vnext-compat.yaml

name: vnext compatibility check

# Runs on every PR regardless of base branch, so this can safely be made a
# required status check without hanging PRs whose base isn't main. Simulates
# squashing the PR onto main, then attempts a dry-run rebase of vnext onto the
# result. If vnext would conflict, the check fails and posts a comment with
# exact commands to resolve it.
#
# Only PRs targeting main do that real work — compat-check is a no-op for
# anything else (including vnext→main release PRs, head_ref == 'vnext'). The
# noop job covers that inverse case, and both paths feed vnext-status so
# there's always a single, consistently-named check to require.

on:
  pull_request:
    types: [opened, reopened, synchronize, edited]

permissions:
  contents: read

concurrency:
  group: ${{ github.workflow }}-${{ github.event.pull_request.number }}
  cancel-in-progress: true

jobs:
  compat-check:
    name: Check vnext compatibility
    if: github.event.pull_request.base.ref == 'main' && github.head_ref != 'vnext'
    runs-on: ubuntu-slim
    permissions:
      contents: read
      pull-requests: write # Required to post conflict resolution instructions as a PR comment
    steps:
      - uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1
        with:
          fetch-depth: 0
          persist-credentials: false

      - name: Simulate squash onto main, then rebase vnext
        id: compat
        env:
          PR_HEAD_SHA: ${{ github.event.pull_request.head.sha }}
          PR_NUMBER: ${{ github.event.pull_request.number }}
        run: |
          git config user.email "github-actions[bot]@users.noreply.github.com"
          git config user.name "github-actions[bot]"

          git fetch origin main vnext

          # Build a throwaway squash commit of the PR on top of main.
          git checkout -b sim-main origin/main
          git merge --squash "$PR_HEAD_SHA"
          git commit --allow-empty -m "sim: squash of PR #${PR_NUMBER}"

          # Attempt to rebase vnext onto the squash commit.
          git checkout -b sim-vnext origin/vnext
          if git rebase sim-main 2>&1; then
            echo "compatible=true" >> "$GITHUB_OUTPUT"
          else
            CONFLICTS=$(git diff --name-only --diff-filter=U 2>/dev/null || true)
            git rebase --abort
            echo "compatible=false" >> "$GITHUB_OUTPUT"
            {
              echo 'conflicted_files<<EOF'
              echo "$CONFLICTS"
              echo 'EOF'
            } >> "$GITHUB_OUTPUT"
          fi

      - name: Post conflict resolution comment
        if: steps.compat.outputs.compatible == 'false' && github.event.pull_request.head.repo.full_name == github.repository
        uses: actions/github-script@3a2844b7e9c422d3c10d287c895573f7108da1b3 # v9.0.0
        env:
          PR_HEAD_REF: ${{ github.head_ref }}
          CONFLICTED_FILES: ${{ steps.compat.outputs.conflicted_files }}
        with:
          script: |
            const branch = process.env.PR_HEAD_REF;
            const files  = (process.env.CONFLICTED_FILES || '').trim();

            const fileBlock = files
              ? `**Conflicting files:**\n\`\`\`\n${files}\n\`\`\`\n`
              : '';

            const fileList = files ? files.split('\n').filter(Boolean) : [];
            const diffCmd  = fileList.length
              ? `git diff origin/main...origin/vnext -- ${fileList.join(' ')}`
              : 'git diff origin/main...origin/vnext';

            const body = [
              '## ⚠️ `vnext` compatibility conflict detected',
              '',
              'If this PR merges to `main`, `vnext` cannot be cleanly rebased on top.',
              'Your changes conflict with something already on `vnext`.',
              '',
              fileBlock,
              '**How to fix:**',
              '',
              'Do **not** rebase your branch onto `vnext` — that would pull unreleased breaking',
              'changes into `main`. Instead:',
              '',
              '1. See exactly what `vnext` changes in the conflicting file(s):',
              '```bash',
              'git fetch origin',
              diffCmd,
              '```',
              '2. Open each conflicting file in your editor. The diff above shows what `vnext` adds',
              '   or changes there — adjust your edits so they no longer overlap with those lines.',
              '3. Commit the adjustment and push:',
              '```bash',
              `git add ${fileList.length ? fileList.join(' ') : '<conflicting files>'}`,
              'git commit -m "fix: resolve vnext compatibility"',
              `git push origin ${branch}`,
              '```',
              '',
              'After pushing, this check will re-run automatically.',
            ].join('\n');

            await github.rest.issues.createComment({
              owner: context.repo.owner,
              repo:  context.repo.repo,
              issue_number: context.issue.number,
              body,
            });

      - name: Emit conflict instructions to job summary (fork PRs)
        if: steps.compat.outputs.compatible == 'false' && github.event.pull_request.head.repo.full_name != github.repository
        env:
          PR_HEAD_REF: ${{ github.head_ref }}
          CONFLICTED_FILES: ${{ steps.compat.outputs.conflicted_files }}
        run: |
          {
            echo "## ⚠️ vnext compatibility conflict detected"
            echo ""
            echo "This PR conflicts with \`vnext\` when rebased onto \`main\`."
            echo ""
            if [ -n "$CONFLICTED_FILES" ]; then
              echo "**Conflicting files:**"
              echo '```'
              echo "$CONFLICTED_FILES"
              echo '```'
              echo ""
              echo "Inspect the diff with:"
              echo '```bash'
              echo "git fetch origin"
              echo "git diff origin/main...origin/vnext -- $CONFLICTED_FILES"
              echo '```'
            else
              echo "Run \`git diff origin/main...origin/vnext\` to inspect the conflict."
            fi
            echo ""
            echo "Edit overlapping lines in those files, then commit and push."
          } >> "$GITHUB_STEP_SUMMARY"

      - name: Fail on conflict
        if: steps.compat.outputs.compatible == 'false'
        run: |
          echo "::error::This PR conflicts with 'vnext' when rebased onto main. See the PR comment for resolution instructions."
          exit 1

  noop:
    name: Skip vnext compatibility check
    if: github.event.pull_request.base.ref != 'main' || github.head_ref == 'vnext'
    runs-on: ubuntu-slim
    steps:
      - run: echo "Base isn't main, or this is the vnext release PR — nothing to check here."

  vnext-status:
    name: vnext compatibility status
    needs: [compat-check, noop]
    if: always()
    runs-on: ubuntu-slim
    permissions: {}
    steps:
      - uses: lowlydba/are-we-good@f506ed6324f55ec5e4ff5d92204a72c7f1c2b4f8 # v1.0.5
        with:
          jobs: ${{ toJSON(needs) }}