Fix valgrind issues

Summary:
NewFixedPrefixTransform is leaked in default options. Broken by b47812fba6

Also included in the diff some code cleanup

Test Plan:
valgrind env_test
also make check

Reviewers: haobo, danguo, yhchiang

Reviewed By: danguo

CC: leveldb

Differential Revision: https://reviews.facebook.net/D17211

db/db_impl.cc

@@ -70,7 +70,6 @@ namespace rocksdb {
 int DBImpl::SuperVersion::dummy = 0;
 void* const DBImpl::SuperVersion::kSVInUse = &DBImpl::SuperVersion::dummy;
 void* const DBImpl::SuperVersion::kSVObsolete = nullptr;
-const std::string kNullString = "NULL";
 
 void DumpLeveldbBuildVersion(Logger * log);
 
@@ -2897,7 +2896,6 @@ Status DBImpl::DoCompactionWork(CompactionState* compact,
   assert(compact);
   compact->CleanupBatchBuffer();
   compact->CleanupMergedBuffer();
-  compact->cur_prefix_ = kNullString;
   bool prefix_initialized = false;
 
   int64_t imm_micros = 0;  // Micros spent doing imm_ compactions

include/rocksdb/compaction_filter.h

@@ -139,6 +139,7 @@ class DefaultCompactionFilterFactory : public CompactionFilterFactory {
 //
 class CompactionFilterFactoryV2 {
  public:
+  // NOTE: CompactionFilterFactoryV2 will not delete prefix_extractor
   explicit CompactionFilterFactoryV2(const SliceTransform* prefix_extractor)
     : prefix_extractor_(prefix_extractor) { }
 
@@ -169,9 +170,8 @@ class CompactionFilterFactoryV2 {
 // return any filter
 class DefaultCompactionFilterFactoryV2 : public CompactionFilterFactoryV2 {
  public:
-  explicit DefaultCompactionFilterFactoryV2(
-    const SliceTransform* prefix_extractor)
-      : CompactionFilterFactoryV2(prefix_extractor) { }
+  explicit DefaultCompactionFilterFactoryV2()
+      : CompactionFilterFactoryV2(nullptr) { }
 
   virtual std::unique_ptr<CompactionFilterV2>
   CreateCompactionFilterV2(

include/rocksdb/options.h

@@ -126,6 +126,8 @@ struct Options {
 
   // Version TWO of the compaction_filter_factory
   // It supports rolling compaction
+  //
+  // Default: a factory that doesn't provide any object
   std::shared_ptr<CompactionFilterFactoryV2> compaction_filter_factory_v2;
 
   // If true, the database will be created if it is missing.

util/options.cc

@@ -32,9 +32,7 @@ Options::Options()
       compaction_filter(nullptr),
       compaction_filter_factory(std::shared_ptr<CompactionFilterFactory>(
           new DefaultCompactionFilterFactory())),
-      compaction_filter_factory_v2(
-            new DefaultCompactionFilterFactoryV2(
-              NewFixedPrefixTransform(8))),
+      compaction_filter_factory_v2(new DefaultCompactionFilterFactoryV2()),
       create_if_missing(false),
       error_if_exists(false),
       paranoid_checks(false),