Add transport wrapper that blocks api calls after context close

The ContextCanceller transport wrapper blocks all API requests
after the provided context is closed. Used with the leader election
step down, a controller can ensure that new requests are not made
after the client has stepped down.

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

@@ -36,6 +36,7 @@ go_test(
         "//staging/src/k8s.io/client-go/kubernetes/scheme:go_default_library",
         "//staging/src/k8s.io/client-go/rest/watch:go_default_library",
         "//staging/src/k8s.io/client-go/tools/clientcmd/api:go_default_library",
+        "//staging/src/k8s.io/client-go/transport:go_default_library",
         "//staging/src/k8s.io/client-go/util/flowcontrol:go_default_library",
         "//staging/src/k8s.io/client-go/util/testing:go_default_library",
         "//vendor/github.com/google/gofuzz:go_default_library",

staging/src/k8s.io/client-go/tools/leaderelection/example/BUILD

@@ -13,6 +13,7 @@ go_library(
         "//staging/src/k8s.io/client-go/tools/clientcmd:go_default_library",
         "//staging/src/k8s.io/client-go/tools/leaderelection:go_default_library",
         "//staging/src/k8s.io/client-go/tools/leaderelection/resourcelock:go_default_library",
+        "//staging/src/k8s.io/client-go/transport:go_default_library",
         "//vendor/k8s.io/klog:go_default_library",
     ],
 )

staging/src/k8s.io/client-go/tools/leaderelection/example/main.go

@@ -19,9 +19,11 @@ package main
 import (
 	"context"
 	"flag"
+	"fmt"
 	"log"
 	"os"
 	"os/signal"
+	"strings"
 	"syscall"
 	"time"
 
@@ -31,6 +33,7 @@ import (
 	"k8s.io/client-go/tools/clientcmd"
 	"k8s.io/client-go/tools/leaderelection"
 	"k8s.io/client-go/tools/leaderelection/resourcelock"
+	"k8s.io/client-go/transport"
 	"k8s.io/klog"
 )
 
@@ -78,6 +81,10 @@ func main() {
 	ctx, cancel := context.WithCancel(context.Background())
 	defer cancel()
 
+	// use a client that will stop allowing new requests once the context ends
+	config.Wrap(transport.ContextCanceller(ctx, fmt.Errorf("the leader is shutting down")))
+	exampleClient := kubernetes.NewForConfigOrDie(config).CoreV1()
+
 	// listen for interrupts or the Linux SIGTERM signal and cancel
 	// our context, which the leader election code will observe and
 	// step down
@@ -116,6 +123,12 @@ func main() {
 		},
 	})
 
+	// because the context is closed, the client should report errors
+	_, err = exampleClient.ConfigMaps(args[1]).Get(args[2], metav1.GetOptions{})
+	if err == nil || !strings.Contains(err.Error(), "the leader is shutting down") {
+		log.Fatalf("%s: expected to get an error when trying to make a client call: %v", id, err)
+	}
+
 	// we no longer hold the lease, so perform any cleanup and then
 	// exit
 	log.Printf("%s: done", id)

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

@@ -17,6 +17,7 @@ limitations under the License.
 package transport
 
 import (
+	"context"
 	"crypto/tls"
 	"crypto/x509"
 	"fmt"
@@ -196,3 +197,31 @@ func Wrappers(fns ...WrapperFunc) WrapperFunc {
 		return base
 	}
 }
+
+// ContextCanceller prevents new requests after the provided context is finished.
+// err is returned when the context is closed, allowing the caller to provide a context
+// appropriate error.
+func ContextCanceller(ctx context.Context, err error) WrapperFunc {
+	return func(rt http.RoundTripper) http.RoundTripper {
+		return &contextCanceller{
+			ctx: ctx,
+			rt:  rt,
+			err: err,
+		}
+	}
+}
+
+type contextCanceller struct {
+	ctx context.Context
+	rt  http.RoundTripper
+	err error
+}
+
+func (b *contextCanceller) RoundTrip(req *http.Request) (*http.Response, error) {
+	select {
+	case <-b.ctx.Done():
+		return nil, b.err
+	default:
+		return b.rt.RoundTrip(req)
+	}
+}

staging/src/k8s.io/client-go/transport/transport_test.go

@@ -17,8 +17,10 @@ limitations under the License.
 package transport
 
 import (
+	"context"
 	"crypto/tls"
 	"errors"
+	"fmt"
 	"net/http"
 	"testing"
 )
@@ -397,3 +399,54 @@ func TestWrappers(t *testing.T) {
 		})
 	}
 }
+
+func Test_contextCanceller_RoundTrip(t *testing.T) {
+	tests := []struct {
+		name string
+		open bool
+		want bool
+	}{
+		{name: "open context should call nested round tripper", open: true, want: true},
+		{name: "closed context should return a known error", open: false, want: false},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			req := &http.Request{}
+			rt := &fakeRoundTripper{Resp: &http.Response{}}
+			ctx := context.Background()
+			if !tt.open {
+				c, fn := context.WithCancel(ctx)
+				fn()
+				ctx = c
+			}
+			errTesting := fmt.Errorf("testing")
+			b := &contextCanceller{
+				rt:  rt,
+				ctx: ctx,
+				err: errTesting,
+			}
+			got, err := b.RoundTrip(req)
+			if tt.want {
+				if err != nil {
+					t.Errorf("unexpected error: %v", err)
+				}
+				if got != rt.Resp {
+					t.Errorf("wanted response")
+				}
+				if req != rt.Req {
+					t.Errorf("expect nested call")
+				}
+			} else {
+				if err != errTesting {
+					t.Errorf("unexpected error: %v", err)
+				}
+				if got != nil {
+					t.Errorf("wanted no response")
+				}
+				if rt.Req != nil {
+					t.Errorf("want no nested call")
+				}
+			}
+		})
+	}
+}