# Automatically backport merged PRs to the last N release branches when the
# "backport" label is applied. Works whether the label is added before or
# after the PR is merged.
#
# Usage:
#   1. Add the "backport" label to a PR targeting main.
#   2. When the PR merges (or if already merged), the workflow detects the
#      latest release/* branches and opens one cherry-pick PR per branch.
#
# The created backport PRs follow existing repo conventions:
#   - Branch:  backport/<pr>-to-<version>
#   - Title:   <original PR title> (#<pr>)
#   - Body:    links back to the original PR and merge commit

name: Backport
on:
  pull_request_target:
    branches:
      - main
    types:
      - closed
      - labeled

permissions: {}

# Prevent duplicate runs for the same PR when both 'closed' and 'labeled'
# fire in quick succession.
concurrency:
  group: backport-${{ github.event.pull_request.number }}

jobs:
  detect:
    name: Detect target branches
    permissions:
      contents: read
    if: >
      github.event.pull_request.merged == true &&
      contains(github.event.pull_request.labels.*.name, 'backport')
    runs-on: ubuntu-latest
    outputs:
      branches: ${{ steps.find.outputs.branches }}
    steps:
      - uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
        with:
          # Need all refs to discover release branches.
          fetch-depth: 0
          persist-credentials: false

      - name: Find latest release branches
        id: find
        run: |
          # List remote release branches matching the exact release/2.X
          # pattern (no suffixes like release/2.31_hotfix), sort by minor
          # version descending, and take the top 3.
          BRANCHES=$(
            git branch -r \
              | grep -E '^\s*origin/release/2\.[0-9]+$' \
              | sed 's|.*origin/||' \
              | sort -t. -k2 -n -r \
              | head -3
          )

          if [ -z "$BRANCHES" ]; then
            echo "No release branches found."
            echo "branches=[]" >> "$GITHUB_OUTPUT"
            exit 0
          fi

          # Convert to JSON array for the matrix.
          JSON=$(echo "$BRANCHES" | jq -Rnc '[inputs | select(length > 0)]')
          echo "branches=$JSON" >> "$GITHUB_OUTPUT"
          echo "Will backport to: $JSON"

  backport:
    name: "Backport to ${{ matrix.branch }}"
    needs: detect
    permissions:
      contents: write
      pull-requests: write
    if: needs.detect.outputs.branches != '[]'
    runs-on: ubuntu-latest
    strategy:
      matrix:
        branch: ${{ fromJson(needs.detect.outputs.branches) }}
      fail-fast: false
    env:
      PR_NUMBER: ${{ github.event.pull_request.number }}
      PR_TITLE: ${{ github.event.pull_request.title }}
      PR_URL: ${{ github.event.pull_request.html_url }}
      MERGE_SHA: ${{ github.event.pull_request.merge_commit_sha }}
      SENDER: ${{ github.event.sender.login }}
      BRANCH: ${{ matrix.branch }}
    steps:
      - uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
        with:
          # Full history required for cherry-pick.
          fetch-depth: 0
          persist-credentials: false

      - name: Cherry-pick and open PR
        env:
          GH_TOKEN: ${{ secrets.GITHUB_TOKEN }}
        run: |
          set -euo pipefail

          # Configure git to authenticate pushes with the job token
          # since persist-credentials is disabled on checkout.
          git remote set-url origin "https://x-access-token:${GH_TOKEN}@github.com/${GITHUB_REPOSITORY}.git"

          RELEASE_VERSION="$BRANCH"
          # Strip the release/ prefix for naming.
          VERSION="${RELEASE_VERSION#release/}"
          BACKPORT_BRANCH="backport/${PR_NUMBER}-to-${VERSION}"

          git config user.name "github-actions[bot]"
          git config user.email "41898282+github-actions[bot]@users.noreply.github.com"

          # Check if backport branch already exists (idempotency for re-runs).
          if git ls-remote --exit-code origin "refs/heads/${BACKPORT_BRANCH}" >/dev/null 2>&1; then
            echo "Backport branch ${BACKPORT_BRANCH} already exists, skipping."
            exit 0
          fi

          # Create the backport branch from the target release branch.
          git checkout -b "$BACKPORT_BRANCH" "origin/${RELEASE_VERSION}"

          # Cherry-pick the merge commit. Use -x to record provenance and
          # -m1 to pick the first parent (the main branch side).
          CONFLICTS=false
          if ! git cherry-pick -x -m1 "$MERGE_SHA"; then
            echo "::warning::Cherry-pick to ${RELEASE_VERSION} had conflicts."
            CONFLICTS=true

            # Abort the failed cherry-pick and create an empty commit
            # explaining the situation.
            git cherry-pick --abort
            git commit --allow-empty -m "Cherry-pick of #${PR_NUMBER} requires manual resolution

          The automatic cherry-pick of ${MERGE_SHA} to ${RELEASE_VERSION} had conflicts.
          Please cherry-pick manually:

              git cherry-pick -x -m1 ${MERGE_SHA}"
          fi

          git push origin "$BACKPORT_BRANCH"

          TITLE="${PR_TITLE} (#${PR_NUMBER})"
          BODY=$(cat <<EOF
          Backport of ${PR_URL}

          Original PR: #${PR_NUMBER} — ${PR_TITLE}
          Merge commit: ${MERGE_SHA}
          Requested by: @${SENDER}
          EOF
          )

          if [ "$CONFLICTS" = true ]; then
            TITLE="${TITLE} (conflicts)"
            BODY="${BODY}

          > [!WARNING]
          > The automatic cherry-pick had conflicts.
          > Please resolve manually by cherry-picking the original merge commit:
          >
          > \`\`\`
          > git fetch origin ${BACKPORT_BRANCH}
          > git checkout ${BACKPORT_BRANCH}
          > git reset --hard origin/${RELEASE_VERSION}
          > git cherry-pick -x -m1 ${MERGE_SHA}
          > # resolve conflicts, then push
          > \`\`\`"
          fi

          # Check if a PR already exists for this branch (idempotency
          # for re-runs).
          EXISTING_PR=$(gh pr list --head "$BACKPORT_BRANCH" --base "$RELEASE_VERSION" --state all --json number --jq '.[0].number // empty')
          if [ -n "$EXISTING_PR" ]; then
            echo "PR #${EXISTING_PR} already exists for ${BACKPORT_BRANCH}, skipping."
            exit 0
          fi

          gh pr create \
            --base "$RELEASE_VERSION" \
            --head "$BACKPORT_BRANCH" \
            --title "$TITLE" \
            --body "$BODY" \
            --assignee "$SENDER" \
            --reviewer "$SENDER"