rebase_check.yml

name: Rebase Check
on: [pull_request]

jobs:
  check_files:
    name: Check for outdated files
    runs-on: ubuntu-latest
    steps:
    - name: Checkout PR
      uses: actions/checkout@v3
      with:
        fetch-depth: 0
    - name: Check if dependencies and checks are up to date.
      run: |
        readonly -a CHECKED_FILES=(
          go.mod
          .github/CODEOWNERS
          .github/workflows/
        )

        # Notes:
        #   * Do not use ${GITHUB_REF}, github.sha, or HEAD because they are
        #     the merged commit of the pull request and main.  There are no
        #     outdated files in the merged commit.
        #   * refs/pull/${pr_number}/head is not available, so use
        #     github.event.pull_request.head.sha which is the "head sha" of
        #     the event that triggered the pull request.
        #   * Do not use github.event.pull_request.base.sha because it is
        #     the base when the pull request was created, not after a rebase.
        #     Ask git merge-base to tell us a suitable base.
        readonly HEAD="${{ github.event.pull_request.head.sha }}"
        readonly BASE="$(git merge-base origin/main "${HEAD}")"

        # Notes:
        #   * Diff between "${BASE}" and origin/main detects outdated files.
        #   * Diff between "${BASE}" and "${HEAD}" detects files changed by PR.
        #   * Merge conflict between "${HEAD}" and main is not detected here,
        #     but GitHub will prevent merging if that's the case.
        #   * comm -23 SET1 SET2 shows what's in SET1 and not in SET2.
        outdated_unchanged="$(
          comm -23 \
            <(git diff --name-only "${BASE}" origin/main -- "${CHECKED_FILES[@]}" | sort) \
            <(git diff --name-only "${BASE}" "${HEAD}" -- "${CHECKED_FILES[@]}" | sort)
        )"
        if [ "${outdated_unchanged}" ]; then
          echo "The following files are outdated: ${outdated_unchanged}"
          echo "Please rebase your pull request to main and rerun checks."
          exit 1
        fi