[libc++] Add format check to CI

Note: contrary to what I said previously, I didn't change .clang-format nor utils/generate_feature_test_macro_components.py script.

Reviewed By: ldionne, #libc

Differential Revision: https://reviews.llvm.org/D92229

Author: mkurdej

libcxx/utils/ci/buildkite-pipeline.yml

@@ -15,6 +15,19 @@
 #
 
 steps:
+  - label: "Format"
+    command: "libcxx/utils/ci/run-buildbot check-format"
+    artifact_paths:
+      - "**/clang-format.patch"
+    agents:
+      queue: "libcxx-builders"
+    retry:
+      automatic:
+        - exit_status: -1  # Agent was lost
+          limit: 2
+    soft_fail:
+        - exit_status: 1
+
   - label: "C++03"
     command: "libcxx/utils/ci/run-buildbot generic-cxx03"
     artifact_paths:

libcxx/utils/ci/run-buildbot

@@ -8,6 +8,7 @@
 #===----------------------------------------------------------------------===##
 
 set -ex
+set -o pipefail
 
 PROGNAME="$(basename "${0}")"
 
@@ -105,6 +106,21 @@ function check-cxx-benchmarks() {
 }
 
 case "${BUILDER}" in
+check-format)
+    clean
+    echo "+++ Checking formatting"
+    # We need to set --extensions so that clang-format checks extensionless files.
+    mkdir -p ${BUILD_DIR}
+    git-clang-format \
+        --binary /usr/bin/clang-format --diff \
+        --extensions ',h,hh,hpp,hxx,c,cc,cxx,cpp' HEAD~1 \
+        -- \
+            libcxx/{benchmarks,include,src,test} \
+            libcxxabi/{fuzz,include,src,test} \
+        | tee ${BUILD_DIR}/clang-format.patch
+    # Check if the diff is empty, fail otherwise.
+    ! grep -q '^--- a' ${BUILD_DIR}/clang-format.patch
+;;
 generic-cxx03)
     export CC=clang
     export CXX=clang++