, reject) => {
    session.post("Profiler.takePreciseCoverage", async (error, coverage) => {
      if (error) return reject(error);

      const filtered = coverage.result.filter(
        (entry) => !entry.url.startsWith("node:") // Exclude NodeJS internals
      );

      collectedCoverage.push(...filtered);

      resolve();
    });
  });
}

[Vinnl]

I think we're running into this too, but it's unpredictable, making it really hard to submit a minimal reproduction. After the upgrade to 20.10, suddenly some lines started to get marked as uncovered. Adding some ignore comments, then re-running the tests, only resulted in other lines getting marked as uncovered. And to make things even harder to reproduce, it's not always the same lines.

What might or might not be helpful, however, is this suspicious result, where I added an ignore comment, and then that comment itself got marked as uncovered. (Possibly related to #238? Though I only saw that for this comment.)

For a non-minimal reproduction, I've got:

git clone git@github.com:mozilla/blurts-server.git && cd blurts-server && git checkout 2acab6b && npm install && cp .env-dist .env

If you then run npm test, you should see the coverage thresholds not being met (see /coverage/lcov-report/index.html) - but adding /* c8 ignore next */ comments turns up new places that are marked as uncovered.

It does look like I can keep adding ignore comments until I eventually get to 100% again, but of course, that's not really satisfactory :)