[client-go] Polymorphic Scale Client

This introduces a polymorphic scale client capable of operating against
scale subresources which return different group-versions of Scale.  The
scale subresources may be in group-versions different than the scale
itself, so that we no longer need a copy of every scalable resource in
the extensions API group.

To discovery which Scale group-versions go to which subresources,
discovery is used.

The scale client maintains its own internal versions and conversions to
several external versions, with a "hub" version that's a copy of the
autoscaling internal version.

It currently supports the following group-versions for Scale subresources:

- extensions/v1beta1.Scale
- autoscaling/v1.Scale

Kubernetes-commit: d61a2d90372c301dd11088df8941acf2bb01c38c

scale/scheme/extensionsv1beta1/conversion.go

@@ -0,0 +1,87 @@
+/*
+Copyright 2017 The Kubernetes Authors.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+package extensionsv1beta1
+
+import (
+	"fmt"
+
+	v1beta1 "k8s.io/api/extensions/v1beta1"
+	metav1 "k8s.io/apimachinery/pkg/apis/meta/v1"
+	"k8s.io/apimachinery/pkg/conversion"
+	"k8s.io/apimachinery/pkg/runtime"
+	scheme "k8s.io/client-go/scale/scheme"
+)
+
+// addConversions registers conversions between the internal version
+// of Scale and supported external versions of Scale.
+func addConversionFuncs(scheme *runtime.Scheme) error {
+	err := scheme.AddConversionFuncs(
+		Convert_scheme_ScaleStatus_To_v1beta1_ScaleStatus,
+		Convert_v1beta1_ScaleStatus_To_scheme_ScaleStatus,
+	)
+	if err != nil {
+		return err
+	}
+
+	return nil
+}
+func Convert_scheme_ScaleStatus_To_v1beta1_ScaleStatus(in *scheme.ScaleStatus, out *v1beta1.ScaleStatus, s conversion.Scope) error {
+	out.Replicas = in.Replicas
+	out.Selector = nil
+	out.TargetSelector = ""
+	if in.Selector != nil {
+		if in.Selector.MatchExpressions == nil || len(in.Selector.MatchExpressions) == 0 {
+			out.Selector = in.Selector.MatchLabels
+		}
+
+		selector, err := metav1.LabelSelectorAsSelector(in.Selector)
+		if err != nil {
+			return fmt.Errorf("invalid label selector: %v", err)
+		}
+		out.TargetSelector = selector.String()
+	}
+
+	return nil
+}
+
+func Convert_v1beta1_ScaleStatus_To_scheme_ScaleStatus(in *v1beta1.ScaleStatus, out *scheme.ScaleStatus, s conversion.Scope) error {
+	out.Replicas = in.Replicas
+
+	// Normally when 2 fields map to the same internal value we favor the old field, since
+	// old clients can't be expected to know about new fields but clients that know about the
+	// new field can be expected to know about the old field (though that's not quite true, due
+	// to kubectl apply). However, these fields are readonly, so any non-nil value should work.
+	if in.TargetSelector != "" {
+		labelSelector, err := metav1.ParseToLabelSelector(in.TargetSelector)
+		if err != nil {
+			out.Selector = nil
+			return fmt.Errorf("failed to parse target selector: %v", err)
+		}
+		out.Selector = labelSelector
+	} else if in.Selector != nil {
+		out.Selector = new(metav1.LabelSelector)
+		selector := make(map[string]string)
+		for key, val := range in.Selector {
+			selector[key] = val
+		}
+		out.Selector.MatchLabels = selector
+	} else {
+		out.Selector = nil
+	}
+
+	return nil
+}