dashboard-preview-comment.yml

name: Dashboard Preview Comment

# Posts (and keeps updated) a PR comment linking to the dashboard-preview
# artifact built by the "Dashboard Preview" workflow.
#
# Why a separate workflow: the build runs on `pull_request` with `contents: read`
# and no PR-write token, so it cannot comment on fork PRs (a fork's token is
# read-only). This job runs on `workflow_run`, i.e. in the base-repo context with
# trusted code (never the PR's), which is the only safe way to comment on forks.
#
# NOTE: workflow_run workflows only trigger when this file exists on the default
# branch. The comment therefore starts appearing on PRs after this is merged to
# main — it will not comment on the PR that introduces it.

on:
  workflow_run:
    workflows: ["Dashboard Preview"]
    types: [completed]

permissions:
  contents: read
  actions: read # read/download artifacts from the triggering run
  pull-requests: write # post the preview comment

# Key by the PR's source (repo + branch), not the run id: a run-id group would be
# unique per run, making cancel-in-progress a no-op. Serializing per source PR lets
# a newer comment job cancel an older in-flight one, so two preview runs completing
# close together can't race into duplicate comments.
concurrency:
  group: >-
    dashboard-preview-comment-${{ github.event.workflow_run.head_repository.full_name }}-${{ github.event.workflow_run.head_branch }}
  cancel-in-progress: true

jobs:
  comment:
    name: Comment preview link
    runs-on: ubuntu-latest
    # Only for PR builds, and skip cancelled runs (which upload no metadata).
    if: >-
      github.event.workflow_run.event == 'pull_request' &&
      github.event.workflow_run.conclusion != 'cancelled'
    steps:
      - name: Download preview metadata
        uses: actions/download-artifact@3e5f45b2cfb9172054b4087a40e8e0b5a5461e7c # v8.0.1
        with:
          name: dashboard-preview-meta
          run-id: ${{ github.event.workflow_run.id }}
          github-token: ${{ github.token }}

      - name: Read PR number
        id: meta
        run: |
          number="$(tr -cd '0-9' < pr-number.txt)"
          if [ -z "$number" ]; then
            echo "No PR number recorded; nothing to comment on."
          fi
          echo "number=$number" >> "$GITHUB_OUTPUT"

      - name: Post or update the preview comment
        if: steps.meta.outputs.number != ''
        uses: actions/github-script@3a2844b7e9c422d3c10d287c895573f7108da1b3 # v9.0.0
        with:
          script: |
            const prNumber = Number('${{ steps.meta.outputs.number }}');
            const run = context.payload.workflow_run;
            const succeeded = run.conclusion === 'success';

            // Prefer a direct link to the artifact; fall back to the run page.
            const arts = await github.rest.actions.listWorkflowRunArtifacts({
              owner: context.repo.owner,
              repo: context.repo.repo,
              run_id: run.id,
            });
            const art = arts.data.artifacts.find(
              (a) => a.name === `dashboard-preview-${prNumber}`,
            );
            const link = art ? `${run.html_url}/artifacts/${art.id}` : run.html_url;

            const marker = '<!-- dashboard-preview-comment -->';
            const body = [
              marker,
              '### 📊 Dashboard preview',
              '',
              succeeded
                ? 'The dashboard was built for this PR.'
                : '⚠️ The build reported failures, but a partial dashboard may still be attached.',
              '',
              `➡️ **[Download \`dashboard-preview-${prNumber}\`](${link})**, unzip, and open \`dashboard.html\` in a browser.`,
              '',
              '> The artifact is contributor-authored HTML — glance at the diff before opening it. It expires after 7 days and is replaced on each new push.',
            ].join('\n');

            // Paginate: on a busy PR the marker comment may sit past the first
            // page, and missing it would create a duplicate instead of updating.
            const comments = await github.paginate(github.rest.issues.listComments, {
              owner: context.repo.owner,
              repo: context.repo.repo,
              issue_number: prNumber,
              per_page: 100,
            });
            const existing = comments.find((c) => c.body && c.body.includes(marker));
            if (existing) {
              await github.rest.issues.updateComment({
                owner: context.repo.owner,
                repo: context.repo.repo,
                comment_id: existing.id,
                body,
              });
            } else {
              await github.rest.issues.createComment({
                owner: context.repo.owner,
                repo: context.repo.repo,
                issue_number: prNumber,
                body,
              });
            }