Add job for commenting benchmark results on the PR

.github/workflows/pr_benchmarks.yml

@@ -1,8 +1,7 @@
-name: Run and Cache Benchmarks
+name: Benchmarks
 
 on:
   pull_request:
-    types: [labeled, opened, reopened, synchronize]
 
 jobs:
   benchmark:
@@ -48,7 +47,7 @@ jobs:
           ref: ${{ github.event.pull_request.base.sha }}
           clean: false
 
-      - name: Benchmark baseline and compare
+      - name: Benchmark baseline and generate comparison message
         env:
           RESULTS_NAME: ${{ env.BASE_REF_SHA }}
         run: |
@@ -59,5 +58,69 @@ jobs:
           # Temporary workaround, until `RESULTS_NAME` var lands into main
           mv -f results/HEAD results/${{ env.BASE_REF_SHA }}
           
+          echo ${{ github.event.pull_request.number }} > pr
+          
           pip3 install rich
-          ./bench.sh compare ${{ env.BASE_REF_SHA }} ${{ env.HEAD_REF_SHA }}
+          cat > message.md <<EOF
+          # Benchmark results
+          
+          <details>
+          <summary>Benchmarks comparing ${{ github.event.pull_request.base.sha }} and ${{ github.sha }}</summary>
+
+          \`\`\`
+          $(./bench.sh compare ${{ env.BASE_REF_SHA }} ${{ env.HEAD_REF_SHA }})
+          \`\`\`
+
+          </details>
+          EOF
+          
+          cat message.md
+
+      - name: Upload benchmark comparison message
+        uses: actions/upload-artifact@v4
+        with:
+          name: message
+          path: benchmarks/message.md
+
+      - name: Upload PR number
+        uses: actions/upload-artifact@v4
+        with:
+          name: pr
+          path: benchmarks/pr
+
+  comment:
+    # Separate job with default write permissions, which can
+    # execute another workflow to actually post a PR comment
+    name: Post benchmarks comment
+    runs-on: ubuntu-latest
+    permissions: write-all
+    needs: [ benchmark ]
+    steps:
+      - name: Download comment message
+        uses: actions/download-artifact@v4
+        with:
+          name: message
+
+      - name: Download pr number
+        uses: actions/download-artifact@v4
+        with:
+          name: pr
+
+      - name: Print message and pr number
+        run: |
+          cat pr
+          echo "PR_NUMBER=$(cat pr)" >> "$GITHUB_ENV"
+          cat message.md
+
+      - name: Post a comment
+        uses: actions/github-script@v7
+        with:
+          script: |
+            const fs = require('fs');
+            const content = fs.readFileSync('message.md', 'utf8');
+            github.rest.issues.createComment({
+              issue_number: process.env.PR_NUMBER,
+              owner: context.repo.owner,
+              repo: context.repo.repo,
+              body: content,
+            })

.github/workflows/pr_comment.yml

@@ -0,0 +1,46 @@
+name: PR Comment
+
+# read-write repo token
+# access to secrets
+on:
+  workflow_run:
+    workflows: ["Benchmarks"]
+    types:
+      - completed
+
+jobs:
+  comment:
+    name: PR Comment
+    runs-on: ubuntu-latest
+    if: >
+      github.event.workflow_run.event == 'pull_request' &&
+      github.event.workflow_run.conclusion == 'success'
+    steps:
+      - name: Dump GitHub context
+        env:
+          GITHUB_CONTEXT: ${{ toJSON(github) }}
+        run: echo "$GITHUB_CONTEXT"
+
+      - name: Download comment message
+        uses: actions/download-artifact@v4
+        with:
+          name: message
+          run-id: ${{ github.event.workflow_run.id }}
+          github-token: ${{ secrets.GITHUB_TOKEN }}
+
+      - name: Print message
+        run: |
+          cat message.md
+
+      - name: Post the comment
+        uses: actions/github-script@v7
+        with:
+          script: |
+            const fs = require('fs');
+            const content = fs.readFileSync('message.md', 'utf8');
+            github.rest.issues.createComment({
+              issue_number: context.issue.number,
+              owner: context.repo.owner,
+              repo: context.repo.repo,
+              body: content,
+            })