Fall back to JSON request encoding after CBOR 415.

If a client is configured to encode request bodies to CBOR, but the server does not support CBOR,
the server will respond with HTTP 415 (Unsupported Media Type). By feeding this response back to the
RESTClient, subsequent requests can fall back to JSON, which is assumed to be acceptable.

Kubernetes-commit: 1745dfdd154b1a838765e70b81c861c644bfcffe

rest/client.go

@@ -24,6 +24,7 @@ import (
 	"os"
 	"strconv"
 	"strings"
+	"sync/atomic"
 	"time"
 
 	"github.com/munnerz/goautoneg"
@@ -89,7 +90,7 @@ type RESTClient struct {
 	versionedAPIPath string
 
 	// content describes how a RESTClient encodes and decodes responses.
-	content ClientContentConfig
+	content requestClientContentConfigProvider
 
 	// creates BackoffManager that is passed to requests.
 	createBackoffMgr func() BackoffManager
@@ -119,11 +120,10 @@ func NewRESTClient(baseURL *url.URL, versionedAPIPath string, config ClientConte
 	return &RESTClient{
 		base:             &base,
 		versionedAPIPath: versionedAPIPath,
-		content:          scrubCBORContentConfigIfDisabled(config),
+		content:          requestClientContentConfigProvider{base: scrubCBORContentConfigIfDisabled(config)},
 		createBackoffMgr: readExpBackoffConfig,
 		rateLimiter:      rateLimiter,
-
-		Client: client,
+		Client:           client,
 	}, nil
 }
 
@@ -237,5 +237,60 @@ func (c *RESTClient) Delete() *Request {
 
 // APIVersion returns the APIVersion this RESTClient is expected to use.
 func (c *RESTClient) APIVersion() schema.GroupVersion {
-	return c.content.GroupVersion
+	return c.content.GetClientContentConfig().GroupVersion
+}
+
+// requestClientContentConfigProvider observes HTTP 415 (Unsupported Media Type) responses to detect
+// that the server does not understand CBOR. Once this has happened, future requests are forced to
+// use JSON so they can succeed. This is convenient for client users that want to prefer CBOR, but
+// also need to interoperate with older servers so requests do not permanently fail. The clients
+// will not default to using CBOR until at least all supported kube-apiservers have enable-CBOR
+// locked to true, so this path will be rarely taken. Additionally, all generated clients accessing
+// built-in kube resources are forced to protobuf, so those will not degrade to JSON.
+type requestClientContentConfigProvider struct {
+	base ClientContentConfig
+
+	// Becomes permanently true if a server responds with HTTP 415 (Unsupported Media Type) to a
+	// request with "Content-Type" header containing the CBOR media type.
+	sawUnsupportedMediaTypeForCBOR atomic.Bool
+}
+
+// GetClientContentConfig returns the ClientContentConfig that should be used for new requests by
+// this client.
+func (p *requestClientContentConfigProvider) GetClientContentConfig() ClientContentConfig {
+	if !clientfeatures.TestOnlyFeatureGates.Enabled(clientfeatures.TestOnlyClientAllowsCBOR) {
+		return p.base
+	}
+
+	if sawUnsupportedMediaTypeForCBOR := p.sawUnsupportedMediaTypeForCBOR.Load(); !sawUnsupportedMediaTypeForCBOR {
+		return p.base
+	}
+
+	if mediaType, _, _ := mime.ParseMediaType(p.base.ContentType); mediaType != runtime.ContentTypeCBOR {
+		return p.base
+	}
+
+	config := p.base
+	// The default ClientContentConfig sets ContentType to CBOR and the client has previously
+	// received an HTTP 415 in response to a CBOR request. Override ContentType to JSON.
+	config.ContentType = runtime.ContentTypeJSON
+	return config
+}
+
+// UnsupportedMediaType reports that the server has responded to a request with HTTP 415 Unsupported
+// Media Type.
+func (p *requestClientContentConfigProvider) UnsupportedMediaType(requestContentType string) {
+	if !clientfeatures.TestOnlyFeatureGates.Enabled(clientfeatures.TestOnlyClientAllowsCBOR) {
+		return
+	}
+
+	// This could be extended to consider the Content-Encoding request header, the Accept and
+	// Accept-Encoding response headers, the request method, and URI (as mentioned in
+	// https://www.rfc-editor.org/rfc/rfc9110.html#section-15.5.16). The request Content-Type
+	// header is sufficient to implement a blanket CBOR fallback mechanism.
+	requestContentType, _, _ = mime.ParseMediaType(requestContentType)
+	switch requestContentType {
+	case runtime.ContentTypeCBOR, string(types.ApplyCBORPatchType):
+		p.sawUnsupportedMediaTypeForCBOR.Store(true)
+	}
 }

rest/request.go

@@ -156,7 +156,7 @@ func NewRequest(c *RESTClient) *Request {
 		timeout = c.Client.Timeout
 	}
 
-	contentConfig := c.content
+	contentConfig := c.content.GetClientContentConfig()
 	contentTypeNotSet := len(contentConfig.ContentType) == 0
 	if contentTypeNotSet {
 		contentConfig.ContentType = "application/json"
@@ -188,7 +188,7 @@ func NewRequestWithClient(base *url.URL, versionedAPIPath string, content Client
 	return NewRequest(&RESTClient{
 		base:             base,
 		versionedAPIPath: versionedAPIPath,
-		content:          content,
+		content:          requestClientContentConfigProvider{base: content},
 		Client:           client,
 	})
 }
@@ -1235,6 +1235,9 @@ func (r *Request) request(ctx context.Context, fn func(*http.Request, *http.Resp
 		if req.ContentLength >= 0 && !(req.Body != nil && req.ContentLength == 0) {
 			metrics.RequestSize.Observe(ctx, r.verb, r.URL().Host, float64(req.ContentLength))
 		}
+		if resp != nil && resp.StatusCode == http.StatusUnsupportedMediaType {
+			r.c.content.UnsupportedMediaType(resp.Request.Header.Get("Content-Type"))
+		}
 		retry.After(ctx, r, resp, err)
 
 		done := func() bool {