openapi3: add support for extensions on the few types left

[fenollp]

Signed-off-by: Pierre Fenoll pierrefenoll@gmail.com

[mcblair]

This shipped in v0.122.0 and when we upgraded to it from v0.121.0 resulted in a silent change in behaviors when marshalling to yaml; paths: {}. I have confirmed that prior to marshalling, the field has a pointer to expected value. I also have confirmed that T.MarshalJSON() behaves as expected, without any loss of T.Paths. We were still using yaml.v2 but I did confirm that the problem is reproducible with yaml.v3.

Reproduce it by running the following code first with v0.121.0 and then with v0.122.0.

oas := openapi3.NewLoader()
existingDoc, err := oas.LoadFromFile("./spec/reference/example.yaml")
if err != nil {
	return fmt.Errorf("failed to load file: %s with err: %s", s.FileName, err.Error())
}
// `existingDoc.Paths` looks good when introspected with debugger.

err = existingDoc.Validate(ctx)
if err != nil {
	return fmt.Errorf("failed to validate file: %s with err: %s", s.FileName, err.Error())
}

marshalledJson, err := existingDoc.MarshalJSON() // Looks as expected.
if err != nil {
	return err
}

marshalledYaml, err := yaml.Marshal(&existingDoc) // Loss of `T.Paths`
if err != nil {
	return err
}

[sonu27]

This was a big breaking change.
I can no longer loop on path items, which were made private, and set directly.
Could m map[string]*PathItem be made public?

[fenollp]

@sonu27 https://pkg.go.dev/github.com/getkin/kin-openapi/openapi3#Paths.Map

[sonu27]

@sonu27 https://pkg.go.dev/github.com/getkin/kin-openapi/openapi3#Paths.Map

I'm using that. The problem is marshalling of yaml. Which I believe there is another bug for