Fix fieldType being dropped by older go-clients

staging/src/k8s.io/apimachinery/pkg/apis/meta/v1/validation/validation.go

@@ -178,7 +178,7 @@ func ValidateManagedFields(fieldsList []metav1.ManagedFieldsEntry, fldPath *fiel
 		default:
 			allErrs = append(allErrs, field.Invalid(fldPath.Child("operation"), fields.Operation, "must be `Apply` or `Update`"))
 		}
-		if fields.FieldsType != "FieldsV1" {
+		if len(fields.FieldsType) > 0 && fields.FieldsType != "FieldsV1" {
 			allErrs = append(allErrs, field.Invalid(fldPath.Child("fieldsType"), fields.FieldsType, "must be `FieldsV1`"))
 		}
 	}

staging/src/k8s.io/apimachinery/pkg/apis/meta/v1/validation/validation_test.go

@@ -242,12 +242,8 @@ func TestValidateFieldManagerInvalid(t *testing.T) {
 	}
 }
 
-func TestValidateMangedFieldsInvalid(t *testing.T) {
+func TestValidateManagedFieldsInvalid(t *testing.T) {
 	tests := []metav1.ManagedFieldsEntry{
-		{
-			Operation: metav1.ManagedFieldsOperationUpdate,
-			// FieldsType is missing
-		},
 		{
 			Operation:  metav1.ManagedFieldsOperationUpdate,
 			FieldsType: "RandomVersion",
@@ -274,6 +270,10 @@ func TestValidateMangedFieldsInvalid(t *testing.T) {
 
 func TestValidateMangedFieldsValid(t *testing.T) {
 	tests := []metav1.ManagedFieldsEntry{
+		{
+			Operation: metav1.ManagedFieldsOperationUpdate,
+			// FieldsType is missing
+		},
 		{
 			Operation:  metav1.ManagedFieldsOperationUpdate,
 			FieldsType: "FieldsV1",

staging/src/k8s.io/apiserver/pkg/endpoints/handlers/fieldmanager/internal/managedfields.go

@@ -107,7 +107,16 @@ func EncodeObjectManagedFields(obj runtime.Object, managed ManagedInterface) err
 func decodeManagedFields(encodedManagedFields []metav1.ManagedFieldsEntry) (managed managedStruct, err error) {
 	managed.fields = make(fieldpath.ManagedFields, len(encodedManagedFields))
 	managed.times = make(map[string]*metav1.Time, len(encodedManagedFields))
-	for _, encodedVersionedSet := range encodedManagedFields {
+
+	for i, encodedVersionedSet := range encodedManagedFields {
+		switch encodedVersionedSet.FieldsType {
+		case "FieldsV1":
+			// Valid case.
+		case "":
+			return managedStruct{}, fmt.Errorf("missing fieldsType in managed fields entry %d", i)
+		default:
+			return managedStruct{}, fmt.Errorf("invalid fieldsType %q in managed fields entry %d", encodedVersionedSet.FieldsType, i)
+		}
 		manager, err := BuildManagerIdentifier(&encodedVersionedSet)
 		if err != nil {
 			return managedStruct{}, fmt.Errorf("error decoding manager from %v: %v", encodedVersionedSet, err)

staging/src/k8s.io/apiserver/pkg/endpoints/handlers/fieldmanager/internal/managedfields_test.go

@@ -27,6 +27,51 @@ import (
 	"sigs.k8s.io/yaml"
 )
 
+// TestHasFieldsType makes sure that we fail if we don't have a
+// FieldsType set properly.
+func TestHasFieldsType(t *testing.T) {
+	var unmarshaled []metav1.ManagedFieldsEntry
+	if err := yaml.Unmarshal([]byte(`- apiVersion: v1
+  fieldsType: FieldsV1
+  fieldsV1:
+    f:field: {}
+  manager: foo
+  operation: Apply
+`), &unmarshaled); err != nil {
+		t.Fatalf("did not expect yaml unmarshalling error but got: %v", err)
+	}
+	if _, err := decodeManagedFields(unmarshaled); err != nil {
+		t.Fatalf("did not expect decoding error but got: %v", err)
+	}
+
+	// Invalid fieldsType V2.
+	if err := yaml.Unmarshal([]byte(`- apiVersion: v1
+  fieldsType: FieldsV2
+  fieldsV1:
+    f:field: {}
+  manager: foo
+  operation: Apply
+`), &unmarshaled); err != nil {
+		t.Fatalf("did not expect yaml unmarshalling error but got: %v", err)
+	}
+	if _, err := decodeManagedFields(unmarshaled); err == nil {
+		t.Fatal("Expect decoding error but got none")
+	}
+
+	// Missing fieldsType.
+	if err := yaml.Unmarshal([]byte(`- apiVersion: v1
+  fieldsV1:
+    f:field: {}
+  manager: foo
+  operation: Apply
+`), &unmarshaled); err != nil {
+		t.Fatalf("did not expect yaml unmarshalling error but got: %v", err)
+	}
+	if _, err := decodeManagedFields(unmarshaled); err == nil {
+		t.Fatal("Expect decoding error but got none")
+	}
+}
+
 // TestRoundTripManagedFields will roundtrip ManagedFields from the wire format
 // (api format) to the format used by sigs.k8s.io/structured-merge-diff and back
 func TestRoundTripManagedFields(t *testing.T) {