[GH-574] ci: Configure and integrate clang-format

git-clang-format ensures that all *new* code being submitted adheres to
the format dictated by clang-format.

The code style is based on LLVM (the creator of clang-format), but with
4 instead of 2 spaces indent width and a maximum of 120 instead of 80
characters per line in order to blend in well with existing code.

Please note:
- The er-coap-13 code gets exempt, as at some point, we probably want to
  to interact with whatever upstream there might be. For those files,
  just do what existing code does (which varies every few lines).
- Default is clang-format-10, which is available in the Ubuntu 20.04
  GitHub runner.

.clang-format

@@ -0,0 +1,5 @@
+---
+Language: Cpp
+BasedOnStyle: LLVM
+IndentWidth: 4
+ColumnLimit: 120

.github/workflows/compliance.yaml

@@ -20,3 +20,22 @@ jobs:
     - name: Check commits with gitlint
       run: |
         tools/ci/run_ci.sh --branch-target origin/${{ github.base_ref }} --run-gitlint
+
+  check-clang-format:
+    name: Run clang-format
+    runs-on: ubuntu-20.04
+
+    steps:
+    - name: Checkout code including full history
+      uses: actions/checkout@v2
+      with:
+        fetch-depth: 0
+
+    - name: Install clang-format
+      run: |
+        sudo apt update
+        sudo apt -qy --no-install-recommends install clang-format-10
+
+    - name: Check commits with clang-format
+      run: |
+        tools/ci/run_ci.sh --branch-target origin/${{ github.base_ref }} --run-clang-format

coap/er-coap-13/.clang-format

@@ -0,0 +1,4 @@
+---
+DisableFormat: true
+SortIncludes: false
+...

tools/ci/run_ci.sh

@@ -24,13 +24,15 @@ OPT_BRANCH_SOURCE=
 OPT_BRANCH_TARGET=master
 OPT_C_EXTENSIONS=""
 OPT_C_STANDARD=""
+OPT_CLANG_FORMAT="clang-format-10"
 OPT_SANITIZER=""
 OPT_SCAN_BUILD=""
 OPT_SONARQUBE=""
 OPT_TEST_COVERAGE_REPORT=""
 OPT_VERBOSE=0
 OPT_WRAPPER_CMD=""
 RUN_BUILD=0
+RUN_CLANG_FORMAT=0
 RUN_CLEAN=0
 RUN_GITLINT=0
 RUN_TESTS=0
@@ -49,6 +51,8 @@ Options:
                             (ENABLE: ON or OFF)
   --c-standard VERSION      Explicitly specify C VERSION to be used
                             (VERSION: 99, 11)
+  --clang-format BINARY     Set specific clang-format binary
+                            (BINARY: defaults to ${OPT_CLANG_FORMAT})
   --sanitizer TYPE          Enable sanitizer
                             (TYPE: address leak thread undefined)
   --scan-build BINARY       Enable Clang code analyzer using specified
@@ -64,6 +68,7 @@ Options:
 
 Available steps (executed by --all):
   --run-gitlint            Check git commits with gitlint
+  --run-clang-format       Check code formatting with clang-format
   --run-clean              Remove all build artifacts
   --run-build              Build all targets
   --run-tests              Build and execute tests
@@ -76,6 +81,24 @@ function usage() {
   exit "${exit_code}"
 }
 
+function run_clang_format() {
+  local patch_file
+
+  patch_file="$(mktemp -t clang-format-patch.XXX)"
+  # shellcheck disable=SC2064
+  trap "{ rm -f -- '${patch_file}'; }" EXIT TERM INT
+
+  "git-${OPT_CLANG_FORMAT}" --diff "${OPT_BRANCH_TARGET}" |
+    { grep -v 'no modified files to format' || true; } > "${patch_file}"
+
+  if [ -s "${patch_file}" ]; then
+    cat "${patch_file}"
+    exit 1
+  fi
+
+  echo "No code formatting errors found"
+}
+
 function run_clean() {
   rm -rf build-wakaama
 }
@@ -153,8 +176,10 @@ if ! PARSED_OPTS=$(getopt -o vah \
                           -l branch-target: \
                           -l c-extensions: \
                           -l c-standard: \
+                          -l clang-format: \
                           -l help \
                           -l run-build \
+                          -l run-clang-format \
                           -l run-clean \
                           -l run-gitlint \
                           -l run-tests \
@@ -188,6 +213,14 @@ while true; do
       OPT_C_STANDARD=$2
       shift 2
       ;;
+    --clang-format)
+      OPT_CLANG_FORMAT=$2
+      shift 2
+      ;;
+    --run-clang-format)
+      RUN_CLANG_FORMAT=1
+      shift
+      ;;
     --run-clean)
       RUN_CLEAN=1
       shift
@@ -233,6 +266,7 @@ while true; do
       shift
       ;;
     -a|--all)
+      RUN_CLANG_FORMAT=1
       RUN_CLEAN=1
       RUN_GITLINT=1
       RUN_BUILD=1
@@ -296,6 +330,10 @@ if [ "${RUN_GITLINT}" -eq 1 ]; then
   run_gitlint
 fi
 
+if [ "${RUN_CLANG_FORMAT}" -eq 1 ]; then
+  run_clang_format
+fi
+
 if [ "${RUN_CLEAN}" -eq 1 ]; then
   run_clean
 fi