fix(pubsub): allow clearing of topic schema

pubsub/pstest/fake.go

@@ -362,8 +362,7 @@ func (s *GServer) UpdateTopic(_ context.Context, req *pb.UpdateTopicRequest) (*p
 			}
 			t.proto.MessageRetentionDuration = req.Topic.MessageRetentionDuration
 		case "schema_settings":
-			// Clear this field.
-			t.proto.SchemaSettings = &pb.SchemaSettings{}
+			t.proto.SchemaSettings = req.Topic.SchemaSettings
 		case "schema_settings.schema":
 			if t.proto.SchemaSettings == nil {
 				t.proto.SchemaSettings = &pb.SchemaSettings{}

pubsub/topic.go

@@ -407,6 +407,11 @@ func (t *Topic) updateRequest(cfg TopicConfigToUpdate) *pb.UpdateTopicRequest {
 		}
 		paths = append(paths, "message_retention_duration")
 	}
+	// Updating SchemaSettings' field masks are more complicated here
+	// since each field should be able to be independently edited, while
+	// preserving the current values for everything else. We also denote
+	// the zero value SchemaSetting to mean clearing or removing schema
+	// from the topic.
 	if cfg.SchemaSettings != nil {
 		pt.SchemaSettings = schemaSettingsToProto(cfg.SchemaSettings)
 		clearSchema := true
@@ -426,9 +431,10 @@ func (t *Topic) updateRequest(cfg TopicConfigToUpdate) *pb.UpdateTopicRequest {
 			paths = append(paths, "schema_settings.last_revision_id")
 			clearSchema = false
 		}
-		// Clear the schema if none of it's value changes.
+		// Clear the schema if all of its values are equal to the zero value.
 		if clearSchema {
 			paths = append(paths, "schema_settings")
+			pt.SchemaSettings = nil
 		}
 	}
 	return &pb.UpdateTopicRequest{

pubsub/topic_test.go

@@ -342,8 +342,8 @@ func TestUpdateTopic_SchemaSettings(t *testing.T) {
 	if err != nil {
 		t.Fatal(err)
 	}
-	if !testutil.Equal(config3.SchemaSettings, settings, opt) {
-		t.Errorf("\ngot  %+v\nwant %+v", config3.SchemaSettings, settings)
+	if config3.SchemaSettings != nil {
+		t.Errorf("got: %+v, want nil", config3.SchemaSettings)
 	}
 }