LDBCommand::InitFromCmdLineArgs() to move from template to function wrapper

Summary:
Build failure with some compiler setting with

tools/reduce_levels_test.cc:97: undefined reference to `rocksdb::LDBCommand* rocksdb::LDBCommand::InitFromCmdLineArgs<rocksdb::LDBCommand* (*)(std::string const&, std::vector<std::string, std::allocator<std::string> > const&, std::map<std::string, std::string, std::less<std::string>, std::allocator<std::pair<std::string const, std::string> > > const&, std::vector<std::string, std::allocator<std::string> > const&)>(std::vector<std::string, std::allocator<std::string> > const&, rocksdb::Options const&, rocksdb::LDBOptions const&, std::vector<rocksdb::ColumnFamilyDescriptor, std::allocator<rocksdb::ColumnFamilyDescriptor> > const*, rocksdb::LDBCommand* (*)(std::string const&, std::vector<std::string, std::allocator<std::string> > const&, std::map<std::string, std::string, std::less<std::string>, std::allocator<std::pair<std::string const, std::string> > > const&, std::vector<std::string, std::allocator<std::string> > const&))'

Fix it by changing to function pointer instead

Test Plan: Run all existing tests

Reviewers: andrewkr, kradhakrishnan, IslamAbdelRahman

Reviewed By: IslamAbdelRahman

Subscribers: adsharma, lightmark, yiwu, leveldb, andrewkr, dhruba

Differential Revision: https://reviews.facebook.net/D58905

include/rocksdb/utilities/ldb_cmd.h

@@ -53,12 +53,19 @@ class LDBCommand {
   static const std::string ARG_CREATE_IF_MISSING;
   static const std::string ARG_NO_VALUE;
 
-  template <typename Selector>
+  // parameters: cmd, cmdParams, option_map, flags
+  // The same interface as SelectCommand()
+  typedef std::function<LDBCommand*(const std::string&,
+                                    const std::vector<std::string>&,
+                                    const std::map<std::string, std::string>&,
+                                    const std::vector<std::string>&)>
+      RocksDBLDBSelectFunc;
+
   static LDBCommand* InitFromCmdLineArgs(
       const std::vector<std::string>& args, const Options& options,
       const LDBOptions& ldb_options,
       const std::vector<ColumnFamilyDescriptor>* column_families,
-      Selector selector = SelectCommand);
+      const RocksDBLDBSelectFunc& selector = SelectCommand);
 
   static LDBCommand* InitFromCmdLineArgs(
       int argc, char** argv, const Options& options,

tools/ldb_cmd.cc

@@ -98,12 +98,11 @@ 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 std::vector<std::string>& args, const Options& options,
     const LDBOptions& ldb_options,
     const std::vector<ColumnFamilyDescriptor>* column_families,
-    Selector selector) {
+    const RocksDBLDBSelectFunc& selector) {
   // --x=y command line arguments are added as x->y map entries.
   std::map<std::string, std::string> option_map;