Adding prefix_extractor string config parameter

Summary:
This diff enables to configure prefix_extractor
string parameter as a CF option.

Test Plan: make all check, ./options_test

Reviewers: sdong, igor

Reviewed By: igor

Subscribers: dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D31653

util/options_helper.cc

@@ -9,6 +9,7 @@
 #include "rocksdb/cache.h"
 #include "rocksdb/filter_policy.h"
 #include "rocksdb/options.h"
+#include "rocksdb/slice_transform.h"
 #include "rocksdb/table.h"
 #include "rocksdb/utilities/convenience.h"
 #include "util/options_helper.h"
@@ -508,6 +509,15 @@ Status GetColumnFamilyOptionsFromMap(
         new_options->min_partial_merge_operands = ParseUint32(o.second);
       } else if (o.first == "inplace_update_support") {
         new_options->inplace_update_support = ParseBoolean(o.first, o.second);
+      } else if (o.first == "prefix_extractor") {
+        const std::string kName = "fixed:";
+        if (o.second.compare(0, kName.size(), kName) != 0) {
+          return Status::InvalidArgument("Invalid Prefix Extractor type: "
+                                         + o.second);
+        }
+        int prefix_length = ParseInt(trim(o.second.substr(kName.size())));
+        new_options->prefix_extractor.reset(
+            NewFixedPrefixTransform(prefix_length));
       } else {
         return Status::InvalidArgument("Unrecognized option: " + o.first);
       }

util/options_test.cc

@@ -129,7 +129,8 @@ TEST(OptionsTest, GetOptionsFromMapTest) {
     {"memtable_prefix_bloom_huge_page_tlb_size", "28"},
     {"bloom_locality", "29"},
     {"max_successive_merges", "30"},
-    {"min_partial_merge_operands", "31"}
+    {"min_partial_merge_operands", "31"},
+    {"prefix_extractor", "fixed:31"}
   };
 
   std::unordered_map<std::string, std::string> db_options_map = {
@@ -220,6 +221,9 @@ TEST(OptionsTest, GetOptionsFromMapTest) {
   ASSERT_EQ(new_cf_opt.bloom_locality, 29U);
   ASSERT_EQ(new_cf_opt.max_successive_merges, 30U);
   ASSERT_EQ(new_cf_opt.min_partial_merge_operands, 31U);
+  ASSERT_TRUE(new_cf_opt.prefix_extractor != nullptr);
+  ASSERT_EQ(std::string(new_cf_opt.prefix_extractor->Name()),
+            "rocksdb.FixedPrefix.31");
 
   cf_options_map["write_buffer_size"] = "hello";
   ASSERT_NOK(GetColumnFamilyOptionsFromMap(