fix: Make bundle size workflow work on fork PRs

.github/workflows/bundle-size-report.yml

@@ -0,0 +1,63 @@
+name: Report bundle size
+
+# Runs after "Measure bundle size" completes. Because workflow_run executes in
+# the context of the base repository, its GITHUB_TOKEN keeps statuses:write
+# even when the triggering run came from a fork PR. This workflow posts a commit
+# status using the trusted workflow_run head SHA and (on success) the measured
+# sizes from the artifact — it never checks out or executes the PR's code.
+on:
+  workflow_run:
+    workflows: ['Measure bundle size']
+    types:
+      - completed
+
+permissions:
+  statuses: write
+  # Required to download the artifact from the triggering workflow run.
+  actions: read
+
+jobs:
+  report:
+    runs-on: ubuntu-latest
+    if: >
+      github.event.workflow_run.event == 'pull_request' ||
+      github.event.workflow_run.event == 'push'
+    steps:
+      # Trusted code only: the reporting script comes from the base repo's
+      # default branch, not from the artifact produced by the PR build.
+      - name: Checkout the base repo
+        uses: actions/checkout@v4
+
+      # Untrusted DATA only, pinned to the run that triggered this workflow.
+      # Skipped on a failed run (no artifact is produced); the report workflow
+      # posts an error status from the run conclusion instead.
+      - name: Download the size report artifact
+        if: github.event.workflow_run.conclusion == 'success'
+        uses: actions/download-artifact@v4
+        with:
+          name: size-report
+          path: bundle-size-data
+          run-id: ${{ github.event.workflow_run.id }}
+          github-token: ${{ github.token }}
+
+      - name: Post the commit status
+        uses: actions/github-script@v7
+        env:
+          # head_sha comes from the trusted workflow_run event, not from the
+          # untrusted artifact, so a fork PR cannot redirect the status onto an
+          # arbitrary commit. It is the PR head (not the merge commit), which is
+          # what we want to annotate.
+          HEAD_SHA: ${{ github.event.workflow_run.head_sha }}
+          CONCLUSION: ${{ github.event.workflow_run.conclusion }}
+          TARGET_URL: ${{ github.event.workflow_run.html_url }}
+        with:
+          script: |
+            const { reportBundleSize } = await import(process.cwd() + '/.github/workflows/bundle-size/status-report.js');
+            await reportBundleSize({
+              github,
+              context,
+              sha: process.env.HEAD_SHA,
+              dataDir: process.cwd() + '/bundle-size-data',
+              conclusion: process.env.CONCLUSION,
+              targetUrl: process.env.TARGET_URL,
+            });

.github/workflows/bundle-size.yml

@@ -8,8 +8,13 @@ on:
     branches:
       - main
 
+# This workflow runs untrusted PR code (npm install + build) and therefore
+# requests no write permissions. Fork PRs would have their token downgraded to
+# read-only anyway. Posting the commit status is delegated to the separate
+# "Report bundle size" workflow (bundle-size-report.yml), which runs with
+# elevated permissions but never executes PR code.
 permissions:
-  statuses: write
+  contents: read
 
 jobs:
   measure:
@@ -27,13 +32,6 @@ jobs:
           cd bundle-size
           npm i --force
 
-      - uses: actions/github-script@v7
-        name: Create a pending status on the commit
-        with:
-          script: |
-            const { statusReportStart } = await import(process.cwd() + '/bundle-size/status-report.js');
-            await statusReportStart({ context, github });
-
       # ------- Base branch size measurement ------------------------------
 
       - name: Checkout the base branch
@@ -73,25 +71,15 @@ jobs:
 
           cat output.json
 
+      # Only the numeric measurements are uploaded. The report workflow derives
+      # the commit SHA from the trusted workflow_run event, not from this
+      # artifact, and handles a missing/failed run via the run conclusion — so
+      # no artifact is needed on the failure path.
       - name: Upload size report artifact
         uses: actions/upload-artifact@v4
         with:
           name: size-report
-          path: bundle-size/output.json
-
-      - name: Update the commit status with calculated results
-        uses: actions/github-script@v7
-        with:
-          script: |
-            const { statusReportSuccess } = await import(process.cwd() + '/bundle-size/status-report.js');
-            await statusReportSuccess({ context, github });
-
-      # ------- Error reporting -------------------------------------------
-
-      - name: Report failure
-        if: failure()
-        uses: actions/github-script@v7
-        with:
-          script: |
-            const { statusReportFailure } = await import(process.cwd() + '/bundle-size/status-report.js');
-            await statusReportFailure({ context, github });
+          path: |
+            bundle-size/output.json
+            bundle-size/output-basebranch.json
+          if-no-files-found: error

.github/workflows/bundle-size/status-report.js

@@ -22,38 +22,40 @@ function formatDiff(prSize, headSize) {
   )} KB)`;
 }
 
-function getStatusMetadata(context) {
+function getStatusMetadata(context, sha, targetUrl) {
   return {
     owner: context.repo.owner,
     repo: context.repo.repo,
-    sha: context.sha,
+    sha,
     context: 'Bundle size',
-    state: 'pending',
-    target_url: `https://github.com/${context.repo.owner}/${context.repo.repo}/actions/runs/${context.runId}`,
+    target_url: targetUrl,
   };
 }
 
-export async function statusReportStart({ github, context }) {
-  await github.rest.repos.createCommitStatus({ ...getStatusMetadata(context), state: 'pending' });
-}
+// Entry point for the "Report bundle size" workflow. Posts a commit status on
+// `sha` (the trusted workflow_run head SHA). On a non-success run it reports an
+// error without touching the artifact. On success it reads the measurement
+// data, whose values only ever flow into numeric formatting, never execution.
+export async function reportBundleSize({ github, context, sha, dataDir, conclusion, targetUrl }) {
+  const metadata = getStatusMetadata(context, sha, targetUrl);
 
-export async function statusReportFailure({ github, context }) {
-  await github.rest.repos.createCommitStatus({
-    ...getStatusMetadata(context),
-    state: 'error',
-    description: 'The workflow encountered an error.',
-  });
-}
+  if (conclusion !== 'success') {
+    await github.rest.repos.createCommitStatus({
+      ...metadata,
+      state: 'error',
+      description: 'The workflow encountered an error.',
+    });
+    return;
+  }
 
-export async function statusReportSuccess({ github, context }) {
-  const basebranch = readJson('./bundle-size/output-basebranch.json');
-  const pr = readJson('./bundle-size/output.json');
+  const basebranch = readJson(`${dataDir}/output-basebranch.json`);
+  const pr = readJson(`${dataDir}/output.json`);
 
   console.log('Base branch:', basebranch);
   console.log('This PR:', pr);
 
   await github.rest.repos.createCommitStatus({
-    ...getStatusMetadata(context),
+    ...metadata,
     state: 'success',
     description: [
       `CSS: ${formatDiff(pr.cssCompressedSize, basebranch.cssCompressedSize)}`,