fix: do not panic in merge.Merge if map value is nil

Checking for `zeroValue` is not enough when accessing `map[string]any`.

Closes #8005

Signed-off-by: Dmitriy Matrenichev <dmitry.matrenichev@siderolabs.com>
(cherry picked from commit 6329222)

pkg/machinery/config/merge/merge.go

@@ -27,12 +27,12 @@ import (
 //   - if it is a struct, merge is performed for each field of the struct.
 //   - if the type implements 'merger' interface, Merge function is called to handle the merge process.
 //   - merger interface should be implemented on the pointer to the type.
-func Merge(left, right interface{}) error {
+func Merge(left, right any) error {
 	return merge(reflect.ValueOf(left), reflect.ValueOf(right), false)
 }
 
 type merger interface {
-	Merge(other interface{}) error
+	Merge(other any) error
 }
 
 var (
@@ -105,7 +105,7 @@ func merge(vl, vr reflect.Value, replace bool) error {
 		}
 
 		for _, k := range vr.MapKeys() {
-			if vl.MapIndex(k) != zeroValue {
+			if !isNilOrZero(vl.MapIndex(k)) {
 				valueType := tl.Elem()
 
 				var v, rightV reflect.Value
@@ -193,3 +193,19 @@ func merge(vl, vr reflect.Value, replace bool) error {
 
 	return nil
 }
+
+// isNilOrZero returns true if the [reflect.Value] is zero [reflect.Value] or something that is nil.
+// We need it because if map contains a key with `nil` value, simply comparing that result to the `zeroValue`
+// is not enough.
+func isNilOrZero(idx reflect.Value) bool {
+	if idx == zeroValue {
+		return true
+	}
+
+	switch idx.Kind() { //nolint:exhaustive
+	case reflect.Interface, reflect.Pointer, reflect.Slice, reflect.Map, reflect.Chan, reflect.Func:
+		return idx.IsNil()
+	default:
+		return false
+	}
+}

pkg/machinery/config/merge/merge_test.go

@@ -33,7 +33,7 @@ type Struct struct {
 
 type CustomSlice []string
 
-func (s *CustomSlice) Merge(other interface{}) error {
+func (s *CustomSlice) Merge(other any) error {
 	otherSlice, ok := other.(CustomSlice)
 	if !ok {
 		return fmt.Errorf("other is not CustomSlice: %v", other)
@@ -45,13 +45,13 @@ func (s *CustomSlice) Merge(other interface{}) error {
 	return nil
 }
 
-type Unstructured map[string]interface{}
+type Unstructured map[string]any
 
 func TestMerge(t *testing.T) {
 	for _, tt := range []struct {
 		name        string
-		left, right interface{}
-		expected    interface{}
+		left, right any
+		expected    any
 	}{
 		{
 			name: "zero",
@@ -249,17 +249,17 @@ func TestMerge(t *testing.T) {
 			name: "unstructured",
 			left: &Unstructured{
 				"a": "aa",
-				"map": map[string]interface{}{
-					"slice": []interface{}{
+				"map": map[string]any{
+					"slice": []any{
 						"s1",
 					},
 					"some": "value",
 				},
 			},
 			right: &Unstructured{
 				"b": "bb",
-				"map": map[string]interface{}{
-					"slice": []interface{}{
+				"map": map[string]any{
+					"slice": []any{
 						"s2",
 					},
 					"other": "thing",
@@ -268,8 +268,8 @@ func TestMerge(t *testing.T) {
 			expected: &Unstructured{
 				"a": "aa",
 				"b": "bb",
-				"map": map[string]interface{}{
-					"slice": []interface{}{
+				"map": map[string]any{
+					"slice": []any{
 						"s1",
 						"s2",
 					},
@@ -278,6 +278,36 @@ func TestMerge(t *testing.T) {
 				},
 			},
 		},
+		{
+			name: "unstructed with nil value",
+			left: Unstructured{
+				"a": nil,
+				"b": []any{
+					"c",
+					"d",
+				},
+			},
+			right: Unstructured{
+				"a": Unstructured{
+					"b": []any{
+						"c",
+						"d",
+					},
+				},
+			},
+			expected: Unstructured{
+				"a": Unstructured{
+					"b": []any{
+						"c",
+						"d",
+					},
+				},
+				"b": []any{
+					"c",
+					"d",
+				},
+			},
+		},
 	} {
 		t.Run(tt.name, func(t *testing.T) {
 			err := merge.Merge(tt.left, tt.right)