Decoding should not clear apiVersion/kind

[liggitt]

What would you like to be added:
When using a typed client, decoding to a versioned struct (not an internal API type), the apiVersion/kind information returned from the server should not be dropped.

Why is this needed:
The GroupVersionKind() method included in the ObjectKind interface is largely useless when dealing with arbitrary runtime.Object instances, since typed instances drop this information here:

kubernetes/staging/src/k8s.io/apimachinery/pkg/runtime/helper.go

Lines 245 to 259 in 69a34f6

	// WithoutVersionDecoder clears the group version kind of a deserialized object.
	type WithoutVersionDecoder struct {
	Decoder
	}
	// Decode does not do conversion. It removes the gvk during deserialization.
	func (d WithoutVersionDecoder) Decode(data []byte, defaults *schema.GroupVersionKind, into Object) (Object, *schema.GroupVersionKind, error) {
	obj, gvk, err := d.Decoder.Decode(data, defaults, into)
	if obj != nil {
	kind := obj.GetObjectKind()
	// clearing the gvk is just a convention of a codec
	kind.SetGroupVersionKind(schema.GroupVersionKind{})
	}
	return obj, gvk, err
	}

This is the decoder used when a client requests a decoder that does not do conversion:

kubernetes/staging/src/k8s.io/apimachinery/pkg/runtime/serializer/codec_factory.go

Lines 175 to 179 in 69a34f6

	// WithoutConversion returns a NegotiatedSerializer that performs no conversion, even if the
	// caller requests it.
	func (f CodecFactory) WithoutConversion() runtime.NegotiatedSerializer {
	return WithoutConversionCodecFactory{f}
	}

I could see clearing group/version/kind information when converting to an internal version, but I don't see the benefit of stripping it on decode if we're only dealing with a versioned struct.

/sig api-machinery
/cc @smarterclayton

Note that #3030 still needs to be resolved before apiVersion/kind could be depended on for individual objects for all API responses, but this would at least solve the issue with an update of an object clearing the apiVersion/kind in an update response (xref kubernetes-sigs/controller-runtime#526)

[YoubingLi]

If the apiVersion is not specified, GroupVersoinKind can be dropped when decoding the data.

Anyway, I will dig more about related issues before filing a PR.

[liggitt]

I think dropping WithoutVersionDecoder might be sufficient to accomplish this. I am looking into fixing #3030 in conversion, and adding tests to ensure conversion populates the apiVersion/kind appropriately

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