Prevent plan reporting endpoint target casing differences as a change

[robselway]

Description

Fix to prevent the plan reporting a difference in endpoint target casing as a change that needs to be made. This fixes a bug that has so far been identified in the Cloudflare provider where uppercase characters in the external-dns.alpha.kubernetes.io/target attribute can cause a CREATE/DELETE loop.

I have addressed the issue as centrally as possible to prevent dependencies on the core plan logic needing to think about this scenario, however the trade-off it a performance hit where !strings.EqualFold() is more expensive than the previous != operation, and would be more expensive than performing a strings.ToLower() at the edge (e.g. in source). Happy to change this if we think it's too much.

Fixes #1906

Checklist

• Unit tests updated

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[Raffo]

@robselway thank you for your PR, this sounds like an easy fix for a nasty problem. Frankly, I don't think performance matter in this case, but I do have a question: is it enough to handle the case problem in this single comparison or would it be handy to convert to lowercase to be useful to other parts of the code? I'm thinking that the answer is "no, this is enough", but I'd love to hear your thoughts given that you took a look at the problem.

[robselway]

Thanks @Raffo. I think this change should be enough to handle it, but it could be worth converting to lowercase in source when we fetch the value from the attribute - it could potentially prevent some other bugs, assuming other providers store values in lowercase like Cloudflare does.

My only reservation is RFC 1035 - 3.3.3 (linked to by @jgrumboe in the original issue) that says:

When data enters the domain system, its original case should be preserved whenever possible.

...

Loss of case sensitive data must be minimized.

So if we're sticking close to the spec, we should minimize any transformations and look to compare strings in a case-insensitive manner wherever possible?

[Raffo]

You are right @robselway . Merging as is.

[Raffo]

/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: Raffo, robselway

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