Add save-pants-cache companion action for cancellation-safe cache saves

[Affanmir]

Summary

BREAKING: init-pants now restores caches only (via actions/cache/restore@v5). A new save-pants-cache companion action must be added at the end of the job to persist caches.

Why: actions/cache@v5 saves via a post-step hook that does not run on workflow cancellation. Workflows using cancel-in-progress: true enter a cold-cache cycle — every cancelled run loses its cache save, so the next run starts cold.

Fix: Split restore and save into separate actions. The save step uses if: always() which fires even on cancellation, breaking the cycle.

Changes

• init-pants/action.yaml — Switch actions/cache@v5 → actions/cache/restore@v5, add outputs for cache keys/paths
• save-pants-cache/action.yaml — New companion action using actions/cache/save@v5
• READMEs updated with migration guide

Usage

      - uses: pantsbuild/actions/init-pants@main
        id: pants-init
        with:
          named-caches-hash: ${{ hashFiles('lockfiles/*.json') }}

      # ... build/test steps ...

      - uses: pantsbuild/actions/save-pants-cache@main
        if: always()
        with:
          setup-cache-path: ${{ steps.pants-init.outputs.setup-cache-path }}
          setup-cache-key: ${{ steps.pants-init.outputs.setup-cache-key }}
          named-caches-path: ${{ steps.pants-init.outputs.named-caches-path }}
          named-caches-key: ${{ steps.pants-init.outputs.named-caches-key }}
          named-caches-hash: ${{ steps.pants-init.outputs.named-caches-hash }}
          lmdb-store-path: ${{ steps.pants-init.outputs.lmdb-store-path }}
          lmdb-store-key: ${{ steps.pants-init.outputs.lmdb-store-key }}
          cache-lmdb-store: ${{ steps.pants-init.outputs.cache-lmdb-store }}

Context

Discovered in CI where persistent cold caches occurred despite no code changes between runs. GHA cache keys are immutable, so actions/cache/save is a safe no-op when the key already exists.

Test plan

• init-pants restores caches correctly (setup, named, LMDB)
• save-pants-cache with if: always() saves caches on normal completion
• Cache saves survive cancel-in-progress cancellation
• Cache keys match between init-pants outputs and save-pants-cache inputs

[benjyw]

It's probably a bad idea to break all the existing users by making the existing action not save the cache. Plus that usage snippet is very verbose, since we have to set up all those paths.

Is it possible to have an optional save action, and it only runs if the earlier job was cancelled?