Add clang-format-diff to github actions (#26445)

Back again with another attempt at #12446 after 6 years.

If this proves to be too aggressive/annoying we can dial it back somehow
or disable it.

Author: sbc100

.clang-format-ignore

@@ -0,0 +1,9 @@
+# ignore third_party code from clang-format checks
+third_party/*
+test/third_party/*
+system/lib/libc/musl
+system/lib/libcxx
+system/lib/libcxxabi
+system/lib/libunwind
+system/lib/mimalloc
+system/lib/llvm-libc

.github/workflows/ci.yml

@@ -98,3 +98,18 @@ jobs:
             echo "-- This failure is only a warning and can be ignored"
             exit 1
           fi
+
+  clang-format-diff:
+    env:
+      LLVM_VERSION: 19
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v4
+      # Fetch all history for all tags and branches
+      with:
+        fetch-depth: 0
+    - name: Install clang-format
+      run: |
+        sudo apt-get install clang-format-19
+        sudo update-alternatives --install /usr/bin/git-clang-format git-clang-format /usr/bin/git-clang-format-19 100
+    - run: tools/maint/clang-format-diff.sh origin/$GITHUB_BASE_REF

tools/maint/clang-format-diff.sh

@@ -0,0 +1,22 @@
+#!/bin/bash
+# Based on binaryen/scripts/clang-format-diff.sh
+# TODO(sbc): Switch to pre-packaged github actions once we find one that
+# is mature enough for our needs.
+
+set -o errexit
+set -o pipefail
+set -o xtrace
+
+if [ -n "$1" ]; then
+  BRANCH="$1"
+else
+  BRANCH="@{upstream}"
+fi
+
+MERGE_BASE=$(git merge-base $BRANCH HEAD)
+FORMAT_ARGS=${MERGE_BASE}
+if [ -n "$LLVM_VERSION" ]; then
+  FORMAT_ARGS="--binary=clang-format-${LLVM_VERSION} ${FORMAT_ARGS}"
+fi
+
+git clang-format ${FORMAT_ARGS} --diff