name: Backport

on:
  pull_request_target:
    types: [closed, labeled]

permissions:
  contents: write
  pull-requests: write
  issues: read

jobs:
  backport:
    name: Backport PR
    runs-on: ubuntu-latest
    # Only run on merged PRs with backport labels.
    # Labels must match pattern: backport-v* (e.g., backport-v0.20.x-branch).
    # This excludes labels like "backport candidate" or "backport-candidate".
    if: |
      github.event.pull_request.merged == true &&
      contains(join(github.event.pull_request.labels.*.name, ','), 'backport-v')

    steps:
      - name: Checkout repository
        uses: actions/checkout@v5
        with:
          fetch-depth: 0
          ref: ${{ github.event.pull_request.base.ref }}

      - name: Validate target branches exist
        id: validate
        shell: bash
        run: |
          # Extract all backport labels
          labels='${{ toJSON(github.event.pull_request.labels.*.name) }}'
          echo "All labels: $labels"

          # Parse labels and extract branch names
          # Only match labels starting with "backport-v" to exclude labels like
          # "backport candidate" or "backport-candidate"
          backport_labels=$(echo "$labels" | jq -r '.[] | select(startswith("backport-v"))')

          if [ -z "$backport_labels" ]; then
            echo "::error::No valid backport labels found (must start with 'backport-v')"
            exit 1
          fi

          echo "Found backport labels:"
          echo "$backport_labels"

          # Check each target branch exists
          missing_branches=()
          valid_branches=()
          while IFS= read -r label; do
            # Extract branch name (everything after "backport-")
            branch_name="${label#backport-}"
            echo "Checking if branch exists: $branch_name"

            # Check if branch exists in remote
            if ! git ls-remote --heads origin "$branch_name" | grep -q "$branch_name"; then
              echo "::warning::Target branch '$branch_name' does not exist (from label '$label')"
              missing_branches+=("$branch_name")
            else
              echo "✓ Branch '$branch_name' exists"
              valid_branches+=("$branch_name")
            fi
          done <<< "$backport_labels"

          # Report validation results
          if [ ${#missing_branches[@]} -gt 0 ]; then
            echo "::warning::The following target branches do not exist and will be skipped: ${missing_branches[*]}"
            echo "::warning::Please check the branch names or create the branches before retrying"
          fi

          # Only fail if ALL branches are invalid
          if [ ${#valid_branches[@]} -eq 0 ]; then
            echo "::error::No valid target branches found. All backport labels reference non-existent branches."
            exit 1
          fi

          echo "✓ Found ${#valid_branches[@]} valid target branch(es): ${valid_branches[*]}"
          if [ ${#missing_branches[@]} -gt 0 ]; then
            echo "⚠ Skipping ${#missing_branches[@]} invalid branch(es): ${missing_branches[*]}"
          fi

      - name: Create backport PRs
        # Uses version v3.4, we pin to a hash here. For more details to 
        # available versions, see: 
        # https://github.com/korthout/backport-action/releases.
        uses: korthout/backport-action@d07416681cab29bf2661702f925f020aaa962997
        with:
          # Automatically detect target branches from labels.
          # Labels must be in format: backport-v0.20.x-branch (must start
          # with "backport-v"). This excludes labels like "backport candidate" 
          # or "backport-candidate". The pattern extracts everything after 
          # "backport-" as the branch name.
          label_pattern: '^backport-(v.+)$'

          # GitHub token for creating PRs.
          github_token: ${{ secrets.GITHUB_TOKEN }}

          # PR title format - shows it's a backport with original PR number.
          pull_title: '[${target_branch}] Backport #${pull_number}: ${pull_title}'

          # PR description template - links back to original PR.
          pull_description: |-
            Backport of #${pull_number}

            ---

            ${pull_description}

          # Automatically add labels to backport PRs.
          # The 'no-changelog' label skips the release notes check in CI.
          add_labels: no-changelog

          # Copy milestone from original PR to backport PR.
          copy_milestone: true

          # Merge strategy - skip merge commits, use cherry-pick only.
          merge_commits: skip

          # If conflicts occur, create a draft PR with conflict markers.
          experimental: '{"conflict_resolution": "draft_commit_conflicts"}'