Implement limited merge function

Author: liggitt

staging/src/k8s.io/client-go/tools/clientcmd/merge.go

@@ -17,13 +17,105 @@ limitations under the License.
 package clientcmd
 
 import (
-	"github.com/imdario/mergo"
+	"fmt"
+	"reflect"
+	"strings"
 )
 
 // recursively merges src into dst:
-// - non-pointer struct fields are recursively merged
+// - non-pointer struct fields with any exported fields are recursively merged
+// - non-pointer struct fields with only unexported fields prefer src if the field is non-zero
 // - maps are shallow merged with src keys taking priority over dst
 // - non-zero src fields encountered during recursion that are not maps or structs overwrite and recursion stops
 func merge[T any](dst, src *T) error {
-	return mergo.Merge(dst, src, mergo.WithOverride)
+	if dst == nil {
+		return fmt.Errorf("cannot merge into nil pointer")
+	}
+	if src == nil {
+		return nil
+	}
+	return mergeValues(nil, reflect.ValueOf(dst).Elem(), reflect.ValueOf(src).Elem())
+}
+
+func mergeValues(fieldNames []string, dst, src reflect.Value) error {
+	dstType := dst.Type()
+	// no-op if we can't read the src
+	if !src.IsValid() {
+		return nil
+	}
+	// sanity check types match
+	if srcType := src.Type(); dstType != srcType {
+		return fmt.Errorf("cannot merge mismatched types (%s, %s) at %s", dstType, srcType, strings.Join(fieldNames, "."))
+	}
+
+	switch dstType.Kind() {
+	case reflect.Struct:
+		if hasExportedField(dstType) {
+			// recursively merge
+			for i, n := 0, dstType.NumField(); i < n; i++ {
+				if err := mergeValues(append(fieldNames, dstType.Field(i).Name), dst.Field(i), src.Field(i)); err != nil {
+					return err
+				}
+			}
+		} else if dst.CanSet() {
+			// If all fields are unexported, overwrite with src.
+			// Using src.IsZero() would make more sense but that's not what mergo did.
+			dst.Set(src)
+		}
+
+	case reflect.Map:
+		if dst.CanSet() && !src.IsZero() {
+			// initialize dst if needed
+			if dst.IsZero() {
+				dst.Set(reflect.MakeMap(dstType))
+			}
+			// shallow-merge overwriting dst keys with src keys
+			for _, mapKey := range src.MapKeys() {
+				dst.SetMapIndex(mapKey, src.MapIndex(mapKey))
+			}
+		}
+
+	case reflect.Slice:
+		if dst.CanSet() && src.Len() > 0 {
+			// overwrite dst with non-empty src slice
+			dst.Set(src)
+		}
+
+	case reflect.Pointer:
+		if dst.CanSet() && !src.IsZero() {
+			// overwrite dst with non-zero values for other types
+			if dstType.Elem().Kind() == reflect.Struct {
+				// use struct pointer as-is
+				dst.Set(src)
+			} else {
+				// shallow-copy non-struct pointer (interfaces, primitives, etc)
+				dst.Set(reflect.New(dstType.Elem()))
+				dst.Elem().Set(src.Elem())
+			}
+		}
+
+	default:
+		if dst.CanSet() && !src.IsZero() {
+			// overwrite dst with non-zero values for other types
+			dst.Set(src)
+		}
+	}
+
+	return nil
+}
+
+// hasExportedField returns true if the given type has any exported fields,
+// or if it has any anonymous/embedded struct fields with exported fields
+func hasExportedField(dstType reflect.Type) bool {
+	for i, n := 0, dstType.NumField(); i < n; i++ {
+		field := dstType.Field(i)
+		if field.Anonymous && field.Type.Kind() == reflect.Struct {
+			if hasExportedField(dstType.Field(i).Type) {
+				return true
+			}
+		} else if len(field.PkgPath) == 0 {
+			return true
+		}
+	}
+	return false
 }