Do not accidentally format files when only checking if they are formatted.

mesonbuild/scripts/clangformat.py

@@ -36,12 +36,17 @@ def parse_pattern_file(fname: Path) -> T.List[str]:
     return patterns
 
 def run_clang_format(exelist: T.List[str], fname: Path, check: bool) -> subprocess.CompletedProcess:
+    if check:
+        original = fname.read_bytes()
     before = fname.stat().st_mtime
-    ret = subprocess.run(exelist + ['-style=file', '-i', str(fname)])
+    args = ['-style=file', '-i', str(fname)]
+    ret = subprocess.run(exelist + args)
     after = fname.stat().st_mtime
     if before != after:
         print('File reformatted: ', fname)
         if check:
+            # Restore the original if only checking.
+            fname.write_bytes(original)
             ret.returncode = 1
     return ret
 

run_unittests.py

@@ -5634,7 +5634,7 @@ class AllPlatformTests(BasePlatformTests):
         self._run(cmd + python_command + [script])
         self.assertEqual('This is text.', self._run(cmd + [app]).strip())
 
-    def test_clang_format(self):
+    def test_clang_format_check(self):
         if self.backend is not Backend.ninja:
             raise unittest.SkipTest(f'Skipping clang-format tests with {self.backend.name} backend')
         if not shutil.which('clang-format'):
@@ -5658,9 +5658,10 @@ class AllPlatformTests(BasePlatformTests):
             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.
+        # The check format should not touch any files. Thus
+        # running format again has some work to do.
         output = self.build('clang-format')
-        self.assertEqual(0, output.count('File reformatted:'))
+        self.assertEqual(1, output.count('File reformatted:'))
         self.build('clang-format-check')
 
     def test_custom_target_implicit_include(self):