Integrate ctx swap, check for wrong ctx names

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

cmd/kubectx/main.go

@@ -20,13 +20,15 @@ func main() {
 	case ListOp:
 		printListContexts(os.Stdout)
 	case SwitchOp:
+		var newCtx string
+		var err error
 		if v.Target == "-" {
-		// TODO implement swap
-		panic("not implemented")
-	}
-		newCtx, err := switchContext(v.Target)
+			newCtx, err = swapContext()
+		} else {
+			newCtx, err = switchContext(v.Target)
+		}
 		if err != nil {
-			printError("faield to switch context: %v", err)
+			printError("failed to switch context: %v", err)
 			os.Exit(1)
 		}
 		fmt.Fprintf(os.Stderr, "Switched to context %q.\n", newCtx)
@@ -40,5 +42,5 @@ func main() {
 }
 
 func printError(format string, args ...interface{}) {
-	fmt.Fprintf(os.Stderr, color.RedString("error: "+format+"\n"), args...)
+	fmt.Fprintf(os.Stderr, color.RedString("error: ")+format+"\n", args...)
 }

cmd/kubectx/switch.go

@@ -32,11 +32,16 @@ func switchContext(name string) (string, error) {
 
 	prev := getCurrentContext(kc)
 
+	// TODO: add a check to ensure user can't switch to non-existing context.
+	if !checkContextExists(kc, name)  {
+		return "", errors.Errorf("no context exists with the name: %q", name)
+	}
+
 	if err := modifyCurrentContext(kc, name); err != nil {
 		return "", err
 	}
 
-	if  err := f.Truncate(0); err != nil {
+	if err := f.Truncate(0); err != nil {
 		return "", errors.Wrap(err, "failed to truncate")
 	}
 
@@ -57,6 +62,58 @@ func switchContext(name string) (string, error) {
 	return name, nil
 }
 
+func checkContextExists(rootNode *yaml.Node, name string) bool {
+	contexts := valueOf(rootNode, "contexts")
+	if contexts == nil {
+		return false
+	}
+	if contexts.Kind != yaml.SequenceNode {
+		return false
+	}
+
+	var ctxNames []string
+	for _, ctx := range contexts.Content {
+		nameVal := valueOf(ctx, "name")
+		if nameVal != nil {
+			ctxNames = append(ctxNames, nameVal.Value)
+		}
+	}
+
+	for _, v := range ctxNames {
+		if v == name {
+			return true
+		}
+	}
+	return false
+}
+
+func valueOf(mapNode *yaml.Node, key string) *yaml.Node {
+	if mapNode.Kind != yaml.MappingNode {
+		return nil
+	}
+	for i, ch := range mapNode.Content {
+		if i%2 == 0 && ch.Kind == yaml.ScalarNode && ch.Value == key {
+			return mapNode.Content[i+1]
+		}
+	}
+	return nil
+}
+
+// swapContext switches to previously switch context.
+func swapContext() (string, error) {
+	stateFile, err := kubectxFilePath()
+	if err != nil {
+		return "", errors.Wrap(err, "failed to determine state file")
+	}
+	prev, err := readLastContext(stateFile)
+	if err != nil {
+		return "", errors.Wrap(err, "failed to read previous context file")
+	}
+	if prev == "" {
+		return "", errors.New("no previous context found")
+	}
+	return switchContext(prev)
+}
 
 // getCurrentContext returns "current-context" value in given
 // kubeconfig object Node, or returns "" if not found.
@@ -64,12 +121,11 @@ 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
-		}
+	v := valueOf(rootNode, "current-context")
+	if v == nil {
+		return ""
 	}
-	return ""
+	return v.Value
 }
 
 func modifyCurrentContext(rootNode *yaml.Node, name string) error {