opa fmt --diff fails unless external diff tool is present

[anderseknert]

Conducting some tests with OPA on a GCP compute instance running Windows Server 2022, and I was surprised to see the opa fmt --diff command fail:

There must be plenty of tools available to display diffs in the Go ecosystem, and perhaps some we depend on already. Perhaps we could use one of those? OTOH this code is from 2017, and if no one else complained in the time since then, I guess that flag is uncommon enough that not many have had to see this.

[srenatus]

There's something weird going on here. The "is there a diff?" logic uses bytes.Equal. Oh wait, we do use diff for formatting later on: https://github.com/open-policy-agent/opa/blob/main/cmd/fmt.go#L209.

Yup we should be able to get rid of this by using some golang code or a new (small) dependency.

[srenatus]

https://play.golang.com/p/XhCf9WJaI_a -- using the first hit on google, https://github.com/sergi/go-diff.

[stale]

This issue has been automatically marked as inactive because it has not had any activity in the last 30 days. Although currently inactive, the issue could still be considered and actively worked on in the future. More details about the use-case this issue attempts to address, the value provided by completing it or possible solutions to resolve it would help to prioritize the issue.

[colinjlacy]

I can take this one

[srenatus]

Since my last comment, I also came across this -- https://cs.opensource.google/go/go/+/master:src/internal/diff/diff.go?q=diff&ss=go%2Fgo -- we'd need to copy-vendor this, but the license lets us do that, I think.

[colinjlacy]

@srenatus it turns out sergi/go-diff is already in the repo, so I can use that if you're still good with taking that approach.

[anderseknert]

If we can use an existing dependency, all the better!