Implement switch via editing yaml in-place

Signed-off-by: Ahmet Alp Balkan <ahmetb@google.com>

cmd/kubectx/main.go

@@ -20,15 +20,25 @@ func main() {
 	case ListOp:
 		printListContexts(os.Stdout)
 	case SwitchOp:
-		// TODO implement
-		panic("not implemented")
+		if v.Target == "-" {
+			// TODO implement swap
+			panic("not implemented")
+		}
+		newCtx, err := switchContext(v.Target)
+		if err != nil {
+			printError("faield to switch context: %v", err)
+			os.Exit(1)
+		}
+		fmt.Fprintf(os.Stderr, "Switched to context %q.\n", newCtx)
 	case UnknownOp:
-		fmt.Printf("%s unsupported operation: %s\n",
-			color.RedString("error:"),
-			strings.Join(v.Args, " "))
+		printError("unsupported operation: %s", strings.Join(v.Args, " "))
 		printHelp(os.Stdout)
 		os.Exit(1)
 	default:
 		fmt.Printf("internal error: operation type %T not handled", op)
 	}
 }
+
+func printError(format string, args ...interface{}) {
+	fmt.Fprintf(os.Stderr, color.RedString("error: "+format+"\n"), args...)
+}

cmd/kubectx/switch.go

@@ -0,0 +1,103 @@
+package main
+
+import (
+	"fmt"
+	"io"
+	"os"
+
+	"github.com/pkg/errors"
+	"gopkg.in/yaml.v3"
+)
+
+// switchContext switches to specified context name.
+func switchContext(name string) (string, error) {
+	cfgPath, err := kubeconfigPath()
+	if err != nil {
+		return "", errors.Wrap(err, "cannot determine kubeconfig path")
+	}
+	f, err := os.OpenFile(cfgPath, os.O_RDWR, 0)
+	if err != nil {
+		return "", errors.Wrap(err, "failed to open file")
+	}
+	defer f.Close()
+
+	kc, err := parseKubeconfigRaw(f)
+	if err != nil {
+		return "", errors.Wrap(err, "yaml parse error")
+	}
+
+	cur := getCurrentContext(kc)
+	fmt.Printf("current-context=%s\n", cur)
+
+	if err := modifyCurrentContext(kc, name); err != nil {
+		return "", err
+	}
+
+	if  err := f.Truncate(0); err != nil {
+		return "", errors.Wrap(err, "failed to truncate")
+	}
+
+	if _, err := f.Seek(0, 0); err != nil {
+		return "", errors.Wrap(err, "failed to seek")
+	}
+
+	if err := saveKubeconfigRaw(f, kc); err != nil {
+		return "", errors.Wrap(err, "failed to save kubeconfig")
+	}
+	return name, nil
+}
+
+
+// getCurrentContext returns "current-context" value in given
+// kubeconfig object Node, or returns "" if not found.
+func getCurrentContext(rootNode *yaml.Node) string {
+	if rootNode.Kind != yaml.MappingNode {
+		return ""
+	}
+	for i, ch := range rootNode.Content {
+		if i%2 == 0 && ch.Value == "current-context" {
+			return rootNode.Content[i+1].Value
+		}
+	}
+	return ""
+}
+
+func modifyCurrentContext(rootNode *yaml.Node, name string) error {
+	if rootNode.Kind != yaml.MappingNode {
+		return errors.New("document is not a map")
+	}
+
+	// find current-context field => modify value (next children)
+	for i, ch := range rootNode.Content {
+		if i%2 == 0 && ch.Value == "current-context" {
+			rootNode.Content[i+1].Value = name
+			return nil
+		}
+	}
+
+	// if current-context ==> create new field
+	keyNode := &yaml.Node{
+		Kind:  yaml.ScalarNode,
+		Value: "current-context",
+		Tag:   "!!str"}
+	valueNode := &yaml.Node{
+		Kind:  yaml.ScalarNode,
+		Value: name,
+		Tag:   "!!str"}
+	rootNode.Content = append(rootNode.Content, keyNode, valueNode)
+	return nil
+}
+
+func parseKubeconfigRaw(r io.Reader) (*yaml.Node, error) {
+	var v yaml.Node
+	if err := yaml.NewDecoder(r).Decode(&v); err != nil {
+		return nil, err
+	}
+	return v.Content[0], nil
+}
+
+func saveKubeconfigRaw(w io.Writer, rootNode *yaml.Node) error {
+	enc := yaml.NewEncoder(w)
+	enc.SetIndent(2)
+	return enc.Encode(rootNode)
+}