Merge pull request #53924 from dims/use-in-cluster-config-before-default

Automatic merge from submit-queue. If you want to cherry-pick this change to another branch, please follow the instructions <a href="https://github.com/kubernetes/community/blob/master/contributors/devel/cherry-picks.md">here</a>.

Try in-cluster config before using localhost:8080

**What this PR does / why we need it**:

When starting an e2e test in a pod in a cluster, if the host is
not specified in the command line, we default to using
'http://127.0.0.1:8080' currently. We should be discovering the
host/port using the in-cluster config and using that if
possible.

**Which issue this PR fixes** *(optional, in `fixes #<issue number>(, fixes #<issue_number>, ...)` format, will close that issue when PR gets merged)*: fixes #

Fixes #53894

**Special notes for your reviewer**:

**Release note**:

```release-note
NONE
```

test/e2e/framework/test_context.go

@@ -19,12 +19,16 @@ package framework
 import (
 	"flag"
 	"fmt"
+	"io/ioutil"
 	"os"
 	"time"
 
+	"github.com/golang/glog"
 	"github.com/onsi/ginkgo/config"
 	"github.com/spf13/viper"
+	restclient "k8s.io/client-go/rest"
 	"k8s.io/client-go/tools/clientcmd"
+	clientcmdapi "k8s.io/client-go/tools/clientcmd/api"
 	"k8s.io/kubernetes/pkg/cloudprovider"
 	"k8s.io/kubernetes/pkg/kubelet/apis/kubeletconfig"
 	"k8s.io/kubernetes/pkg/kubemark"
@@ -296,12 +300,61 @@ func ViperizeFlags() {
 	AfterReadingAllFlags(&TestContext)
 }
 
+func createKubeConfig(clientCfg *restclient.Config) *clientcmdapi.Config {
+	clusterNick := "cluster"
+	userNick := "user"
+	contextNick := "context"
+
+	config := clientcmdapi.NewConfig()
+
+	credentials := clientcmdapi.NewAuthInfo()
+	credentials.Token = clientCfg.BearerToken
+	credentials.ClientCertificate = clientCfg.TLSClientConfig.CertFile
+	if len(credentials.ClientCertificate) == 0 {
+		credentials.ClientCertificateData = clientCfg.TLSClientConfig.CertData
+	}
+	credentials.ClientKey = clientCfg.TLSClientConfig.KeyFile
+	if len(credentials.ClientKey) == 0 {
+		credentials.ClientKeyData = clientCfg.TLSClientConfig.KeyData
+	}
+	config.AuthInfos[userNick] = credentials
+
+	cluster := clientcmdapi.NewCluster()
+	cluster.Server = clientCfg.Host
+	cluster.CertificateAuthority = clientCfg.CAFile
+	if len(cluster.CertificateAuthority) == 0 {
+		cluster.CertificateAuthorityData = clientCfg.CAData
+	}
+	cluster.InsecureSkipTLSVerify = clientCfg.Insecure
+	config.Clusters[clusterNick] = cluster
+
+	context := clientcmdapi.NewContext()
+	context.Cluster = clusterNick
+	context.AuthInfo = userNick
+	config.Contexts[contextNick] = context
+	config.CurrentContext = contextNick
+
+	return config
+}
+
 // AfterReadingAllFlags makes changes to the context after all flags
 // have been read.
 func AfterReadingAllFlags(t *TestContextType) {
 	// Only set a default host if one won't be supplied via kubeconfig
 	if len(t.Host) == 0 && len(t.KubeConfig) == 0 {
-		t.Host = defaultHost
+		// Check if we can use the in-cluster config
+		if clusterConfig, err := restclient.InClusterConfig(); err == nil {
+			if tempFile, err := ioutil.TempFile(os.TempDir(), "kubeconfig-"); err == nil {
+				kubeConfig := createKubeConfig(clusterConfig)
+				clientcmd.WriteToFile(*kubeConfig, tempFile.Name())
+				t.KubeConfig = tempFile.Name()
+				glog.Infof("Using a temporary kubeconfig file from in-cluster config : %s", tempFile.Name())
+			}
+		}
+		if len(t.KubeConfig) == 0 {
+			glog.Warningf("Unable to find in-cluster config, using default host : %s", defaultHost)
+			t.Host = defaultHost
+		}
 	}
 	// Reset the cluster IP range flag to CLUSTER_IP_RANGE env var, if defined.
 	if clusterIPRange := os.Getenv("CLUSTER_IP_RANGE"); clusterIPRange != "" {