Validate optional fields correctly

fieldbehavior/fieldbehavior.go

@@ -62,13 +62,16 @@ func CopyFields(dst, src proto.Message, behaviorsToCopy ...annotations.FieldBeha
 }
 
 func isMessageFieldPresent(m protoreflect.Message, f protoreflect.FieldDescriptor) bool {
-	return isPresent(m.Get(f), f)
+	return isPresent(m.Get(f), f, m.Has(f))
 }
 
-func isPresent(v protoreflect.Value, f protoreflect.FieldDescriptor) bool {
+func isPresent(v protoreflect.Value, f protoreflect.FieldDescriptor, populated bool) bool {
 	if !v.IsValid() {
 		return false
 	}
+	if f.HasOptionalKeyword() && populated {
+		return true
+	}
 	if f.IsList() {
 		return v.List().Len() > 0
 	}

fieldbehavior/fieldbehavior_test.go

@@ -100,6 +100,17 @@ func TestValidateRequiredFieldsWithMask(t *testing.T) {
 			"missing required field: shipment.origin_site",
 		)
 	})
+	t.Run("missing optional field annotated with required", func(t *testing.T) {
+		t.Parallel()
+		assert.Error(
+			t,
+			ValidateRequiredFieldsWithMask(
+				&examplefreightv1.Site{},
+				&fieldmaskpb.FieldMask{Paths: []string{"personnel_count"}},
+			),
+			"missing required field: personnel_count",
+		)
+	})
 	t.Run("missing nested not in mask", func(t *testing.T) {
 		t.Parallel()
 		assert.NilError(

proto/einride/example/freight/v1/site.proto

@@ -30,4 +30,6 @@ message Site {
   string display_name = 5 [(google.api.field_behavior) = REQUIRED];
   // The geographic location of the site.
   google.type.LatLng lat_lng = 6;
+  // Personnel count on site.
+  optional int64 personnel_count = 7 [(google.api.field_behavior) = REQUIRED];
 }