Make wrapping a client transport more pleasant

Properly wrapping a transport can be tricky. Make the normal case
(adding a non-nil transport wrapper to a config) easier with a helper.
Also enforce a rough ordering, which in the future we can use to
simplify the WrapTransport mechanism down into an array of functions
we execute in order and avoid wrapping altogether.

Kubernetes-commit: 1f590e697ef64812620c787720b4b5942027e4a1

rest/config.go

@@ -34,6 +34,7 @@ import (
 	"k8s.io/apimachinery/pkg/runtime/schema"
 	"k8s.io/client-go/pkg/version"
 	clientcmdapi "k8s.io/client-go/tools/clientcmd/api"
+	"k8s.io/client-go/transport"
 	certutil "k8s.io/client-go/util/cert"
 	"k8s.io/client-go/util/flowcontrol"
 	"k8s.io/klog"
@@ -95,13 +96,16 @@ type Config struct {
 
 	// Transport may be used for custom HTTP behavior. This attribute may not
 	// be specified with the TLS client certificate options. Use WrapTransport
-	// for most client level operations.
+	// to provide additional per-server middleware behavior.
 	Transport http.RoundTripper
 	// WrapTransport will be invoked for custom HTTP behavior after the underlying
 	// transport is initialized (either the transport created from TLSClientConfig,
 	// Transport, or http.DefaultTransport). The config may layer other RoundTrippers
 	// on top of the returned RoundTripper.
-	WrapTransport func(rt http.RoundTripper) http.RoundTripper
+	//
+	// A future release will change this field to an array. Use config.Wrap()
+	// instead of setting this value directly.
+	WrapTransport transport.WrapperFunc
 
 	// QPS indicates the maximum QPS to the master from this client.
 	// If it's zero, the created RESTClient will use DefaultQPS: 5

rest/config_test.go

@@ -27,12 +27,13 @@ import (
 	"strings"
 	"testing"
 
-	"k8s.io/api/core/v1"
+	v1 "k8s.io/api/core/v1"
 	"k8s.io/apimachinery/pkg/runtime"
 	"k8s.io/apimachinery/pkg/runtime/schema"
 	"k8s.io/apimachinery/pkg/util/diff"
 	"k8s.io/client-go/kubernetes/scheme"
 	clientcmdapi "k8s.io/client-go/tools/clientcmd/api"
+	"k8s.io/client-go/transport"
 	"k8s.io/client-go/util/flowcontrol"
 
 	fuzz "github.com/google/gofuzz"
@@ -236,6 +237,9 @@ func TestAnonymousConfig(t *testing.T) {
 		func(fn *func(http.RoundTripper) http.RoundTripper, f fuzz.Continue) {
 			*fn = fakeWrapperFunc
 		},
+		func(fn *transport.WrapperFunc, f fuzz.Continue) {
+			*fn = fakeWrapperFunc
+		},
 		func(r *runtime.NegotiatedSerializer, f fuzz.Continue) {
 			serializer := &fakeNegotiatedSerializer{}
 			f.Fuzz(serializer)
@@ -316,6 +320,9 @@ func TestCopyConfig(t *testing.T) {
 		func(fn *func(http.RoundTripper) http.RoundTripper, f fuzz.Continue) {
 			*fn = fakeWrapperFunc
 		},
+		func(fn *transport.WrapperFunc, f fuzz.Continue) {
+			*fn = fakeWrapperFunc
+		},
 		func(r *runtime.NegotiatedSerializer, f fuzz.Continue) {
 			serializer := &fakeNegotiatedSerializer{}
 			f.Fuzz(serializer)

rest/transport.go

@@ -103,14 +103,15 @@ func (c *Config) TransportConfig() (*transport.Config, error) {
 		if err != nil {
 			return nil, err
 		}
-		wt := conf.WrapTransport
-		if wt != nil {
-			conf.WrapTransport = func(rt http.RoundTripper) http.RoundTripper {
-				return provider.WrapTransport(wt(rt))
-			}
-		} else {
-			conf.WrapTransport = provider.WrapTransport
-		}
+		conf.Wrap(provider.WrapTransport)
 	}
 	return conf, nil
 }
+
+// Wrap adds a transport middleware function that will give the caller
+// an opportunity to wrap the underlying http.RoundTripper prior to the
+// first API call being made. The provided function is invoked after any
+// existing transport wrappers are invoked.
+func (c *Config) Wrap(fn transport.WrapperFunc) {
+	c.WrapTransport = transport.Wrappers(c.WrapTransport, fn)
+}