Use application/cbor-seq media type in streaming CBOR responses.

The media type application/cbor describes exactly one encoded item. As a new (to Kubernetes) format
with no existing clients, streaming/watch responses will use the application/cbor-seq media
type. CBOR watch responses conform to the specification of CBOR Sequences and are encoded as the
concatenation of zero or more items with no additional framing.

staging/src/k8s.io/apimachinery/pkg/runtime/types.go

@@ -43,10 +43,11 @@ type TypeMeta struct {
 }
 
 const (
-	ContentTypeJSON     string = "application/json"
-	ContentTypeYAML     string = "application/yaml"
-	ContentTypeProtobuf string = "application/vnd.kubernetes.protobuf"
-	ContentTypeCBOR     string = "application/cbor"
+	ContentTypeJSON         string = "application/json"
+	ContentTypeYAML         string = "application/yaml"
+	ContentTypeProtobuf     string = "application/vnd.kubernetes.protobuf"
+	ContentTypeCBOR         string = "application/cbor"     // RFC 8949
+	ContentTypeCBORSequence string = "application/cbor-seq" // RFC 8742
 )
 
 // RawExtension is used to hold extensions in external versions.

staging/src/k8s.io/apiserver/pkg/endpoints/handlers/negotiation/negotiate.go

@@ -26,6 +26,8 @@ import (
 
 	"k8s.io/apimachinery/pkg/runtime"
 	"k8s.io/apimachinery/pkg/runtime/schema"
+	"k8s.io/apiserver/pkg/features"
+	utilfeature "k8s.io/apiserver/pkg/util/feature"
 )
 
 // MediaTypesForSerializer returns a list of media and stream media types for the server.
@@ -33,6 +35,10 @@ func MediaTypesForSerializer(ns runtime.NegotiatedSerializer) (mediaTypes, strea
 	for _, info := range ns.SupportedMediaTypes() {
 		mediaTypes = append(mediaTypes, info.MediaType)
 		if info.StreamSerializer != nil {
+			if utilfeature.DefaultFeatureGate.Enabled(features.CBORServingAndStorage) && info.MediaType == runtime.ContentTypeCBOR {
+				streamMediaTypes = append(streamMediaTypes, runtime.ContentTypeCBORSequence)
+				continue
+			}
 			// stream=watch is the existing mime-type parameter for watch
 			streamMediaTypes = append(streamMediaTypes, info.MediaType+";stream=watch")
 		}

staging/src/k8s.io/apiserver/pkg/endpoints/handlers/watch.go

@@ -88,7 +88,17 @@ func serveWatchHandler(watcher watch.Interface, scope *RequestScope, mediaTypeOp
 	}
 	// TODO: next step, get back mediaTypeOptions from negotiate and return the exact value here
 	mediaType := serializer.MediaType
-	if mediaType != runtime.ContentTypeJSON {
+	switch mediaType {
+	case runtime.ContentTypeJSON:
+		// as-is
+	case runtime.ContentTypeCBOR:
+		// If a client indicated it accepts application/cbor (exactly one data item) on a
+		// watch request, set the conformant application/cbor-seq media type the watch
+		// response. RFC 9110 allows an origin server to deviate from the indicated
+		// preference rather than send a 406 (Not Acceptable) response (see
+		// https://www.rfc-editor.org/rfc/rfc9110.html#section-12.1-5).
+		mediaType = runtime.ContentTypeCBORSequence
+	default:
 		mediaType += ";stream=watch"
 	}
 

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

@@ -129,6 +129,7 @@ func NewRESTClient(baseURL *url.URL, versionedAPIPath string, config ClientConte
 
 func scrubCBORContentConfigIfDisabled(content ClientContentConfig) ClientContentConfig {
 	if clientfeatures.FeatureGates().Enabled(clientfeatures.ClientsAllowCBOR) {
+		content.Negotiator = clientNegotiatorWithCBORSequenceStreamDecoder{content.Negotiator}
 		return content
 	}
 
@@ -294,3 +295,38 @@ func (p *requestClientContentConfigProvider) UnsupportedMediaType(requestContent
 		p.sawUnsupportedMediaTypeForCBOR.Store(true)
 	}
 }
+
+// clientNegotiatorWithCBORSequenceStreamDecoder is a ClientNegotiator that delegates to another
+// ClientNegotiator to select the appropriate Encoder or Decoder for a given media type. As a
+// special case, it will resolve "application/cbor-seq" (a CBOR Sequence, the concatenation of zero
+// or more CBOR data items) as an alias for "application/cbor" (exactly one CBOR data item) when
+// selecting a stream decoder.
+type clientNegotiatorWithCBORSequenceStreamDecoder struct {
+	negotiator runtime.ClientNegotiator
+}
+
+func (n clientNegotiatorWithCBORSequenceStreamDecoder) Encoder(contentType string, params map[string]string) (runtime.Encoder, error) {
+	return n.negotiator.Encoder(contentType, params)
+}
+
+func (n clientNegotiatorWithCBORSequenceStreamDecoder) Decoder(contentType string, params map[string]string) (runtime.Decoder, error) {
+	return n.negotiator.Decoder(contentType, params)
+}
+
+func (n clientNegotiatorWithCBORSequenceStreamDecoder) StreamDecoder(contentType string, params map[string]string) (runtime.Decoder, runtime.Serializer, runtime.Framer, error) {
+	if !clientfeatures.FeatureGates().Enabled(clientfeatures.ClientsAllowCBOR) {
+		return n.negotiator.StreamDecoder(contentType, params)
+	}
+
+	switch contentType {
+	case runtime.ContentTypeCBORSequence:
+		return n.negotiator.StreamDecoder(runtime.ContentTypeCBOR, params)
+	case runtime.ContentTypeCBOR:
+		// This media type is only appropriate for exactly one data item, not the zero or
+		// more events of a watch stream.
+		return nil, nil, nil, runtime.NegotiateError{ContentType: contentType, Stream: true}
+	default:
+		return n.negotiator.StreamDecoder(contentType, params)
+	}
+
+}

test/integration/client/client_test.go

@@ -1430,6 +1430,30 @@ func TestClientCBOREnablement(t *testing.T) {
 		return err
 	}
 
+	DoWatchRequestWithGenericTypedClient := func(t *testing.T, config *rest.Config) error {
+		clientset, err := kubernetes.NewForConfig(config)
+		if err != nil {
+			t.Fatal(err)
+		}
+
+		// Generated clients for built-in types include the PreferProtobuf option, which
+		// forces Protobuf encoding on a per-request basis.
+		client := gentype.NewClientWithListAndApply[*v1.Namespace, *v1.NamespaceList, *corev1ac.NamespaceApplyConfiguration](
+			"namespaces",
+			clientset.CoreV1().RESTClient(),
+			clientscheme.ParameterCodec,
+			"",
+			func() *v1.Namespace { return &v1.Namespace{} },
+			func() *v1.NamespaceList { return &v1.NamespaceList{} },
+		)
+		w, err := client.Watch(context.TODO(), metav1.ListOptions{LabelSelector: "a,!a"})
+		if err != nil {
+			return err
+		}
+		w.Stop()
+		return nil
+	}
+
 	type testCase struct {
 		name                    string
 		served                  bool
@@ -1650,6 +1674,20 @@ func TestClientCBOREnablement(t *testing.T) {
 			wantStatusError:         false,
 			doRequest:               DoRequestWithGenericTypedClient,
 		},
+		{
+			name:                    "generated client watch accept cbor and json get cbor-seq",
+			served:                  true,
+			allowed:                 true,
+			preferred:               false,
+			configuredContentType:   "application/json",
+			configuredAccept:        "application/cbor;q=1,application/json;q=0.9",
+			wantRequestContentType:  "",
+			wantRequestAccept:       "application/cbor;q=1,application/json;q=0.9",
+			wantResponseContentType: "application/cbor-seq",
+			wantResponseStatus:      http.StatusOK,
+			wantStatusError:         false,
+			doRequest:               DoWatchRequestWithGenericTypedClient,
+		},
 		{
 			name:                    "generated client accept cbor and json get json cbor not served",
 			served:                  false,

test/integration/client/dynamic_client_test.go

@@ -408,6 +408,20 @@ func TestDynamicClientCBOREnablement(t *testing.T) {
 		return err
 	}
 
+	DoWatch := func(t *testing.T, config *rest.Config) error {
+		client, err := dynamic.NewForConfig(config)
+		if err != nil {
+			t.Fatal(err)
+		}
+
+		w, err := client.Resource(corev1.SchemeGroupVersion.WithResource("namespaces")).Watch(context.TODO(), metav1.ListOptions{LabelSelector: "a,!a"})
+		if err != nil {
+			return err
+		}
+		w.Stop()
+		return nil
+	}
+
 	testCases := []struct {
 		name                    string
 		serving                 bool
@@ -540,6 +554,17 @@ func TestDynamicClientCBOREnablement(t *testing.T) {
 			wantStatusError:         true,
 			doRequest:               DoApply,
 		},
+		{
+			name:                    "watch accepts both gets cbor-seq",
+			serving:                 true,
+			allowed:                 true,
+			preferred:               false,
+			wantRequestAccept:       "application/json;q=0.9,application/cbor;q=1",
+			wantResponseContentType: "application/cbor-seq",
+			wantResponseStatus:      http.StatusOK,
+			wantStatusError:         false,
+			doRequest:               DoWatch,
+		},
 	}
 
 	for _, serving := range []bool{true, false} {