check-base-branch-before-merge.yml

name: Check Base Branch Before Merge

on:
  pull_request_review:
    types: [submitted]

jobs:
  check-base-branch-before-merge:
    if: github.event.review.state == 'APPROVED'
    runs-on: ubuntu-latest

    steps:
      - name: Checkout code
        uses: actions/checkout@v4

      - name: Check base branch
        run: |
          git fetch origin --prune
          
          CURRENT_BRANCH=${{ github.event.pull_request.head.ref }}
          TARGET_BRANCH="${{ github.event.pull_request.base.ref }}"

          COMMON_ANCESTOR=$(git merge-base origin/$CURRENT_BRANCH origin/$TARGET_BRANCH)
          COMMITS_BEHIND=$(git rev-list --count origin/$TARGET_BRANCH ^$COMMON_ANCESTOR)

          if [ "$COMMITS_BEHIND" -eq 0 ]; then
            echo "Your branch is up-to-date with the target branch."
          elif [ "$COMMITS_BEHIND" -eq 1 ]; then
            echo "Your branch is 1 commit behind the target branch."
            exit 1
          else
            echo "Your branch is $COMMITS_BEHIND commits behind the target branch."
            exit 1
          fi