Fix ament_clang_format output with empty files

[nwn]

Clang format doesn't output anything for empty files that are passed to it. This means an empty source code file causes our file indices to get offset from the parsed replacements, resulting in the printed replacements being generated for the wrong file. This behaviour has been verified on clang-format versions 10–21. (This doesn't impact the correctness of the --reformat option, only the correctness of the rendered output without that flag.)

This change detects empty files and fixes the indices accordingly. It also outputs a special output for them.

Example:

Files:

// malformed.cpp
void this_should_get_formatted(     );

// proper.cpp
#include <but/this/file/gets/output/instead>

// empty.cpp (must have size 0)

Before this change:

$ ament_clang_format test/empty.cpp test/proper.cpp test/malformed.cpp 
No code style divergence in file 'test/empty.cpp'

Code style divergence in file 'test/proper.cpp':

[test/proper.cpp:1:31]: Replace [tput/] with []
- #include <but/this/file/gets/output/instead>
+ #include <but/this/file/gets/ouinstead>

1 files with 1 code style divergences

After this change:

$ ament_clang_format test/empty.cpp test/proper.cpp test/malformed.cpp 
Skipping empty file 'test/empty.cpp'

No code style divergence in file 'test/proper.cpp'

Code style divergence in file 'test/malformed.cpp':

[test/malformed.cpp:1:31]: Replace [     ] with []
- void this_should_get_formatted(     );
+ void this_should_get_formatted();

1 files with 1 code style divergences

[cfveeden]

LGTM

[christophebedard]

Pulls: #580
Gist: https://gist.githubusercontent.com/christophebedard/7803d2e56e602540255c390c9beef55c/raw/74341a7e44219679321555654be52a632d24a994/ros2.repos
BUILD args:
TEST args:
ROS Distro: rolling
Job: ci_launcher
ci_launcher ran: https://ci.ros2.org/job/ci_launcher/19077

• Linux
• Linux-aarch64
• Linux-rhel
• Windows

[christophebedard]

A test would be ideal, but there are no tests for ament_clang_format right now other than linter tests on the package itself. However, you could try extracting this logic to a separate function and then unit-testing it

[tony-p]

@christophebedard this PR adds a basic executiion unit test for ament_clang_format #563

[mergify]

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[nwn]

@christophebedard I've added two unit tests to cover the case of empty files, similarly to the example provided in the PR description.