name: Restore Nix cache
description: |
  Restore /nix from the shared CI cache and runs garbage collection
  before storing if save is set to true.
inputs:
  save:
    description: Whether this job should save the cache after running.
    required: false
    default: "false"
outputs:
  cache-key-hash:
    description: >
      Hash of the Nix inputs (flake, toolchain, lockfiles) that key the /nix
      cache. Dependent caches (e.g. the Rust build cache) should fold this into
      their key so they are invalidated whenever the Nix-provided toolchain
      changes.
    value: ${{ steps.compute-key.outputs.hash }}
runs:
  using: composite
  steps:
    - name: Compute Nix cache key hash
      id: compute-key
      shell: bash
      run: echo "hash=${{ hashFiles('flake.nix', 'flake.lock', 'nix/**', 'rust-toolchain.toml', 'Cargo.lock') }}" >> "$GITHUB_OUTPUT"

    - name: Restore /nix from cache
      uses: nix-community/cache-nix-action@7df957e333c1e5da7721f60227dbba6d06080569 # v7.0.2
      with:
        primary-key: nix-${{ runner.os }}-ci-${{ steps.compute-key.outputs.hash }}
        restore-prefixes-first-match: nix-${{ runner.os }}-ci-
        save: ${{ inputs.save }}
        # Target store size of 0 means always run garbage collection.
        # Note that pinning step below prevents the usable cache to be cleaned.
        gc-max-store-size: 0

    - name: Pin dev-shell closure as GC root
      if: inputs.save == 'true'
      shell: bash
      # `--profile` registers the dev-shell closure as an indirect GC root,
      # so the pre-upload garbage collection keeps it (and its dependencies)
      # while dropping everything else.
      run: nix develop --profile "$RUNNER_TEMP/ci-devshell" --command true