Post release benchmark comparisons on pull requests

.github/workflows/ci.yml

@@ -20,6 +20,63 @@ jobs:
       env:
         RESULT: ${{ needs.benchmark-rails.result }}
       run: test "$RESULT" = success || test "$RESULT" = skipped
+  benchmark-comment:
+    name: Comment on release benchmarks
+    if: >-
+      ${{ !cancelled() && github.event_name == 'pull_request' &&
+          github.event.pull_request.head.repo.full_name == github.repository &&
+          startsWith(github.head_ref, 'release-') }}
+    needs: benchmark-rails
+    runs-on: ubuntu-latest
+    permissions:
+      actions: read
+      pull-requests: write
+    concurrency:
+      group: release-benchmark-comment-${{ github.event.pull_request.number }}
+      cancel-in-progress: false
+    steps:
+    - name: Download benchmark results
+      uses: actions/download-artifact@634f93cb2916e3fdff6788551b99b062d0335ce0 # v5
+      with:
+        pattern: performance-rails-*
+        path: artifacts
+    - name: Post release benchmark results
+      shell: bash
+      env:
+        GH_TOKEN: ${{ github.token }}
+        PR_NUMBER: ${{ github.event.pull_request.number }}
+        HEAD_SHA: ${{ github.event.pull_request.head.sha }}
+      run: |
+        current_head=$(gh api "repos/$GITHUB_REPOSITORY/pulls/$PR_NUMBER" --jq .head.sha)
+        if [ "$current_head" != "$HEAD_SHA" ]; then
+          echo "Skipping results for an outdated pull request revision."
+          exit 0
+        fi
+
+        {
+          printf '<!-- view-component-release-benchmarks -->\n## Release benchmark results\n\n'
+          printf '[Workflow run](%s/%s/actions/runs/%s)\n' "$GITHUB_SERVER_URL" "$GITHUB_REPOSITORY" "$GITHUB_RUN_ID"
+          for rails in 8.1 main; do
+            printf '\n## Rails %s\n\n' "$rails"
+            report="artifacts/performance-rails-$rails/report.md"
+            if [ -f "$report" ]; then
+              cat "$report"
+            else
+              echo "Benchmark results are unavailable. See the workflow run for errors."
+              echo "::warning::No benchmark report for Rails $rails" >&2
+            fi
+          done
+        } > benchmark-comment.md
+
+        comment_id=$(gh api --paginate --slurp "repos/$GITHUB_REPOSITORY/issues/$PR_NUMBER/comments?per_page=100" \
+          --jq '[.[][] | select(.user.login == "github-actions[bot]" and (.body | startswith("<!-- view-component-release-benchmarks -->")))] | first | .id // empty')
+        if [ -n "$comment_id" ]; then
+          gh api --method PATCH "repos/$GITHUB_REPOSITORY/issues/comments/$comment_id" \
+            --field body=@benchmark-comment.md --silent
+        else
+          gh api --method POST "repos/$GITHUB_REPOSITORY/issues/$PR_NUMBER/comments" \
+            --field body=@benchmark-comment.md --silent
+        fi
   benchmark-rails:
     name: benchmark (Rails ${{ matrix.rails_version }})
     if: github.event_name == 'pull_request'
@@ -30,6 +87,7 @@ jobs:
         rails_version: ["8.1", "main"]
     env:
       BUNDLE_GEMFILE: gemfiles/rails_${{ matrix.rails_version }}.gemfile
+      RELEASE_PR: ${{ github.event.pull_request.head.repo.full_name == github.repository && startsWith(github.head_ref, 'release-') }}
     steps:
     - uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1
       with:
@@ -39,6 +97,16 @@ jobs:
         persist-credentials: false
         ref: ${{ github.event.pull_request.base.sha }}
         path: tmp/base
+        fetch-depth: ${{ env.RELEASE_PR == 'true' && '0' || '1' }}
+    - name: Use previous release as benchmark base
+      if: env.RELEASE_PR == 'true'
+      id: release
+      working-directory: tmp/base
+      shell: bash
+      run: |
+        tag=$(git describe --first-parent --tags --abbrev=0 --match 'v[0-9]*')
+        git checkout --detach "$tag"
+        echo "tag=$tag" >> "$GITHUB_OUTPUT"
     - name: Setup Ruby
       uses: ruby/setup-ruby@26e5b7d197ebf60545ee9667bcbace0be11ccaec # v1
       with:
@@ -48,12 +116,16 @@ jobs:
       shell: bash
       env:
         PASSES: 5
+        BASE_REF: ${{ steps.release.outputs.tag || github.event.pull_request.base.sha }}
       run: |
         for round in 1 2 3; do
           VIEW_COMPONENT_LIB=tmp/base/lib bundle exec ruby performance/measure.rb "tmp/performance/base/$round.json"
           bundle exec ruby performance/measure.rb "tmp/performance/head/$round.json"
         done
-        bundle exec ruby performance/compare.rb tmp/performance/base tmp/performance/head | tee -a "$GITHUB_STEP_SUMMARY"
+        {
+          printf 'Base: `%s`; head: `%s` (pull request merge commit).\n\n' "$BASE_REF" "$GITHUB_SHA"
+          bundle exec ruby performance/compare.rb tmp/performance/base tmp/performance/head
+        } | tee -a "$GITHUB_STEP_SUMMARY" tmp/performance/report.md
     - name: Upload performance results
       uses: actions/upload-artifact@330a01c490aca151604b8cf639adc76d48f6c5d4 # v5
       if: always()

docs/CHANGELOG.md

@@ -10,6 +10,10 @@ nav_order: 6
 
 ## main
 
+* Comment on release pull requests with benchmark results compared to the previous release.
+
+    *Joel Hawksley*
+
 * Reduce source-file reads and dependency-scanning allocations during cold cache digests and boot-time precomputation.
 
     *Joel Hawksley*

docs/CONTRIBUTING.md

@@ -93,7 +93,9 @@ bundle exec appraisal rails-8.0 rake
 
 CI compares each pull request against its base branch on Rails 8.1 and Rails main. For each rendering scenario in `performance/measure.rb`, it divides the component render time by the render time of an equivalent partial. It also measures cold component digests relative to cold partial digests, clearing Action View's digest caches and using a new lookup context for every call.
 
-The digest and boot scenarios use 300 generated ERB components and 300 generated Slim components. Each renders another component and includes a shared helper that references multiple components, exercising overlapping and cyclic dependencies. The boot scenario divides the time spent in ViewComponent (requiring it, loading components, compiling them, and precomputing digests) by the rest of boot. CI fails if any scenario's ratio is more than 5% higher than the base branch's ratio.
+The digest and boot scenarios use 300 generated ERB components and 300 generated Slim components. Each renders another component and includes a shared helper that references multiple components, exercising overlapping and cyclic dependencies. The boot scenario divides the time spent in ViewComponent (requiring it, loading components, compiling them, and precomputing digests) by the rest of boot. CI fails if any scenario's ratio is more than 5% higher than the baseline's ratio.
+
+For same-repository `release-*` pull requests, the baseline is the most recent release tag on the base branch's first-parent history instead. CI posts both Rails comparisons in one pull request comment, including regressions, and updates that comment on subsequent runs.
 
 To run the comparison locally: