Prune as the post action is not useful

The action will run pnpm store prune as a post action, but based on the documented example for caching as in README:

on:
  - push
  - pull_request

jobs:
  cache-and-install:
    runs-on: ubuntu-latest

    steps:
      - name: Checkout
        uses: actions/checkout@v4

      - uses: pnpm/action-setup@v4
        name: Install pnpm
        with:
          version: 10
          run_install: false

      - name: Install Node.js
        uses: actions/setup-node@v4
        with:
          node-version: 20
          cache: 'pnpm'

      - name: Install dependencies
        run: pnpm install

The caching step in the post step of action/setup-node will actually happen before the post step pnpm/action-setup which does the pruning, so the cache will not be pruned.

If you try to still benefit from the pruning in this aciton, and put any caching before pnpm/action-setup (so that the post caching happens after action-setup post), you won't benefit from the cache as the install step of this action nukes the install directory. Doing caching before pnpm is installed also means that you cannot get pnpm store path to get what directory to cache.

Workaround

To cache the pnpm store, after pruning, you could something like this

on: [push, pull_request]

jobs:
  cache-and-install:
    runs-on: ubuntu-latest

    steps:
      - name: Checkout
        uses: actions/checkout@v4

      - uses: pnpm/action-setup@v4
        name: Install pnpm
        with:
          version: 10
          run_install: false

      - name: Install Node.js
        uses: actions/setup-node@v4
        with:
          node-version: 20

      - name: Get pnpm cache directory
        id: pnpm-store-path
        run: echo "path=$(pnpm store path)" >> $GITHUB_OUTPUT

      - name: Cache pnpm store
        uses: actions/cache@v4
        with:
          path: ${{ steps.pnpm-store-path.outputs.path }}
          key: ${{ runner.os }}-pnpm-store-${{ hashFiles('**/pnpm-lock.yaml') }}
          restore-keys: |
            ${{ runner.os }}-pnpm-store-

      - name: Install dependencies
        run: pnpm install

      - name: Prune store
        run: pnpm store prune

Suggestion

I think it would be great if pnpm/action-setup had built-in cache, especially since with the devEngines.runtime support, you don't really need to use setup-node anymore. That seems like quite the effort, and not sure something I could pull off, so what's easier to achieve would be to remove the pruning from the post action, and update the documentation to recommend to run it separately. I will try to see if I can put together a PR with this.

EDIT: Or, the pruning could happen right after install I guess! That should not have any negative effect on any following actions, but any caching that may happen later will be caching a pruned store at least?

Read the caching example in README and the installation flow referenced at src/install-pnpm/run.ts. Trace the GitHub Actions post-step ordering, then decide where pruning belongs and update the documented caching workflow so the resulting store is pruned as intended.