Make GitHub Actions enforce clang-format-clean code

.github/workflows/clang_format.yml

@@ -0,0 +1,45 @@
+# Copyright (c) 2023 Sebastian Pipping <sebastian@pipping.org>
+# Licensed under Apache License Version 2.0
+
+name: Enforce clang-format
+
+on:
+  pull_request:
+  push:
+  schedule:
+    - cron: '0 3 * * 5'  # Every Friday at 3am
+  workflow_dispatch:
+
+# Minimum permissions for security
+permissions:
+  contents: read
+
+jobs:
+  clang-format:
+    name: Enforce clang-format
+    runs-on: ubuntu-22.04
+    env:
+      CLANG_MAJOR_VERSION: 17
+    steps:
+      - uses: actions/checkout@b4ffde65f46336ab88eb53be808477a3936bae11  # v4.1.1
+
+      - name: Add Clang/LLVM repositories
+        run: |-
+          set -x
+          source /etc/os-release
+          wget -O - https://apt.llvm.org/llvm-snapshot.gpg.key | sudo apt-key add -
+          sudo add-apt-repository "deb http://apt.llvm.org/${UBUNTU_CODENAME}/ llvm-toolchain-${UBUNTU_CODENAME}-${CLANG_MAJOR_VERSION} main"
+
+      - name: Install clang-format
+        run: |-
+          set -x
+          sudo apt-get update
+          sudo apt-get install --yes --no-install-recommends -V \
+              clang-format-${CLANG_MAJOR_VERSION}
+
+      - name: Enforce clang-format
+        run: |-
+          set -x
+          clang-format-${CLANG_MAJOR_VERSION} --version
+          clang-format-${CLANG_MAJOR_VERSION} -i *.c
+          git diff --exit-code  # i.e. reject non-empty diff