feat: add extensible transform API for custom field transformations

[OrlovEvgeny]

This enables users to extend compose-go with custom protocols and validation logic without forking the project, addressing the need for custom port protocols like http/https and domain name support

• Add RegisterDefaultValue() functions
• Add EnhancedPortDefaults as example of extensible port transformation
• Maintain backward compatibility with existing code

example:

// EnhancedPortDefaults is an example of an enhanced port defaults function
// that supports additional protocols and domain names in the published field.
// This function can be used as a replacement for the default portDefaults
// by registering it with RegisterDefaultValue.
//
// Example usage:
//   transform.RegisterDefaultValue("services.*.ports.*", transform.EnhancedPortDefaults)
//
// This function supports:
// - Additional protocols: "http", "https", "tcp", "udp"
// - Domain names in the published field (e.g., "example.com:80")
// - All existing functionality of the original portDefaults
func EnhancedPortDefaults(data any, _ tree.Path, _ bool) (any, error) {
	switch v := data.(type) {
	case map[string]any:
		// Set default protocol if not specified
		if _, ok := v["protocol"]; !ok {
			v["protocol"] = "tcp"
		}
		// Set default mode if not specified
		if _, ok := v["mode"]; !ok {
			v["mode"] = "ingress"
		}
		// Additional logic can be added here to handle domain names
		// in the published field or other custom transformations
		return v, nil
	default:
		return data, nil
	}
}

Related issue: psviderski/uncloud#44

[ndeloof]

not needed, as a non-public field there's no backward compatibility to be addressed here

[ndeloof]

RegisterDefaultTransformer sounds weird to me. What about RegisterDefaultValue

[ndeloof]

why have this function while DefaultValues is public ?

[ndeloof]

Can you please squash your commits, rebase and sign-off, so we can merge ?

[OrlovEvgeny]

Hey @ndeloof

Can you please squash your commits, rebase and sign-off, so we can merge ?
Done

[ndeloof]

seems you included way more than the expected changes in your commit :')

[OrlovEvgeny]

I see,It seems something broke after I rebased main
let me fix it

[OrlovEvgeny]

@ndeloof fixed

[ndeloof]

unnecessary

[OrlovEvgeny]

removed

[OrlovEvgeny]

Linter complains about "stuttering" when using type as transform.TransformFunc
https://github.com/compose-spec/compose-go/actions/runs/16293608125/job/46012331031#step:4:33

I refactored some naming:
renamed TransformFunc to Func in transform/canonical.go
what do you think about it?

[glours]

Sounds good!
I wonder if we should not add a test, to avoid breaking change on the feature and give an exemple for developers who want to use it, WDYT?

[OrlovEvgeny]

Sounds good! I wonder if we should not add a test, to avoid breaking change on the feature and give an exemple for developers who want to use it, WDYT?

Yes, I think that's a good idea

I added a test and an example function exampleEnhancedPortDefaults

[OrlovEvgeny]

Hi team @glours @ndeloof , just wanted to check if you expect me to make any more changes here?

[glours]

LGTM, thanks a lot for this contribution 🙏