build: run clang-format on CI

We already include the tool inside tools/clang-format, so we should
start using it on CI. This attempts to run the linter only on the
commits present in an opened PR.

Signed-off-by: Darshan Sen <raisinten@gmail.com>

PR-URL: #42681
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

.github/workflows/linters.yml

@@ -49,6 +49,40 @@ jobs:
         run: npx envinfo
       - name: Lint C/C++ files
         run: make lint-cpp
+  format-cpp:
+    if: ${{ github.event.pull_request.draft == false && github.base_ref == 'master' }}
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v3
+        with:
+          fetch-depth: 0
+          persist-credentials: false
+      - name: Use Node.js ${{ env.NODE_VERSION }}
+        uses: actions/setup-node@v3
+        with:
+          node-version: ${{ env.NODE_VERSION }}
+      - name: Set up Python ${{ env.PYTHON_VERSION }}
+        uses: actions/setup-python@v3
+        with:
+          python-version: ${{ env.PYTHON_VERSION }}
+      - name: Environment Information
+        run: npx envinfo
+      - name: Format C/C++ files
+        run: |
+          make format-cpp-build
+          CLANG_FORMAT_START="$(git merge-base HEAD refs/remotes/origin/$GITHUB_BASE_REF)" \
+            make format-cpp
+          git --no-pager diff --exit-code && EXIT_CODE="$?" || EXIT_CODE="$?"
+          if [ "$EXIT_CODE" != "0" ]
+          then
+            echo
+            echo 'ERROR: Please run:'
+            echo
+            echo '  CLANG_FORMAT_START="$(git merge-base HEAD <target-branch-name>)" make format-cpp'
+            echo
+            echo 'to format the commits in your branch.'
+            exit "$EXIT_CODE"
+          fi
   lint-js-and-md:
     if: github.event.pull_request.draft == false
     runs-on: ubuntu-latest