feat(experimental): support merge reports for non-sharded multi-environment runs (take 2)

[hi-ogawa]

Description

• Closes Support merging reports for non-sharded multi-environment runs #9954
• Closes feat: support merge reports for non-sharded multi-environment runs #9967 (supersedes)

Alternative take for #9967 (comment)

This PR adds new blob reporter option label so --merge-reports can distinguish results from the same test files across different environments, such as linux/macos/windows CI jobs.

Usage example

Set VITEST_BLOB_LABEL when creating blobs:

VITEST_BLOB_LABEL=linux vitest run --reporter=blob
VITEST_BLOB_LABEL=macos vitest run --reporter=blob

Or pass the label via the blob reporter option:

// vitest.config.ts
export default defineConfig({
  test: {
    reporters: [
      ['blob', { label: process.platform }],
    ],
  },
})

The label is included in the default blob filename (blob-linux.json) to avoid collisions. When merging, each labeled run is shown separately:

$ vitest --merge-reports
 ✓  linux  src/basic.test.ts (2 tests)
 ✓  macos  src/basic.test.ts (2 tests)

How it works

The label is a blob reporter option, with VITEST_BLOB_LABEL as an environment-variable shortcut for CI usage. During config resolution, Vitest extracts the label from ['blob', { label }] or process.env.VITEST_BLOB_LABEL and stores it as internal core config (mergeReportsLabel).

During the original test run, this label is not exposed as a normal built-in reporter label. This keeps terminal output unchanged for runs that create blobs with another reporter enabled, for example reporters: ['default', ['blob', { label: 'linux' }]].

The label is stored in blob task metadata and salts the file identity used for merged reports, so state deduplication can keep same-filepath files from different environments separate. The default blob output filename also uses the label as blob-(label)-(shard).json when no explicit output file is configured.

Reporters read the label from merged blob metadata and display it when replaying --merge-reports.

The difference from #9967

Concern	#9967 (label as project.name)	This PR (label as blob metadata)
Mechanism	Clone TestProject per label at read time	Store label in blob task metadata and salt file identity
readBlobs changes	Significant — label discovery, ID rewrite, env module rewrite	No project cloning or read-time project rewrite
file.projectName	Mutated to include label	projectName unchanged
API	[["blob", { label }]] reporter option or VITEST_BLOB_LABEL	[["blob", { label }]] reporter option or VITEST_BLOB_LABEL (same)
Original run output	Label could appear as a project-style label	Built-in reporters do not show the blob label during the original run

Demo

I setup CI to upload html reports for test/cli and test/core. It's viewable on newly introduced viewer #10023.

https://viewer.vitest.dev/?url=https://github.com/vitest-dev/vitest/actions/runs/24275938517/artifacts/6383832383

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[sheremet-va]

I like the approach, but I feel like meta.label might be too generic. Typecheck is a static one where users can't even modify meta, and in benchmark you don't have access to meta.

Should we make it more internal to avoid clashes with user meta? It could also be removed from json.meta: #10078

[hi-ogawa]

Agree meta.label can definitely crash. I'll rename it to sound like something vitest owned.

[hi-ogawa]

Hash was aligned so pre-parse mode was broken. I consolidated ast-collect with the same createFileTask from runner package.

[hi-ogawa]

We discussed that

• receive the label via blob reporter option ['blob', { label: ... }]
• stash this as core level internal config by extracting it during resolveConfig
• during the original test run, this label shouldn't show up as label for builtin reporters.

[hi-ogawa]

The PR is updated with blob reporter level option. I've also revived VITEST_BLOB_LABEL environment support so it can one shot from CLI VITEST_BLOB_LABEL=linux vitest --reporter=blob.