kubenet: implement fake execer for testcases

When the IP isn't in the internal map, GetPodNetworkStatus() needs
to call the execer for the 'nsenter' program.  That  means the execer
needs to be !nil, which it wasn't before.

pkg/kubelet/network/kubenet/kubenet_linux_test.go

@@ -17,6 +17,8 @@ limitations under the License.
 package kubenet
 
 import (
+	"fmt"
+
 	kubecontainer "k8s.io/kubernetes/pkg/kubelet/container"
 	"k8s.io/kubernetes/pkg/kubelet/network"
 	nettest "k8s.io/kubernetes/pkg/kubelet/network/testing"
@@ -38,9 +40,6 @@ func TestGetPodNetworkStatus(t *testing.T) {
 	podIPMap[kubecontainer.ContainerID{ID: "1"}] = "10.245.0.2/32"
 	podIPMap[kubecontainer.ContainerID{ID: "2"}] = "10.245.0.3/32"
 
-	fhost := nettest.NewFakeHost(nil)
-	fakeKubenet := newFakeKubenetPlugin(podIPMap, nil, fhost)
-
 	testCases := []struct {
 		id          string
 		expectError bool
@@ -66,6 +65,34 @@ func TestGetPodNetworkStatus(t *testing.T) {
 		//TODO: add test cases for retrieving ip inside container network namespace
 	}
 
+	fakeCmds := make([]exec.FakeCommandAction, 0)
+	for _, t := range testCases {
+		// the fake commands return the IP from the given index, or an error
+		fCmd := exec.FakeCmd{
+			CombinedOutputScript: []exec.FakeCombinedOutputAction{
+				func() ([]byte, error) {
+					ip, ok := podIPMap[kubecontainer.ContainerID{ID: t.id}]
+					if !ok {
+						return nil, fmt.Errorf("Pod IP %q not found", t.id)
+					}
+					return []byte(ip), nil
+				},
+			},
+		}
+		fakeCmds = append(fakeCmds, func(cmd string, args ...string) exec.Cmd {
+			return exec.InitFakeCmd(&fCmd, cmd, args...)
+		})
+	}
+	fexec := exec.FakeExec{
+		CommandScript: fakeCmds,
+		LookPathFunc: func(file string) (string, error) {
+			return fmt.Sprintf("/fake-bin/%s", file), nil
+		},
+	}
+
+	fhost := nettest.NewFakeHost(nil)
+	fakeKubenet := newFakeKubenetPlugin(podIPMap, &fexec, fhost)
+
 	for i, tc := range testCases {
 		out, err := fakeKubenet.GetPodNetworkStatus("", "", kubecontainer.ContainerID{ID: tc.id})
 		if tc.expectError {