fix(kyaml/yaml): minor nil safety fix for RNode.Content etc (#5985)

* Fix kyaml/yaml field access deref nil value for methods that look "nil-safe"

This change is addressing observed panics within kustomize that obscure the
actual failure. The primary observed problem case involves RNode.Content.

* Fix test case

* Fixes from review

kyaml/yaml/rnode.go

@@ -718,10 +718,11 @@ func (rn *RNode) MustString() string {
 
 // Content returns Node Content field.
 func (rn *RNode) Content() []*yaml.Node {
-	if rn == nil {
+	yNode := rn.YNode()
+	if yNode == nil {
 		return nil
 	}
-	return rn.YNode().Content
+	return yNode.Content
 }
 
 // Fields returns the list of field names for a MappingNode.
@@ -756,7 +757,11 @@ func (rn *RNode) FieldRNodes() ([]*RNode, error) {
 // Field returns a fieldName, fieldValue pair for MappingNodes.
 // Returns nil for non-MappingNodes.
 func (rn *RNode) Field(field string) *MapNode {
-	if rn.YNode().Kind != yaml.MappingNode {
+	yNode := rn.YNode()
+	if yNode == nil {
+		return nil
+	}
+	if yNode.Kind != yaml.MappingNode {
 		return nil
 	}
 	var result *MapNode
@@ -892,7 +897,11 @@ func (rn *RNode) ElementValuesList(keys []string) ([][]string, error) {
 // Element returns the element in the list which contains the field matching the value.
 // Returns nil for non-SequenceNodes or if no Element matches.
 func (rn *RNode) Element(key, value string) *RNode {
-	if rn.YNode().Kind != yaml.SequenceNode {
+	yNode := rn.YNode()
+	if yNode == nil {
+		return nil
+	}
+	if yNode.Kind != yaml.SequenceNode {
 		return nil
 	}
 	elem, err := rn.Pipe(MatchElement(key, value))
@@ -906,7 +915,11 @@ func (rn *RNode) Element(key, value string) *RNode {
 // corresponding values[i].
 // Returns nil for non-SequenceNodes or if no Element matches.
 func (rn *RNode) ElementList(keys []string, values []string) *RNode {
-	if rn.YNode().Kind != yaml.SequenceNode {
+	yNode := rn.YNode()
+	if yNode == nil {
+		return nil
+	}
+	if yNode.Kind != yaml.SequenceNode {
 		return nil
 	}
 	elem, err := rn.Pipe(MatchElementList(keys, values))
@@ -960,12 +973,17 @@ func (rn *RNode) GetAssociativeKey() string {
 
 // MarshalJSON creates a byte slice from the RNode.
 func (rn *RNode) MarshalJSON() ([]byte, error) {
+	yNode := rn.YNode()
+	if yNode == nil {
+		return []byte("null"), nil
+	}
+
 	s, err := rn.String()
 	if err != nil {
 		return nil, err
 	}
 
-	if rn.YNode().Kind == SequenceNode {
+	if yNode.Kind == SequenceNode {
 		var a []interface{}
 		if err := Unmarshal([]byte(s), &a); err != nil {
 			return nil, err
@@ -977,6 +995,7 @@ func (rn *RNode) MarshalJSON() ([]byte, error) {
 	if err := Unmarshal([]byte(s), &m); err != nil {
 		return nil, err
 	}
+
 	return json.Marshal(m)
 }
 

kyaml/yaml/rnode_test.go

@@ -2356,3 +2356,70 @@ metadata:
 	require.NoError(t, err)
 	require.Equal(t, "hello-world-foo", fooAppName) // no field named 'foo.appname'
 }
+
+func TestRNode_nilSafety(t *testing.T) {
+	// Both of these scenarios should cause rn.YNode() to return nil.
+	nodesToTest := [...]struct {
+		name string
+		rn   *RNode
+	}{
+		{"nil *RNode receiver", nil},
+		{"RNode with nil internal node", &RNode{value: nil}},
+	}
+
+	t.Run("Content", func(t *testing.T) {
+		for _, tc := range nodesToTest {
+			t.Run(tc.name, func(t *testing.T) {
+				assert.NotPanics(t, func() {
+					assert.Nil(t, tc.rn.Content())
+				})
+			})
+		}
+	})
+
+	t.Run("Field", func(t *testing.T) {
+		for _, tc := range nodesToTest {
+			t.Run(tc.name, func(t *testing.T) {
+				assert.NotPanics(t, func() {
+					assert.Nil(t, tc.rn.Field("any-field"))
+				})
+			})
+		}
+	})
+
+	t.Run("Element", func(t *testing.T) {
+		for _, tc := range nodesToTest {
+			t.Run(tc.name, func(t *testing.T) {
+				assert.NotPanics(t, func() {
+					assert.Nil(t, tc.rn.Element("any-key", "any-value"))
+				})
+			})
+		}
+	})
+
+	t.Run("ElementList", func(t *testing.T) {
+		for _, tc := range nodesToTest {
+			t.Run(tc.name, func(t *testing.T) {
+				assert.NotPanics(t, func() {
+					assert.Nil(t, tc.rn.ElementList([]string{"any-key"}, []string{"any-value"}))
+				})
+			})
+		}
+	})
+
+	t.Run("MarshalJSON", func(t *testing.T) {
+		for _, tc := range nodesToTest {
+			t.Run(tc.name, func(t *testing.T) {
+				var (
+					jsonData []byte
+					err      error
+				)
+				assert.NotPanics(t, func() {
+					jsonData, err = tc.rn.MarshalJSON()
+				})
+				require.NoError(t, err)
+				assert.Equal(t, []byte("null"), jsonData)
+			})
+		}
+	})
+}