name: CI image

concurrency:
  group: ci-image-${{ github.sha }}
  cancel-in-progress: false

# Resolves the CI image the calling workflow's jobs run in, building and pushing
# it only when it isn't published yet.
on:
  workflow_call:
    outputs:
      image:
        description: Image reference to pass to a job's `container:`.
        value: ${{ jobs.ci-image.outputs.image }}

jobs:
  ci-image:
    runs-on: ubuntu-latest
    timeout-minutes: 60
    outputs:
      image: ${{ steps.image-ref.outputs.image }}
    steps:
      - uses: actions/checkout@v5
        with:
          ref: ${{ github.event.pull_request.head.sha || github.head_ref }}

      # Tagging by content hash means a PR that touches the Dockerfile or the
      # toolchain builds and tests against its own image, while every other PR
      # reuses the one already in the registry.
      - name: Compute image reference
        id: image-ref
        run: |
          repo="$(echo "${{ github.repository }}" | tr '[:upper:]' '[:lower:]')"
          tag="$(cat .github/docker/ci.Dockerfile rust-toolchain.toml | sha256sum | cut -c1-16)"
          echo "image=ghcr.io/${repo}/ci:${tag}" >> "$GITHUB_OUTPUT"
          echo "CI image: ghcr.io/${repo}/ci:${tag}"

      - name: Log in to GHCR
        uses: docker/login-action@v3
        with:
          registry: ghcr.io
          username: ${{ github.actor }}
          password: ${{ secrets.GITHUB_TOKEN }}

      - name: Check whether the image is already published
        id: probe
        run: |
          if docker manifest inspect "${{ steps.image-ref.outputs.image }}" > /dev/null 2>&1; then
            echo "exists=true" >> "$GITHUB_OUTPUT"
            echo "Image already published, skipping build."
          else
            echo "exists=false" >> "$GITHUB_OUTPUT"
            echo "Image not published yet, building it."
          fi

      - name: Set up Docker Buildx
        if: steps.probe.outputs.exists == 'false'
        uses: docker/setup-buildx-action@v3

      # A pull_request from a fork gets a read-only GITHUB_TOKEN, so this step
      # fails there. It only runs when the fork changed the image, which is the
      # case that needs a maintainer's eyes anyway.
      - name: Build and push CI image
        if: steps.probe.outputs.exists == 'false'
        uses: docker/build-push-action@v5
        with:
          context: .
          file: ./.github/docker/ci.Dockerfile
          push: true
          tags: ${{ steps.image-ref.outputs.image }}
          # Links the package to the repo, so it shows up there and inherits its
          # access rules.
          labels: org.opencontainers.image.source=https://github.com/${{ github.repository }}
          cache-from: type=gha,scope=ci-image
          cache-to: type=gha,mode=max,scope=ci-image