flatten nested lists for flatten in visitor

pkg/kubectl/genericclioptions/resource/visitor.go

@@ -376,12 +376,28 @@ func (v FlattenListVisitor) Visit(fn VisitorFunc) error {
 		if info.Object == nil {
 			return fn(info, nil)
 		}
-		items, err := meta.ExtractList(info.Object)
-		if err != nil {
+		if !meta.IsListType(info.Object) {
 			return fn(info, nil)
 		}
-		if errs := runtime.DecodeList(items, v.mapper.decoder); len(errs) > 0 {
-			return utilerrors.NewAggregate(errs)
+
+		items := []runtime.Object{}
+		itemsToProcess := []runtime.Object{info.Object}
+
+		for i := 0; i < len(itemsToProcess); i++ {
+			currObj := itemsToProcess[i]
+			if !meta.IsListType(currObj) {
+				items = append(items, currObj)
+				continue
+			}
+
+			currItems, err := meta.ExtractList(currObj)
+			if err != nil {
+				return err
+			}
+			if errs := runtime.DecodeList(currItems, v.mapper.decoder); len(errs) > 0 {
+				return utilerrors.NewAggregate(errs)
+			}
+			itemsToProcess = append(itemsToProcess, currItems...)
 		}
 
 		// If we have a GroupVersionKind on the list, prioritize that when asking for info on the objects contained in the list

pkg/kubectl/genericclioptions/resource/visitor_test.go

@@ -24,6 +24,7 @@ import (
 	"testing"
 	"time"
 
+	"github.com/davecgh/go-spew/spew"
 	"github.com/stretchr/testify/assert"
 )
 
@@ -163,3 +164,19 @@ func TestVisitorHttpGet(t *testing.T) {
 		})
 	}
 }
+
+func TestFlattenListVisitor(t *testing.T) {
+	b := newDefaultBuilder().
+		FilenameParam(false, &FilenameOptions{Recursive: false, Filenames: []string{"../../../../test/fixtures/pkg/kubectl/builder/deeply-nested.yaml"}}).
+		Flatten()
+
+	test := &testVisitor{}
+
+	err := b.Do().Visit(test.Handle)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if len(test.Infos) != 6 {
+		t.Fatal(spew.Sdump(test.Infos))
+	}
+}

test/fixtures/pkg/kubectl/builder/deeply-nested.yaml

@@ -0,0 +1,21 @@
+apiVersion: v1
+kind: List
+items:
+- apiVersion: v1
+  kind: Pod
+- apiVersion: v1
+  kind: Pod
+- apiVersion: v1
+  kind: List
+  items:
+  - apiVersion: v1
+    kind: Pod
+  - apiVersion: v1
+    kind: Pod
+  - apiVersion: v1
+    kind: List
+    items:
+    - apiVersion: v1
+      kind: Pod
+    - apiVersion: v1
+      kind: Pod