Fix ament_clang_format output with empty files

ament_clang_format/ament_clang_format/main.py

@@ -119,8 +119,20 @@ def main(argv: list[str] = sys.argv[1:]) -> Literal[0, 1]:
         report[filename] = []
 
     xmls = output.split(b"<?xml version='1.0'?>")[1:]
+    if len(files) > len(xmls):
+        # Skip empty files since clang-format doesn't output anything for them.
+        for index in range(len(files)):
+            if os.stat(files[index]).st_size == 0:
+                xmls.insert(index, None)
+
     changed_files = []
     for filename, xml in zip(files, xmls):
+        if not xml:
+            print("Skipping empty file '%s'" % filename)
+            if not args.reformat:
+                print('')
+            continue
+
         try:
             root = ElementTree.fromstring(xml)
         except ElementTree.ParseError as e:

ament_clang_format/test/test_empty_files.py

@@ -0,0 +1,54 @@
+# Copyright 2026 Open Source Robotics Foundation, Inc.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+from ament_clang_format.main import main
+
+
+EMPTY_CONTENTS = ''
+PROPER_CONTENTS = '#include <this/file/should/be/unchanged>\n'
+MALFORMED_CONTENTS_BEFORE = 'void this_should_get_formatted(     );\n'
+MALFORMED_CONTENTS_AFTER = 'void this_should_get_formatted();\n'
+
+
+def test_empty_files(tmp_path, capsys):
+    """Check that formatting suggestions are correct in the presence of an empty file."""
+    empty_path = tmp_path / 'empty.cpp'
+    proper_path = tmp_path / 'proper.cpp'
+    malformed_path = tmp_path / 'malformed.cpp'
+
+    empty_path.write_text(EMPTY_CONTENTS, encoding='utf_8')
+    proper_path.write_text(PROPER_CONTENTS, encoding='utf_8')
+    malformed_path.write_text(MALFORMED_CONTENTS_BEFORE, encoding='utf_8')
+
+    main([str(empty_path), str(proper_path), str(malformed_path)])
+
+    captured = capsys.readouterr()
+    assert MALFORMED_CONTENTS_AFTER in captured.err
+
+
+def test_empty_files_reformat(tmp_path):
+    """Check that reformatting applies correctly in the presence of an empty file."""
+    empty_path = tmp_path / 'empty.cpp'
+    proper_path = tmp_path / 'proper.cpp'
+    malformed_path = tmp_path / 'malformed.cpp'
+
+    empty_path.write_text(EMPTY_CONTENTS, encoding='utf_8')
+    proper_path.write_text(PROPER_CONTENTS, encoding='utf_8')
+    malformed_path.write_text(MALFORMED_CONTENTS_BEFORE, encoding='utf_8')
+
+    main(['--reformat', str(empty_path), str(proper_path), str(malformed_path)])
+
+    assert empty_path.read_text(encoding='utf_8') == EMPTY_CONTENTS
+    assert proper_path.read_text(encoding='utf_8') == PROPER_CONTENTS
+    assert malformed_path.read_text(encoding='utf_8') == MALFORMED_CONTENTS_AFTER