Add test-only client feature gates for CBOR.

As with the apiserver feature gate for CBOR as a serving and storage encoding, the client feature
gates for CBOR are being initially added through a test-only feature gate instance that is not wired
to environment variables or to command-line flags and is intended only to be enabled
programmatically from integration tests. The test-only instance will be removed as part of alpha
graduation and replaced by conventional client feature gating.

Author: benluddy

staging/src/k8s.io/client-go/dynamic/scheme.go

@@ -21,7 +21,9 @@ import (
 	"k8s.io/apimachinery/pkg/apis/meta/v1/unstructured"
 	"k8s.io/apimachinery/pkg/runtime"
 	"k8s.io/apimachinery/pkg/runtime/schema"
+	"k8s.io/apimachinery/pkg/runtime/serializer/cbor"
 	"k8s.io/apimachinery/pkg/runtime/serializer/json"
+	"k8s.io/client-go/features"
 )
 
 var basicScheme = runtime.NewScheme()
@@ -35,11 +37,8 @@ func init() {
 	metav1.AddToGroupVersion(parameterScheme, versionV1)
 }
 
-// basicNegotiatedSerializer is used to handle discovery and error handling serialization
-type basicNegotiatedSerializer struct{}
-
-func (s basicNegotiatedSerializer) SupportedMediaTypes() []runtime.SerializerInfo {
-	return []runtime.SerializerInfo{
+func newBasicNegotiatedSerializer() basicNegotiatedSerializer {
+	supportedMediaTypes := []runtime.SerializerInfo{
 		{
 			MediaType:        "application/json",
 			MediaTypeType:    "application",
@@ -54,6 +53,27 @@ func (s basicNegotiatedSerializer) SupportedMediaTypes() []runtime.SerializerInf
 			},
 		},
 	}
+	if features.TestOnlyFeatureGates.Enabled(features.TestOnlyClientAllowsCBOR) {
+		supportedMediaTypes = append(supportedMediaTypes, runtime.SerializerInfo{
+			MediaType:        "application/cbor",
+			MediaTypeType:    "application",
+			MediaTypeSubType: "cbor",
+			Serializer:       cbor.NewSerializer(unstructuredCreater{basicScheme}, unstructuredTyper{basicScheme}),
+			StreamSerializer: &runtime.StreamSerializerInfo{
+				Serializer: cbor.NewSerializer(basicScheme, basicScheme, cbor.Transcode(false)),
+				Framer:     cbor.NewFramer(),
+			},
+		})
+	}
+	return basicNegotiatedSerializer{supportedMediaTypes: supportedMediaTypes}
+}
+
+type basicNegotiatedSerializer struct {
+	supportedMediaTypes []runtime.SerializerInfo
+}
+
+func (s basicNegotiatedSerializer) SupportedMediaTypes() []runtime.SerializerInfo {
+	return s.supportedMediaTypes
 }
 
 func (s basicNegotiatedSerializer) EncoderForVersion(encoder runtime.Encoder, gv runtime.GroupVersioner) runtime.Encoder {

staging/src/k8s.io/client-go/dynamic/simple.go

@@ -29,6 +29,7 @@ import (
 	"k8s.io/apimachinery/pkg/runtime/schema"
 	"k8s.io/apimachinery/pkg/types"
 	"k8s.io/apimachinery/pkg/watch"
+	"k8s.io/client-go/features"
 	"k8s.io/client-go/rest"
 	"k8s.io/client-go/util/consistencydetector"
 	"k8s.io/client-go/util/watchlist"
@@ -45,9 +46,17 @@ var _ Interface = &DynamicClient{}
 // appropriate dynamic client defaults set.
 func ConfigFor(inConfig *rest.Config) *rest.Config {
 	config := rest.CopyConfig(inConfig)
-	config.AcceptContentTypes = "application/json"
+
 	config.ContentType = "application/json"
-	config.NegotiatedSerializer = basicNegotiatedSerializer{} // this gets used for discovery and error handling types
+	config.AcceptContentTypes = "application/json"
+	if features.TestOnlyFeatureGates.Enabled(features.TestOnlyClientAllowsCBOR) {
+		config.AcceptContentTypes = "application/json;q=0.9,application/cbor;q=1"
+		if features.TestOnlyFeatureGates.Enabled(features.TestOnlyClientPrefersCBOR) {
+			config.ContentType = "application/cbor"
+		}
+	}
+
+	config.NegotiatedSerializer = newBasicNegotiatedSerializer()
 	if config.UserAgent == "" {
 		config.UserAgent = rest.DefaultKubernetesUserAgent()
 	}

staging/src/k8s.io/client-go/features/features.go

@@ -18,9 +18,11 @@ package features
 
 import (
 	"errors"
+	"fmt"
+	"sync"
+	"sync/atomic"
 
 	utilruntime "k8s.io/apimachinery/pkg/util/runtime"
-	"sync/atomic"
 )
 
 // NOTE: types Feature, FeatureSpec, prerelease (and its values)
@@ -141,3 +143,43 @@ var (
 	// should use AddFeaturesToExistingFeatureGates followed by ReplaceFeatureGates.
 	featureGates = &atomic.Value{}
 )
+
+// TestOnlyFeatureGates is a distinct registry of pre-alpha client features that must not be
+// included in runtime wiring to command-line flags or environment variables. It exists as a risk
+// mitigation to allow only programmatic enablement of CBOR serialization for integration testing
+// purposes.
+//
+// TODO: Once all required integration test coverage is complete, this will be deleted and the
+// test-only feature gates will be replaced by normal feature gates.
+var TestOnlyFeatureGates = &testOnlyFeatureGates{
+	features: map[Feature]bool{
+		TestOnlyClientAllowsCBOR:  false,
+		TestOnlyClientPrefersCBOR: false,
+	},
+}
+
+type testOnlyFeatureGates struct {
+	lock     sync.RWMutex
+	features map[Feature]bool
+}
+
+func (t *testOnlyFeatureGates) Enabled(feature Feature) bool {
+	t.lock.RLock()
+	defer t.lock.RUnlock()
+
+	enabled, ok := t.features[feature]
+	if !ok {
+		panic(fmt.Sprintf("test-only feature %q not recognized", feature))
+	}
+	return enabled
+}
+
+func (t *testOnlyFeatureGates) Set(feature Feature, enabled bool) error {
+	t.lock.Lock()
+	defer t.lock.Unlock()
+	if _, ok := t.features[feature]; !ok {
+		return fmt.Errorf("test-only feature %q not recognized", feature)
+	}
+	t.features[feature] = enabled
+	return nil
+}

staging/src/k8s.io/client-go/features/known_features.go

@@ -41,6 +41,27 @@ const (
 	// owner: @nilekhc
 	// alpha: v1.30
 	InformerResourceVersion Feature = "InformerResourceVersion"
+
+	// owner: @benluddy
+	// kep: https://kep.k8s.io/4222
+	//
+	// If disabled, clients configured to accept "application/cbor" will instead accept
+	// "application/json" with the same relative preference, and clients configured to write
+	// "application/cbor" or "application/apply-patch+cbor" will instead write
+	// "application/json" or "application/apply-patch+yaml", respectively.
+	//
+	// This feature is currently PRE-ALPHA and MUST NOT be enabled outside of integration tests.
+	TestOnlyClientAllowsCBOR Feature = "TestOnlyClientAllowsCBOR"
+
+	// owner: @benluddy
+	// kep: https://kep.k8s.io/4222
+	//
+	// If enabled AND TestOnlyClientAllowsCBOR is also enabled, the default request content type
+	// (if not explicitly configured) and the dynamic client's request content type both become
+	// "application/cbor".
+	//
+	// This feature is currently PRE-ALPHA and MUST NOT be enabled outside of integration tests.
+	TestOnlyClientPrefersCBOR Feature = "TestOnlyClientPrefersCBOR"
 )
 
 // defaultKubernetesFeatureGates consists of all known Kubernetes-specific feature keys.