ci(perf): post sticky comment for fork PRs via workflow_run

.github/workflows/performance-comment.yml

@@ -0,0 +1,109 @@
+name: Performance Comment
+
+# Posts the perf comparison as a sticky PR comment.
+#
+# This runs in the *base* repository's context via `workflow_run`, which
+# means it has a writable `GITHUB_TOKEN` even for pull requests opened from
+# forks. The Performance workflow itself runs in the (potentially untrusted)
+# PR context and only produces an artifact — no PR write permissions there.
+#
+# Security model:
+#   * The artifact from the PR-context workflow is treated as untrusted
+#     input. We only consume the JSON benchmark outputs (`base.json` /
+#     `pr.json`) and the PR number, and we regenerate `report.md` here
+#     using the base repository's checked-out `perf.compare` so the
+#     markdown posted under the writable token is never attacker-supplied.
+#   * The PR number is validated as a positive integer before being used.
+
+on:
+  workflow_run:
+    workflows: ["Performance"]
+    types:
+      - completed
+
+permissions:
+  contents: read
+  pull-requests: write
+  actions: read
+
+jobs:
+  comment:
+    name: Post sticky PR comment
+    runs-on: ubuntu-latest
+    if: github.event.workflow_run.event == 'pull_request'
+
+    steps:
+      - name: Check out base repository (trusted)
+        uses: actions/checkout@v4
+
+      - name: Download perf-results artifact
+        id: download
+        # Don't fail this job if the upstream Performance run was cancelled
+        # or failed before producing the artifact — we just skip posting.
+        continue-on-error: true
+        uses: actions/download-artifact@v4
+        with:
+          name: perf-results
+          path: perf-results
+          run-id: ${{ github.event.workflow_run.id }}
+          github-token: ${{ secrets.GITHUB_TOKEN }}
+
+      - name: Resolve PR number
+        id: pr
+        if: steps.download.outcome == 'success'
+        run: |
+          set -euo pipefail
+          pr=""
+          if [ -f perf-results/pr-number.txt ]; then
+            pr="$(tr -d '[:space:]' < perf-results/pr-number.txt)"
+          fi
+
+          if ! [[ "$pr" =~ ^[1-9][0-9]*$ ]]; then
+            echo "pr-number.txt missing or not a positive integer (got: '${pr}'); skipping comment." >&2
+            echo "number=" >> "$GITHUB_OUTPUT"
+            exit 0
+          fi
+
+          echo "number=$pr" >> "$GITHUB_OUTPUT"
+
+      - name: Set up Python
+        if: steps.pr.outputs.number != ''
+        uses: actions/setup-python@v5
+        with:
+          python-version: "3.12"
+
+      - name: Regenerate report from JSON (trusted code)
+        id: report
+        if: steps.pr.outputs.number != ''
+        run: |
+          set -euo pipefail
+          if [ ! -s perf-results/base.json ] || [ ! -s perf-results/pr.json ]; then
+            echo "base.json or pr.json missing from artifact; skipping comment." >&2
+            echo "found=false" >> "$GITHUB_OUTPUT"
+            exit 0
+          fi
+
+          # Use the base repo's perf.compare (trusted) to render the markdown
+          # from the PR-supplied JSON data, so we never post attacker-supplied
+          # markdown under the base repo's writable token.
+          python -m perf.compare \
+            --baseline perf-results/base.json \
+            --candidate perf-results/pr.json \
+            --threshold "${PERF_REGRESSION_THRESHOLD:-15}" \
+            --output report.md \
+            || true
+
+          if [ -s report.md ]; then
+            echo "found=true" >> "$GITHUB_OUTPUT"
+          else
+            echo "found=false" >> "$GITHUB_OUTPUT"
+            echo "report.md was not produced; skipping comment." >&2
+          fi
+
+      - name: Post sticky PR comment
+        if: steps.pr.outputs.number != '' && steps.report.outputs.found == 'true'
+        uses: marocchino/sticky-pull-request-comment@v2
+        with:
+          header: perf-comparison
+          number: ${{ steps.pr.outputs.number }}
+          path: report.md

.github/workflows/performance.yml

@@ -94,6 +94,10 @@ jobs:
             --output "$GITHUB_WORKSPACE/report.md"
           echo "exit_code=$?" >> "$GITHUB_OUTPUT"
 
+      - name: Record PR number
+        if: always() && github.event_name == 'pull_request'
+        run: echo "${{ github.event.pull_request.number }}" > "$GITHUB_WORKSPACE/pr-number.txt"
+
       - name: Upload artifacts
         if: always()
         uses: actions/upload-artifact@v4
@@ -103,17 +107,8 @@ jobs:
             pr.json
             base.json
             report.md
-
-      - name: Post sticky PR comment
-        # Skip for PRs from forks — the default GITHUB_TOKEN has read-only
-        # access in that case and the comment API will reject the call. The
-        # JSON artifacts and the workflow log still contain the comparison.
-        if: always() && github.event_name == 'pull_request' && github.event.pull_request.head.repo.full_name == github.repository
-        continue-on-error: true
-        uses: marocchino/sticky-pull-request-comment@v2
-        with:
-          header: perf-comparison
-          path: report.md
+            pr-number.txt
+          if-no-files-found: ignore
 
       - name: Fail on regression
         if: steps.compare.outputs.exit_code != '0'