name: Check Merge Fast-Forward Only

permissions:
  pull-requests: write

on:
  push:
  pull_request_target:
  pull_request_review:
    types: [submitted]

jobs:
  check_merge:
    runs-on: ubuntu-latest
    steps:
      - name: Checkout repository
        uses: actions/checkout@v3
        with:
          fetch-depth: 0

      - name: Set up Git
        run: |
          git config user.name "GitHub Action"
          git config user.email "noreply@example.com"

      - name: Check merge --ff-only
        run: |
          git fetch origin master:master
          git checkout master
          if [[ "${{ github.event_name }}" == "pull_request"* ]]; then
            git fetch origin pull/${{ github.event.pull_request.number }}/head
            git merge --ff-only ${{ github.event.pull_request.head.sha }}
          else
            git merge --ff-only ${{ github.sha }}
          fi

      - name: add labels
        uses: actions-ecosystem/action-add-labels@v1
        if: failure()
        with:
          labels: |
            needs rebase

      - name: comment
        uses: mshick/add-pr-comment@v2
        if: failure()
        with:
          message: |
            This pull request has conflicts, please rebase.