Source map resolution difference between Istanbul and V8 coverage plugins

[just-boris]

Describe the bug

In our project setup, we have a pre-processing step, so the test run against a lib folder instead of src. This causes differences in behavior between istanbul and v8 coverage providers.

When running tests with istanbul it follows the source maps and the resolves the file to the original index.ts source:

 % Coverage report from istanbul
----------|---------|----------|---------|---------|-------------------
File      | % Stmts | % Branch | % Funcs | % Lines | Uncovered Line #s
----------|---------|----------|---------|---------|-------------------
All files |     100 |      100 |     100 |     100 |
 index.ts |     100 |      100 |     100 |     100 |
----------|---------|----------|---------|---------|-------------------

But it does not happen with v8, the report shows transpiled index.js

 % Coverage report from v8
----------|---------|----------|---------|---------|-------------------
File      | % Stmts | % Branch | % Funcs | % Lines | Uncovered Line #s
----------|---------|----------|---------|---------|-------------------
All files |     100 |      100 |     100 |     100 |
 index.js |     100 |      100 |     100 |     100 |
----------|---------|----------|---------|---------|-------------------

This becomes an issue when uploading reports to tools like Codecov, because they cannot match up the coverage report against the source

Reproduction

Minimal repro can be found here: https://github.com/just-boris/vitest-coverage-issue-5341

System Info

System:
    OS: macOS 13.6.4
    CPU: (10) arm64 Apple M1 Pro
    Memory: 594.05 MB / 32.00 GB
    Shell: 5.9 - /bin/zsh
  Binaries:
    Node: 18.19.0 - ~/n/bin/node
    npm: 10.2.3 - ~/n/bin/npm
  Browsers:
    Chrome: 122.0.6261.94
    Edge: 122.0.2365.63
    Safari: 17.3.1
  npmPackages:
    @vitest/coverage-istanbul: ^1.3.1 => 1.3.1
    @vitest/coverage-v8: ^1.3.1 => 1.3.1
    vitest: ^1.3.1 => 1.3.1

Used Package Manager

npm

Validations

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• Read the Contributing Guidelines.
• Read the docs.
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• The provided reproduction is a minimal reproducible example of the bug.

[AriPerkkio]

The root cause is in line below. V8 provider is setting the executed filename as map.sources without checking whether original source map had any other sources set.

vitest/packages/coverage-v8/src/provider.ts

Line 333 in 6d1b145

sources: [url],

Instead it should respect the original map.sources if it exists and resolve full path for that. With some quick testing the coverage looks much better. In the picture below the left side is current behavior and right one is with fixed sources. I added some uncovered if-branches to the original test code just to see that coverage reporting works after these changes: