name: Sync default branches

on:
  push:
    branches:
      - main
      - master
  pull_request:
    paths:
      - .github/workflows/sync-default-branches.yml

permissions: {}

defaults:
  run:
    shell: bash -xeuo pipefail {0}

concurrency:
  group: "sync-default-branches-${{ github.ref }}"
  cancel-in-progress: true

jobs:
  sync:
    runs-on: ubuntu-slim
    permissions:
      contents: write
    steps:
      - name: Configure Git user
        uses: Homebrew/actions/git-user-config@main
        with:
          username: github-actions[bot]

      - name: Determine source and target branches
        id: branches
        run: |
          if [[ "${GITHUB_REF_NAME}" == "main" ]]; then
            target="master"
            source="main"
          else
            target="main"
            source="master"
          fi
          echo "target=${target}" >> "$GITHUB_OUTPUT"
          echo "source=${source}" >> "$GITHUB_OUTPUT"

      - uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
        with:
          fetch-depth: 1
          persist-credentials: true

      - name: Get target SHA
        id: sha
        run: |
          TARGET_SHA=$(git ls-remote origin "refs/heads/${SOURCE_BRANCH}" | cut -f1)
          echo "target=${TARGET_SHA}" >> "$GITHUB_OUTPUT"
        env:
          SOURCE_BRANCH: ${{ steps.branches.outputs.source }}

      - name: Push target branch
        if: github.ref == 'refs/heads/main' || github.ref == 'refs/heads/master'
        run: git push origin "${TARGET_SHA}:refs/heads/${TARGET_BRANCH}" --force
        env:
          TARGET_SHA: ${{ steps.sha.outputs.target }}
          TARGET_BRANCH: ${{ steps.branches.outputs.target }}