[ldb] Templatize the Selector

Summary:
So a customized ldb tool can pass it's own Selector.
Such a selector is expected to call LDBCommand::SelectCommand
and then add some of its own customized commands

Test Plan: make ldb

Reviewers: sdong, IslamAbdelRahman

Reviewed By: IslamAbdelRahman

Subscribers: andrewkr, dhruba

Differential Revision: https://reviews.facebook.net/D57249

include/rocksdb/utilities/ldb_cmd.h

@@ -53,10 +53,12 @@ class LDBCommand {
   static const std::string ARG_CREATE_IF_MISSING;
   static const std::string ARG_NO_VALUE;
 
+  template <typename Selector>
   static LDBCommand* InitFromCmdLineArgs(
       const std::vector<std::string>& args, const Options& options,
       const LDBOptions& ldb_options,
-      const std::vector<ColumnFamilyDescriptor>* column_families);
+      const std::vector<ColumnFamilyDescriptor>* column_families,
+      Selector selector = SelectCommand);
 
   static LDBCommand* InitFromCmdLineArgs(
       int argc, char** argv, const Options& options,
@@ -105,6 +107,11 @@ class LDBCommand {
 
   static const char* DELIM;
 
+  static LDBCommand* SelectCommand(
+      const std::string& cmd, const std::vector<std::string>& cmdParams,
+      const std::map<std::string, std::string>& option_map,
+      const std::vector<std::string>& flags);
+
  protected:
   LDBCommandExecuteResult exec_state_;
   std::string db_path_;
@@ -223,11 +230,6 @@ class LDBCommand {
    * Otherwise an exception is thrown.
    */
   bool StringToBool(std::string val);
-
-  static LDBCommand* SelectCommand(
-      const std::string& cmd, const std::vector<std::string>& cmdParams,
-      const std::map<std::string, std::string>& option_map,
-      const std::vector<std::string>& flags);
 };
 
 class LDBCommandRunner {

tools/ldb_cmd.cc

@@ -86,7 +86,8 @@ LDBCommand* LDBCommand::InitFromCmdLineArgs(
   for (int i = 1; i < argc; i++) {
     args.push_back(argv[i]);
   }
-  return InitFromCmdLineArgs(args, options, ldb_options, column_families);
+  return InitFromCmdLineArgs(args, options, ldb_options, column_families,
+                             SelectCommand);
 }
 
 /**
@@ -99,10 +100,12 @@ LDBCommand* LDBCommand::InitFromCmdLineArgs(
  * Command name is not included in args.
  * Returns nullptr if the command-line cannot be parsed.
  */
+template <typename Selector>
 LDBCommand* LDBCommand::InitFromCmdLineArgs(
     const vector<string>& args, const Options& options,
     const LDBOptions& ldb_options,
-    const std::vector<ColumnFamilyDescriptor>* column_families) {
+    const std::vector<ColumnFamilyDescriptor>* column_families,
+    Selector selector) {
   // --x=y command line arguments are added as x->y map entries.
   map<string, string> option_map;
 
@@ -137,12 +140,7 @@ LDBCommand* LDBCommand::InitFromCmdLineArgs(
 
   string cmd = cmdTokens[0];
   vector<string> cmdParams(cmdTokens.begin()+1, cmdTokens.end());
-  LDBCommand* command = LDBCommand::SelectCommand(
-    cmd,
-    cmdParams,
-    option_map,
-    flags
-  );
+  LDBCommand* command = selector(cmd, cmdParams, option_map, flags);
 
   if (command) {
     command->SetDBOptions(options);

tools/reduce_levels_test.cc

@@ -93,8 +93,8 @@ Status ReduceLevelTest::OpenDB(bool create_if_missing, int num_levels) {
 bool ReduceLevelTest::ReduceLevels(int target_level) {
   std::vector<std::string> args = rocksdb::ReduceDBLevelsCommand::PrepareArgs(
       dbname_, target_level, false);
-  LDBCommand* level_reducer =
-      LDBCommand::InitFromCmdLineArgs(args, Options(), LDBOptions(), nullptr);
+  LDBCommand* level_reducer = LDBCommand::InitFromCmdLineArgs(
+      args, Options(), LDBOptions(), nullptr, LDBCommand::SelectCommand);
   level_reducer->Run();
   bool is_succeed = level_reducer->GetExecuteState().IsSucceed();
   delete level_reducer;