Use in-cluster config when kubeconfig file is not found (#2996)

* Use in-cluster config if KUBECONFIG env is not set.
Or when kubeconfig file is not found in user's home directory.
* Fixed e2e cli tests failure
* Resolve review comments
* Join kubeconfig not found and failed to load in-cluster config errors


---------

Signed-off-by: Md Soharab Ansari <soharab.ansari@infracloud.io>

pkg/fission-cli/cmd/client.go

@@ -22,6 +22,7 @@ import (
 	"os"
 	"os/user"
 	"path/filepath"
+	"strings"
 
 	"k8s.io/client-go/kubernetes"
 	"k8s.io/client-go/rest"
@@ -99,32 +100,44 @@ func GetClientConfig(kubeContext string) (clientcmd.ClientConfig, error) {
 
 func NewClient(opts ClientOptions) (*Client, error) {
 	client := &Client{}
-	var err error
+	var err1 error
 	var cmdConfig clientcmd.ClientConfig
 	if len(opts.Namespace) > 0 {
 		client.Namespace = opts.Namespace
 	} else {
-		cmdConfig, err = GetClientConfig(opts.KubeContext)
-		if err != nil {
-			return nil, err
+		cmdConfig, err1 = GetClientConfig(opts.KubeContext)
+		if err1 != nil {
+			console.Verbose(2, err1.Error())
 		}
-		namespace, _, err := cmdConfig.Namespace()
-		if err != nil {
-			return nil, err
-		}
-		client.Namespace = namespace
-	}
 
-	console.Verbose(2, "Kubeconfig default namespace %q", client.Namespace)
+		if cmdConfig != nil {
+			namespace, _, err := cmdConfig.Namespace()
+			if err != nil {
+				return nil, err
+			}
+			client.Namespace = namespace
+			console.Verbose(2, "Kubeconfig default namespace %q", namespace)
+		}
+	}
 
 	if opts.RestConfig != nil {
 		client.RestConfig = opts.RestConfig
-	} else {
+	} else if cmdConfig != nil {
 		restConfig, err := cmdConfig.ClientConfig()
 		if err != nil {
 			return nil, err
 		}
 		client.RestConfig = restConfig
+	} else {
+		console.Verbose(2, "Using in-cluster config")
+		restConfig, err2 := rest.InClusterConfig()
+		if err2 != nil {
+			return nil, errors.Join(err1, err2)
+		}
+
+		client.RestConfig = restConfig
+		client.Namespace = getInClusterConfigNamespace()
+		console.Verbose(2, "In-cluster config default namespace %q", client.Namespace)
 	}
 
 	clientGen := crd.NewClientGeneratorWithRestConfig(client.RestConfig)
@@ -142,3 +155,21 @@ func NewClient(opts ClientOptions) (*Client, error) {
 
 	return client, nil
 }
+
+// Fetch default namespace for inClusterConfig
+func getInClusterConfigNamespace() string {
+	// This way assumes you've set the POD_NAMESPACE environment variable using the downward API.
+	// This check has to be done first for backwards compatibility with the way InClusterConfig was originally set up
+	if ns, ok := os.LookupEnv("POD_NAMESPACE"); ok {
+		return ns
+	}
+
+	// Fall back to the namespace associated with the service account token, if available
+	if data, err := os.ReadFile("/var/run/secrets/kubernetes.io/serviceaccount/namespace"); err == nil {
+		if ns := strings.TrimSpace(string(data)); len(ns) > 0 {
+			return ns
+		}
+	}
+
+	return "default"
+}