# Fails a required status check if a pull request from one `repo-sync/*`
# branch targets another `repo-sync/*` branch.  Combined with branch
# protection on `repo-sync/**` requiring this check, this prevents one sync
# PR from being merged into another sync PR's branch (which would bypass the
# restacking flow and corrupt the sync stack).
#
# PRs from non-`repo-sync/*` branches into `repo-sync/*` branches (e.g. for
# conflict resolution) are intentionally allowed.  Direct pushes to
# `repo-sync/*` branches are also unaffected, since only the `pull_request`
# trigger is used here.

name: Sync PR Checks
on:
  pull_request:
    types: [opened, reopened, edited, synchronize, ready_for_review]

jobs:
  check:
    name: Verify PR base is the default branch
    runs-on: ubuntu-slim
    steps:
      - name: Fail if merging a repo-sync branch into another repo-sync branch
        env:
          BASE_REF: ${{ github.base_ref }}
          HEAD_REF: ${{ github.head_ref }}
        run: |
          if [[ "$BASE_REF" == repo-sync/* && "$HEAD_REF" == repo-sync/* ]]; then
            echo "::error::repo-sync/* PRs cannot be merged into other repo-sync/* branches."
            echo "Wait for repo-sync to retarget this PR to the default branch before merging."
            exit 1
          fi
          echo "Base branch '$BASE_REF' and head branch '$HEAD_REF' are OK."