prevent_merge_when_branch_behind.yml

name: Require Branch to Be Up-to-Date

# Trigger this workflow on pull request events targeting a specific branch.
on:
  pull_request:
    branches:
      - production
      - development
      - fix_prevent_merge_when_branch_behind
  workflow_dispatch: # enables manual triggering

jobs:
  check-branch:
    runs-on: ubuntu-latest
    steps:
      - name: Determine Comparison Branch
        id: comparison
        run: |
          if [[ "${{ github.event.pull_request.base.ref }}" == "production" && "${{ github.event.pull_request.head.ref }}" == "development" ]]; then
            echo "branch=production" >> $GITHUB_ENV
          else
            echo "branch=development" >> $GITHUB_ENV
          fi

      - name: Checkout pull request branch
        uses: actions/checkout@v3
        with:
          ref: ${{ github.event.pull_request.head.ref }}
    
      - name: Fetch comparison branch
        run: |
          git fetch --unshallow
          git fetch origin ${{ env.branch }}

      - name: Compare branch with ${{ env.branch }}
        run: |
          if git merge-base --is-ancestor origin/${{ env.branch }} HEAD; then
            echo "::notice ::Branch is up-to-date with ${{ env.branch }}."
          else
            echo "::error ::Merge Blocked: Your branch is behind the latest commits on ${{ env.branch }}. Please update your branch before attempting to merge."
            exit 1
          fi