name: backport
on:
  pull_request_target:
    types: [closed, labeled]

permissions:
  contents: read
  pull-requests: write

jobs:
  backport:
    name: Backport merged PR
    runs-on: ubuntu-latest
    if: >
      github.event.pull_request.merged == true &&
      (
        github.event.action == 'closed' ||
        startsWith(github.event.label.name, 'backport ')
      )
    steps:
      - name: Generate backport App token
        id: backport-app
        uses: actions/create-github-app-token@v3
        with:
          client-id: ${{ vars.XCAT_BACKPORT_APP_CLIENT_ID }}
          private-key: ${{ secrets.XCAT_BACKPORT_APP_PRIVATE_KEY }}
          permission-contents: write
          permission-pull-requests: write
          permission-workflows: write
      - uses: actions/checkout@v6
        with:
          token: ${{ steps.backport-app.outputs.token }}
      - name: Create backport pull requests
        id: backport
        uses: korthout/backport-action@v3
        with:
          github_token: ${{ steps.backport-app.outputs.token }}
          auto_merge_enabled: true
      - name: Approve backport pull requests
        if: steps.backport.outputs.created_pull_numbers != ''
        uses: actions/github-script@v8
        env:
          BACKPORT_APP_SLUG: ${{ steps.backport-app.outputs.app-slug }}
          CREATED_PULL_NUMBERS: ${{ steps.backport.outputs.created_pull_numbers }}
        with:
          github-token: ${{ github.token }}
          script: |
            const expectedAuthor = `${process.env.BACKPORT_APP_SLUG}[bot]`;
            const pullNumbers = process.env.CREATED_PULL_NUMBERS
              .trim()
              .split(/\s+/)
              .map(Number);

            for (const pull_number of pullNumbers) {
              if (!Number.isSafeInteger(pull_number) || pull_number <= 0) {
                throw new Error(`Invalid backport pull request number: ${pull_number}`);
              }

              const { data: pull } = await github.rest.pulls.get({
                ...context.repo,
                pull_number,
              });

              if (pull.user?.login !== expectedAuthor) {
                throw new Error(
                  `Refusing to approve #${pull_number}: expected author ${expectedAuthor}`,
                );
              }
              if (pull.head.repo?.full_name !== `${context.repo.owner}/${context.repo.repo}`) {
                throw new Error(
                  `Refusing to approve #${pull_number}: head repository does not match`,
                );
              }
              if (pull.state !== 'open' || pull.draft) {
                throw new Error(
                  `Refusing to approve #${pull_number}: pull request is not open and ready`,
                );
              }
              if (pull.auto_merge === null || pull.auto_merge === undefined) {
                throw new Error(
                  `Refusing to approve #${pull_number}: auto-merge is not enabled`,
                );
              }

              await github.rest.pulls.createReview({
                ...context.repo,
                pull_number,
                commit_id: pull.head.sha,
                event: 'APPROVE',
                body: 'Automated approval for a conflict-free backport of an already merged pull request.',
              });
            }
      - name: Fail when a backport needs intervention
        if: >
          steps.backport.outputs.was_successful_by_target != '' &&
          steps.backport.outputs.was_successful != 'true'
        run: exit 1