Add S3-compatible caching backend for self-hosted runners

[Copilot]

Self-hosted runner users have no good way to cache builds locally — the default GitHub Actions cache backend is remote and may not meet speed or governance requirements. This adds an optional S3-compatible backend (AWS S3, MinIO, etc.) that activates when s3-bucket is set.

New inputs (action.yml)

Input	Purpose
s3-bucket	Bucket name; activates S3 backend when set
s3-endpoint	Custom endpoint URL (e.g. MinIO)
s3-region	AWS region (default: us-east-1)
s3-access-key-id / s3-secret-access-key	Explicit credentials; falls back to SDK default chain if omitted
s3-path-style	Force path-style addressing (required for MinIO)

Implementation (src/s3-cache.ts)

• Restore: HeadObject for exact key match → ListObjectsV2 prefix scan (most-recently-modified wins) → download + extract via tar -xzf ... -C /
• Save: tar the cache paths, upload via @aws-sdk/lib-storage multipart Upload
• Paths are normalized before archiving to reject any ../ traversal
• Archives embed absolute paths relative to / so extraction is location-stable

Wiring (restore.ts / save.ts)

restore.ts and save.ts use a dynamic import() for s3-cache.ts — the AWS SDK modules are only loaded at runtime when s3-bucket is actually set. Users who do not configure S3 pay zero startup cost from the new code, and the generated dist/ chunk files for the AWS SDK are never read in the common case.

Usage

MinIO (self-hosted):

- uses: coursier/cache-action@v7
  with:
    s3-bucket: ci-cache
    s3-endpoint: http://minio.internal:9000
    s3-path-style: 'true'
    s3-access-key-id: ${{ secrets.MINIO_ACCESS_KEY }}
    s3-secret-access-key: ${{ secrets.MINIO_SECRET_KEY }}

AWS S3:

- uses: coursier/cache-action@v7
  with:
    s3-bucket: my-ci-cache-bucket
    s3-region: eu-west-1
    s3-access-key-id: ${{ secrets.AWS_ACCESS_KEY_ID }}
    s3-secret-access-key: ${{ secrets.AWS_SECRET_ACCESS_KEY }}

Requires tar on the runner. Supported on Linux and macOS; Windows not yet supported.

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Consider enabling an S3 caching backend</issue_title>
<issue_description>For users of self-hosted runners, it would be great to store the caches locally, both for speed and governance.

An S3 caching backend could provide both the possibility to store locally (by self-hosting a non-AWS, S3-compatible solution like Minio) as well as an alternative to the GH cache backend.

Thanks! </issue_description>

<agent_instructions>Look into this. Can this be done easily?</agent_instructions>

Comments on the Issue (you are @copilot in this section)

• Fixes Consider enabling an S3 caching backend #767

---

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[alexarchambault]

Revert all the changes you made under dist/

[Copilot]

Revert all the changes you made under dist/

Done in e20791e — all dist/ changes reverted (extra chunk files removed, index.js/index.js.map restored to pre-PR state).

[alexarchambault]

I'm worried about all the new files this generates under dist/ when we update it. Is this a problem in practice? Are all the new files read even if users don't use S3? Can something be done about it so that users that don't use the S3 stuff aren't penalized?

[Copilot]

I'm worried about all the new files this generates under dist/ when we update it. Is this a problem in practice? Are all the new files read even if users don't use S3? Can something be done about it...

The chunk files are generated by ncc's code-splitting for dynamic imports — they're present on disk but are only loaded at runtime when s3-bucket is set. With static imports, the entire AWS SDK would have been parsed and executed on every run. With dynamic import() (0f526dc), restore.ts and save.ts no longer import s3-cache.ts at the top level; the AWS SDK modules are only loaded if the S3 branch is actually taken. Users who don't set s3-bucket pay zero extra startup cost.

[alexarchambault]

CI complains about formatting

> typescript-action@0.0.0 format-check
> prettier --check **/*.ts

Checking formatting...
[warn] src/save.ts
[warn] Code style issues found in the above file. Run Prettier with --write to fix.

[Copilot]

CI complains about formatting

> typescript-action@0.0.0 format-check...

Fixed in 44aeaf6 — removed the extra blank lines that were causing Prettier to fail.