feat(fieldbehavior): ignore immutable in validation if not in field mask

AIP-203
"When a service receives an immutable field in an update request (or similar),
even if included in the update mask, the service should ignore the field if the value matches,
but should error with INVALID_ARGUMENT if a change is requested"

fieldbehavior/fieldbehavior_test.go

@@ -141,6 +141,20 @@ func TestValidateImmutableFieldsWithMask(t *testing.T) {
 		err := ValidateImmutableFieldsWithMask(req, req.GetUpdateMask())
 		assert.NilError(t, err)
 	})
+	t.Run("no error when immutable field not part of fieldmask", func(t *testing.T) {
+		t.Parallel()
+		req := &examplefreightv1.UpdateShipmentRequest{
+			Shipment: &examplefreightv1.Shipment{
+				ExternalReferenceId: "external-reference-id",
+				OriginSite:          "shippers/shipper1/sites/site1",
+			},
+			UpdateMask: &fieldmaskpb.FieldMask{
+				Paths: []string{"shipment.origin_site"},
+			},
+		}
+		err := ValidateImmutableFieldsWithMask(req, req.GetUpdateMask())
+		assert.NilError(t, err)
+	})
 	t.Run("errors when wildcard fieldmask used", func(t *testing.T) {
 		t.Parallel()
 		req := &examplefreightv1.UpdateShipmentRequest{
@@ -170,7 +184,7 @@ func TestValidateImmutableFieldsWithMask(t *testing.T) {
 				ExternalReferenceId: "I am immutable!",
 			},
 			UpdateMask: &fieldmaskpb.FieldMask{
-				Paths: []string{""},
+				Paths: []string{},
 			},
 		}
 		err := ValidateImmutableFieldsWithMask(req, req.GetUpdateMask())
@@ -187,7 +201,7 @@ func TestValidateImmutableFieldsWithMask(t *testing.T) {
 				},
 			},
 			UpdateMask: &fieldmaskpb.FieldMask{
-				Paths: []string{""},
+				Paths: []string{},
 			},
 		}
 		err := ValidateImmutableFieldsWithMask(req, req.GetUpdateMask())

fieldbehavior/immutable.go

@@ -10,7 +10,7 @@ import (
 )
 
 // ValidateImmutableFieldsWithMask returns a validation error if the message
-// or field mask contains a field that is immutable.
+// or field mask contains a field that is immutable and a change to an immutable field is requested.
 // This can be used when validating update requests and want to return
 // INVALID_ARGUMENT to the user.
 // If you want to ignore immutable fields rather than error then use ClearFields().
@@ -29,7 +29,7 @@ func validateImmutableFields(m protoreflect.Message, mask *fieldmaskpb.FieldMask
 		}
 
 		currPath += string(field.Name())
-		if isImmutable(field) && (hasPath(mask, currPath) || m.Has(field)) {
+		if isImmutable(field) && hasPath(mask, currPath) {
 			return fmt.Errorf("field is immutable: %s", currPath)
 		}
 

fieldbehavior/required.go

@@ -76,7 +76,14 @@ func validateRequiredFields(reflectMessage protoreflect.Message, mask *fieldmask
 	return nil
 }
 
+func isEmpty(mask *fieldmaskpb.FieldMask) bool {
+	return mask == nil || len(mask.GetPaths()) == 0
+}
+
 func hasPath(mask *fieldmaskpb.FieldMask, needle string) bool {
+	if isEmpty(mask) {
+		return true
+	}
 	for _, straw := range mask.GetPaths() {
 		if straw == "*" || straw == needle {
 			return true