✨ Add support for +default markers

[liggitt]

This adds annotation support for +default markers for specifying default values in JSON format, as is done in core Kubernetes types and described in https://github.com/kubernetes/enhancements/tree/master/keps/sig-api-machinery/1929-built-in-default#summary

If both +default and +kubebuilder:default are specified, the kubebuilder-specific annotation wins.

Fixes #939

[liggitt]

/hold

forgot about the ref(type) support in the KEP... that's going to be a pain to reimplement :-/

[liggitt]

cc @alexzielenski for thoughts on the best way to add support for +default=ref(...) to kubebuilder

[alexzielenski]

cc @alexzielenski for thoughts on the best way to add support for +default=ref(...) to kubebuilder

In our code generators we use ParseSymbolReference to parse the ref string into an symbol name and package path. But our output is not the final YAML schema like it is for kubebuilder - it is Go code to express a spec.Schema.

To add ref support to this I don't see a way except a large refactor to generate code that expresses a spec.Schema like kube-openapi, or if there is a way to statically evaluate a constant using Go tools which I have not seen.

[liggitt]

To add ref support to this I don't see a way except a large refactor to generate code that expresses a spec.Schema like kube-openapi, or if there is a way to statically evaluate a constant using Go tools which I have not seen.

yeah... it's really thorny... I might start by limiting support here to inlined json-formatted values. We don't actually use ref() in any in-tree objects... we might not want to as long as it's difficult for CRD generators to make use of them

[liggitt]

Reworked this to delegate specialized marker parsing to the value holder, structured to ignore +default=ref(...) for now, and added test coverage for that.

/hold cancel

[alexzielenski]

I might start by limiting support here to inlined json-formatted values.

Sounds reasonable. I've looked around a little, and closest thing I found to static evaluation of constants from symbol path is go doc:

❯ go doc k8s.io/api/core/v1.TerminationMessageReadFile
package v1 // import "k8s.io/kubernetes/staging/src/k8s.io/api/core/v1"

const (
        // TerminationMessageReadFile is the default behavior and will set the container status message to
        // the contents of the container's terminationMessagePath when the container exits.
        TerminationMessageReadFile TerminationMessagePolicy = "File"
        // TerminationMessageFallbackToLogsOnError will read the most recent contents of the container logs
        // for the container status message when the container exits with an error and the
        // terminationMessagePath has no contents.
        TerminationMessageFallbackToLogsOnError TerminationMessagePolicy = "FallbackToLogsOnError"
)

Unfortunately it is not machine readable, not isolated to what was asked for, and returns go source so if the constant was an expression like 24 * time.Hour then we'd not be able to handle it simply by parsing this.

[sbueringer]

Just a few nits, otherwise lgtm

[sbueringer]

Thx!

/lgtm
/assign @alvaroaleman

[k8s-ci-robot]

LGTM label has been added.

Git tree hash: 3a1e540dedac1ba2aae86a0a41d631d8ff0255d4

[alvaroaleman]

thanks!

[k8s-ci-robot]

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