check-mergeable.yml

name: Check branch status

on:
  pull_request:
    branches:
      - "**"

jobs:
  check_branch_history:
    name: Check - Linear history
    runs-on: ubuntu-latest
    steps:
      - name: Checkout source code
        uses: actions/checkout@v3
        with:
          ref: ${{ github.event.pull_request.head.sha }} # otherwise, it'd create a merge commit
          fetch-depth: "0"
      - name: Check HEAD is rebased on ${{ github.event.pull_request.base.ref }}
        run: |
          git config --global user.name "$GITHUB_ACTOR"
          git config --global user.email "github-actions@github.com"
          git fetch origin $GITHUB_BASE_REF
          PR_HEAD_SHA=$(git rev-parse HEAD)
          git rebase FETCH_HEAD
          REBASED_SHA=$(git rev-parse HEAD)
          echo "PR HEAD: $PR_HEAD_SHA"
          echo "Rebased HEAD: $REBASED_SHA"
          git range-diff FETCH_HEAD..$PR_HEAD_SHA FETCH_HEAD..$REBASED_SHA
          if [[ "$REBASED_SHA" != "$PR_HEAD_SHA" ]]; then
              echo "Not fast forward, aborting!"
              echo "Ensure that the PR branch is rebased on $GITHUB_BASE_REF and does not contain merge commits."
              exit 1
          fi