WIP feat: support new comparator because of design

add a pegasus comparator to support a normal users' sort.

Using new pegasus comparator will solve many scan timeout problems.
Pegasus can be compatible apps(tables) using the old comparator.
pegasus comparator will be the new table's default comparator.

more details at: apache#729

rdsn/src/dsn.layer2.thrift

@@ -70,6 +70,12 @@ enum app_status
     AS_RECALLING
 }
 
+enum comparator_type
+{
+    DEFAULT,
+    PEGASUS
+}
+
 struct app_info
 {
     1:app_status    status = app_status.AS_INVALID;
@@ -100,4 +106,11 @@ struct app_info
     // New fields for bulk load
     // Whether this app is executing bulk load
     14:optional bool    is_bulk_loading = false;
+
+    // New fields for rocksdb comparator, Should not change it after app(table) created.
+    // "comparator_type.DEFAULT": BytesComparator,
+    // "comparator_type.PEGASUS": a new comparator defined by pegasus.
+    // new table will use "comparator_type.PEGASUS",
+    // if no comparator set, use "comparator_type.DEFAULT" for compatible.
+    15:optional comparator_type comparator = comparator_type.DEFAULT;
 }

rdsn/src/meta/server_state.cpp

@@ -1161,6 +1161,7 @@ void server_state::create_app(dsn::message_ex *msg)
             info.status = app_status::AS_CREATING;
             info.create_second = dsn_now_ms() / 1000;
             info.init_partition_count = request.options.partition_count;
+            info.comparator = dsn::comparator_type::PEGASUS;
 
             app = app_state::create(info);
             app->helpers->pending_response = msg;