name: base-up-to-date

on:
    pull_request:
        types:
            - opened
            - reopened
            - synchronize

permissions:
    contents: read

concurrency:
    group: base-up-to-date-pr-${{ github.event.pull_request.number }}
    cancel-in-progress: true

jobs:
    base-up-to-date:
        name: base-up-to-date
        runs-on: ubuntu-latest
        timeout-minutes: 5
        steps:
            - name: Checkout pull request head
              uses: actions/checkout@v4
              with:
                  ref: ${{ github.event.pull_request.head.sha }}
                  fetch-depth: 0

            - name: Check target branch is included
              shell: bash
              env:
                  BASE_REF: ${{ github.event.pull_request.base.ref }}
                  HEAD_SHA: ${{ github.event.pull_request.head.sha }}
              run: |
                  set -euo pipefail

                  target_ref='refs/remotes/origin/base-up-to-date-target'
                  git fetch --no-tags origin "refs/heads/$BASE_REF:$target_ref"
                  base_sha="$(git rev-parse "$target_ref^{commit}")"

                  if git merge-base --is-ancestor "$base_sha" "$HEAD_SHA"; then
                      printf 'PR head %s contains target branch %s at %s.\n' \
                          "$HEAD_SHA" "$BASE_REF" "$base_sha"
                      exit 0
                  fi

                  echo "::error title=Base branch is out of date::PR head $HEAD_SHA does not contain the latest $BASE_REF commit $base_sha."
                  printf 'Update the PR branch with the latest %s commit, then push a new commit.\n' "$BASE_REF"
                  exit 1