gstack/.github/workflows/pr-title-sync.yml

name: PR Title Sync

# WHY pull_request_target (not pull_request): the default GITHUB_TOKEN is
# READ-ONLY on fork PRs under `pull_request`, so the title-sync backstop could
# never `gh pr edit` a fork/agent PR. `pull_request_target` runs in the base-repo
# context with a write token, which fixes fork coverage.
#
# WHY this is SAFE (pull_request_target is the most dangerous trigger):
#   - We check out the BASE repo (no `ref:`), so the only code we execute is
#     trusted base-repo infra (bin/gstack-pr-title-rewrite.sh). We NEVER check
#     out or run PR-head/fork code.
#   - Every attacker-controlled PR field (title, head repo, head sha) arrives via
#     `env:` and is referenced as a shell-quoted "$VAR". We NEVER inline a
#     `${{ github.event.pull_request.* }}` expression inside the run: script
#     (that would execute a crafted title as shell).
#   - The PR-head VERSION is read as DATA via the API (raw media type), from the
#     head repo at the head sha — never by checking out the head.
# test/pr-title-sync-workflow-safety.test.ts is the static tripwire for all of
# the above and fails CI if any of it regresses.

on:
  pull_request_target:
    types: [opened, synchronize, edited]
    paths:
      - 'VERSION'

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

jobs:
  sync:
    name: Sync PR title to VERSION
    runs-on: ubicloud-standard-2
    permissions:
      contents: read
      pull-requests: write
    if: github.actor != 'github-actions[bot]'
    steps:
      # Base repo only — trusted infra (the rewrite helper). No PR-head checkout.
      - name: Checkout base repo (trusted)
        uses: actions/checkout@v7
        with:
          fetch-depth: 1

      - name: Rewrite PR title to match VERSION
        env:
          GH_TOKEN: ${{ secrets.GITHUB_TOKEN }}
          PR_NUM: ${{ github.event.pull_request.number }}
          # Attacker-controlled on fork PRs — env-only, never inlined into run:.
          OLD_TITLE: ${{ github.event.pull_request.title }}
          BASE_REPO: ${{ github.repository }}
          HEAD_REPO: ${{ github.event.pull_request.head.repo.full_name }}
          HEAD_SHA: ${{ github.event.pull_request.head.sha }}
        run: |
          set -euo pipefail
          chmod +x ./bin/gstack-pr-title-rewrite.sh

          if [ "$HEAD_REPO" = "$BASE_REPO" ]; then IS_FORK=0; else IS_FORK=1; fi

          # Read the PR-head VERSION as data (raw bytes), from the head repo at
          # the head sha. Guard the assignment itself: under `set -e` a bare
          # `VERSION=$(...)` would abort the step before any later [ -z ] check.
          if ! VERSION=$(gh api -H "Accept: application/vnd.github.raw" \
              "repos/$HEAD_REPO/contents/VERSION?ref=$HEAD_SHA" 2>/dev/null | tr -d '[:space:]'); then
            VERSION=""
          fi

          if [ -z "$VERSION" ]; then
            # Same-repo read failure should never happen — fail loudly so we
            # notice. A fork miss (public-contents quirk, private fork) is a
            # convenience gap, not a gate — warn and skip so the check stays green.
            if [ "$IS_FORK" = "0" ]; then
              echo "::error::Could not read VERSION from same-repo PR head ($HEAD_SHA)."
              exit 1
            fi
            echo "::warning::Could not read VERSION from fork $HEAD_REPO ($HEAD_SHA); skipping title sync."
            exit 0
          fi

          # The helper rejects a malformed VERSION (exit 2). Same policy: loud for
          # same-repo, soft for forks. Never echo the raw (attacker-controlled)
          # title — Actions still parses ::workflow-command:: from stdout.
          if ! NEW_TITLE=$(./bin/gstack-pr-title-rewrite.sh "$VERSION" "$OLD_TITLE"); then
            if [ "$IS_FORK" = "0" ]; then
              echo "::error::Could not compute title for VERSION '$VERSION' on PR #$PR_NUM."
              exit 1
            fi
            echo "::warning::Could not compute title for fork PR #$PR_NUM; skipping."
            exit 0
          fi

          if [ "$NEW_TITLE" = "$OLD_TITLE" ]; then
            echo "PR #$PR_NUM title already correct; no change."
            exit 0
          fi
          gh pr edit "$PR_NUM" --title "$NEW_TITLE"
          echo "PR #$PR_NUM title synced to VERSION."