ARROW-4363: [CI] [C++] Add CMake format checks

[pitrou]

• Add a top-level script that reformats all known CMake files in the source tree
• Add a lint step in Travis that checks CMake files are properly formatted

Requires cmake_format and Python 3.

[wesm]

This is nice! Can you add instructions to https://github.com/apache/arrow/blob/master/cpp/README.md#continuous-integration about this new hurdle to a clean CI build?

BTW I'm going to port cpp/README.md to Sphinx this week with any luck, but I will do that later

[xhochy]

+1, LGTM

Thanks! I really like that we also have a consitent formatting for CMake now.

[kszucs]

We should put this into the lint docker image and add to the pre-commit hooks.

[pitrou]

Hmm... how do I run that docker image and where are the executed steps?

[wesm]

See https://github.com/apache/arrow/blob/master/docker-compose.yml#L189

Can we please document the docker-compose setup? https://issues.apache.org/jira/browse/ARROW-3364

[pitrou]

Ok, I've tried to update the docker-compose setup, but there are too many things to fix right now (most notably, it wasn't switched to clang 7.0 and there's a weird hack for iwyu).

[wesm]

Argh. I wish I had noticed that this changed the casing on all usages of functions in BuildUtils.cmake. You can pass --command-case=unchanged to disable changes to command names

[kou]

I like lower case for all function definitions and calls. Should we mix cases for function definitions and calls?

It seems that @wesm uses a rule that we use upper case for functions defined by us and lower case for other functions. (I'm not sure this is right.)
Generally, it's difficult to follow implicit rules.

[wesm]

Well, I'd be OK with making everything lower-case if you all would like that

[pitrou]

I don't mind either way. But whatever the rule, it should remain simple to express ;-)
(e.g. "if 'arrow' is in the name, it should be upper-case")

[xhochy]

I'm happy with everything as long as it's automated ;)

On the other side, I prefer to just use the default settings when using automated formatters. Thus the formatting is normally consistent how others also use it.

[pitrou]

I'll also admit the uppercase provides a nice visual cue for Arrow-specific functions.