5 files changed, 40 insertions, 12 deletions

diff --git a/docs/markdown/Code-formatting.md b/docs/markdown/Code-formatting.md
index c8d83de..386c787 100644
--- a/docs/markdown/Code-formatting.md
+++ b/docs/markdown/Code-formatting.md
@@ -54,5 +54,5 @@ src/*.cpp
 Note that `.clang-format-ignore` has the same format as used by
 [`run-clang-format.py`](https://github.com/Sarcasm/run-clang-format).
 
-Modified files will be printed on the console which can be used for example by
-CI to ensure all files are correctly formatted.
+A new target `clang-format-check` has been added. It returns an error code if
+any file needs to be reformatted. This is intended to be used by CI.
diff --git a/docs/markdown/snippets/clang-format.md b/docs/markdown/snippets/clang-format.md
index 8cb88e0..a390d13 100644
--- a/docs/markdown/snippets/clang-format.md
+++ b/docs/markdown/snippets/clang-format.md
@@ -39,5 +39,5 @@ Example of `.clang-format-ignore` file:
 src/*.cpp
 ```
 
-Modified files will be printed on the console which can be used for example by
-CI to ensure all files are correctly formatted.
+A new target `clang-format-check` has been added. It returns an error code if
+any file needs to be reformatted. This is intended to be used by CI.
diff --git a/mesonbuild/backend/ninjabackend.py b/mesonbuild/backend/ninjabackend.py
index b31339e..594e297 100644
--- a/mesonbuild/backend/ninjabackend.py
+++ b/mesonbuild/backend/ninjabackend.py
@@ -3042,8 +3042,12 @@ https://gcc.gnu.org/bugzilla/show_bug.cgi?id=47485'''))
         # Alias that runs the target defined above
         self.create_target_alias('meson-scan-build')
 
-    def generate_clangtool(self, name):
+    def generate_clangtool(self, name, extra_arg=None):
         target_name = 'clang-' + name
+        extra_args = []
+        if extra_arg:
+            target_name += f'-{extra_arg}'
+            extra_args.append(f'--{extra_arg}')
         if not os.path.exists(os.path.join(self.environment.source_dir, '.clang-' + name)) and \
                 not os.path.exists(os.path.join(self.environment.source_dir, '_clang-' + name)):
             return
@@ -3052,7 +3056,8 @@ https://gcc.gnu.org/bugzilla/show_bug.cgi?id=47485'''))
         if ('', target_name) in self.build.run_target_names:
             return
         cmd = self.environment.get_build_command() + \
-            ['--internal', 'clang' + name, self.environment.source_dir, self.environment.build_dir]
+            ['--internal', 'clang' + name, self.environment.source_dir, self.environment.build_dir] + \
+            extra_args
         elem = NinjaBuildElement(self.all_outputs, 'meson-' + target_name, 'CUSTOM_COMMAND', 'PHONY')
         elem.add_item('COMMAND', cmd)
         elem.add_item('pool', 'console')
@@ -3063,6 +3068,7 @@ https://gcc.gnu.org/bugzilla/show_bug.cgi?id=47485'''))
         if not environment.detect_clangformat():
             return
         self.generate_clangtool('format')
+        self.generate_clangtool('format', 'check')
 
     def generate_clangtidy(self):
         import shutil
diff --git a/mesonbuild/scripts/clangformat.py b/mesonbuild/scripts/clangformat.py
index 2cf757f..ceb36ac 100644
--- a/mesonbuild/scripts/clangformat.py
+++ b/mesonbuild/scripts/clangformat.py
@@ -12,6 +12,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
+import argparse
 import subprocess
 import itertools
 import fnmatch
@@ -34,15 +35,17 @@ def parse_pattern_file(fname: Path) -> T.List[str]:
         pass
     return patterns
 
-def run_clang_format(exelist: T.List[str], fname: Path) -> subprocess.CompletedProcess:
+def run_clang_format(exelist: T.List[str], fname: Path, check: bool) -> subprocess.CompletedProcess:
     before = fname.stat().st_mtime
     ret = subprocess.run(exelist + ['-style=file', '-i', str(fname)])
     after = fname.stat().st_mtime
     if before != after:
         print('File reformatted: ', fname)
+        if check:
+            ret.returncode = 1
     return ret
 
-def clangformat(exelist: T.List[str], srcdir: Path, builddir: Path) -> int:
+def clangformat(exelist: T.List[str], srcdir: Path, builddir: Path, check: bool) -> int:
     patterns = parse_pattern_file(srcdir / '.clang-format-include')
     if not patterns:
         patterns = ['**/*']
@@ -61,17 +64,23 @@ def clangformat(exelist: T.List[str], srcdir: Path, builddir: Path) -> int:
             if f.is_dir() or f.suffix not in suffixes or \
                 any(fnmatch.fnmatch(strf, i) for i in ignore):
                 continue
-            futures.append(e.submit(run_clang_format, exelist, f))
+            futures.append(e.submit(run_clang_format, exelist, f, check))
         returncode = max([x.result().returncode for x in futures])
     return returncode
 
 def run(args: T.List[str]) -> int:
-    srcdir = Path(args[0])
-    builddir = Path(args[1])
+    parser = argparse.ArgumentParser()
+    parser.add_argument('--check', action='store_true')
+    parser.add_argument('sourcedir')
+    parser.add_argument('builddir')
+    options = parser.parse_args(args)
+
+    srcdir = Path(options.sourcedir)
+    builddir = Path(options.builddir)
 
     exelist = detect_clangformat()
     if not exelist:
         print('Could not execute clang-format "%s"' % ' '.join(exelist))
         return 1
 
-    return clangformat(exelist, srcdir, builddir)
+    return clangformat(exelist, srcdir, builddir, options.check)
diff --git a/run_unittests.py b/run_unittests.py
index b95ead2..03e931a 100755
--- a/run_unittests.py
+++ b/run_unittests.py
@@ -5602,10 +5602,23 @@ class AllPlatformTests(BasePlatformTests):
         shutil.copytree(testdir, newdir)
         self.new_builddir()
         self.init(newdir)
+
+        # Should reformat 1 file but not return error
         output = self.build('clang-format')
         self.assertEqual(1, output.count('File reformatted:'))
+
+        # Reset source tree then try again with clang-format-check, it should
+        # return an error code this time.
+        windows_proof_rmtree(newdir)
+        shutil.copytree(testdir, newdir)
+        with self.assertRaises(subprocess.CalledProcessError):
+            output = self.build('clang-format-check')
+            self.assertEqual(1, output.count('File reformatted:'))
+
+        # All code has been reformatted already, so it should be no-op now.
         output = self.build('clang-format')
         self.assertEqual(0, output.count('File reformatted:'))
+        self.build('clang-format-check')
 
 
 class FailureTests(BasePlatformTests):