cmd: Add --diff option for caddy fmt

[francislavoie]

Just thought this might be fun to have.

When using no options or using --overwrite, it can be difficult to find out what was actually "wrong" according to the formatter.

Example output where a couple of the lines had spaces instead of tabs:

  :80 {
  	root * /srv
  	file_server
  
  	handle_errors {
-       @404 {
-           expression {http.error.status_code} == 404
+ 		@404 {
+ 			expression {http.error.status_code} == 404
  		}
  		rewrite @404 /not-found.txt
  	}
  }

[AliAlhajji]

I'm curious what would be the output if there isn't a new line at the end of the file

[francislavoie]

It'll show it:

  	}
  }
+

[mholt]

This is cool. I do wonder if it'd be useful to output an actual patch format so that diffing tools can visualize it for the user.

[francislavoie]

There's https://github.com/google/go-patchutils that we could use to do that, but it pulls in additional deps, not sure we need the complexity. I don't think we really gain that much from a full-on patch here. If you want a real patch, make it yourself with your machine's diff or w/e, not that difficult. Basically caddy fmt Caddyfile | diff -u Caddyfile - or whatever.

[mholt]

I see...

Do we really need this at all then? If the pipe to diff can be done anyway nearly just as easily?

[francislavoie]

Piping on Windows is uh, not nice 😁

I wrote this because I was on Windows and wanted a way to compare quickly in environments where I can't easily add random tooling. Some Windows users who see the warning in their logs might appreciate having this too for example. Or people in Docker who don't feel like getting a Caddy binary outside Docker just to compare their fmt.

[AliAlhajji]

Piping on Windows is uh, not nice 😁

I wrote this because I was on Windows and wanted a way to compare quickly in environments where I can't easily add random tooling. Some Windows users who see the warning in their logs might appreciate having this too for example. Or people in Docker who don't feel like getting a Caddy binary outside Docker just to compare their fmt.

As a Windows user I can relate to your Windows experience. Actually it was the new line at the end that kept causing the warning.