pr-overlap.yml

name: PR overlap

# Advisory only: with many agent sessions working concurrently, two open PRs that
# touch the same files are usually duplicated or conflicting work discovered too
# late (post-merge). This surfaces the intersection as a warning annotation +
# step summary while both PRs are still open; it never fails the build.

on:
  pull_request:
    branches: [main]
    types: [opened, reopened, ready_for_review, synchronize]

# Least privilege: listing open PRs and their files only needs read access.
# No checkout, no third-party actions — the job is a single `gh api` script.
permissions:
  contents: read
  pull-requests: read

concurrency:
  group: ${{ github.workflow }}-${{ github.ref }}
  cancel-in-progress: true

jobs:
  overlap:
    name: warn when another open PR touches the same files
    runs-on: ubuntu-latest
    timeout-minutes: 5
    env:
      GH_TOKEN: ${{ github.token }}
      # Numeric / repo-slug values, passed through env (not inlined into the
      # script) so no event-controlled text is interpolated into bash.
      PR_NUMBER: ${{ github.event.pull_request.number }}
      REPO: ${{ github.repository }}
    steps:
      - name: Compare changed files against every other open PR
        run: |
          set -euo pipefail
          mine="$(mktemp)"
          theirs="$(mktemp)"
          gh api "repos/${REPO}/pulls/${PR_NUMBER}/files" --paginate \
            --jq '.[].filename' | sort -u > "$mine"
          found=0
          while read -r number; do
            [ "$number" = "$PR_NUMBER" ] && continue
            gh api "repos/${REPO}/pulls/${number}/files" --paginate \
              --jq '.[].filename' | sort -u > "$theirs"
            shared="$(comm -12 "$mine" "$theirs")"
            if [ -n "$shared" ]; then
              found=1
              count="$(printf '%s\n' "$shared" | wc -l | tr -d '[:space:]')"
              echo "::warning title=PR overlap::PR #${number} also changes ${count} of this PR's files - check for duplicated or conflicting work."
              {
                echo "### Overlap with PR #${number}"
                echo ""
                printf '%s\n' "$shared" | sed 's/^/- /'
                echo ""
              } >> "$GITHUB_STEP_SUMMARY"
            fi
          done < <(gh api "repos/${REPO}/pulls?state=open&per_page=100" --paginate \
            --jq '.[].number')
          if [ "$found" = 0 ]; then
            echo "No open PR shares files with this one." >> "$GITHUB_STEP_SUMMARY"
          fi