[RocksDB][Performance Branch]Iterator Cleanup method only tries to find obsolete files if it has the last reference to a version

Summary: When deconstructing an iterator, no need to check obsolete file if it doesn't hold last reference of any version.

Test Plan: make all check

Reviewers: haobo, igor, dhruba, kailiu

Reviewed By: haobo

CC: leveldb

Differential Revision: https://reviews.facebook.net/D14595

db/db_impl.cc

@@ -2515,9 +2515,10 @@ static void CleanupIteratorState(void* arg1, void* arg2) {
       deletion_state.memtables_to_free.push_back(m);
     }
   }
-  state->version->Unref();
+  if (state->version->Unref()) {
     // fast path FindObsoleteFiles
     state->db->FindObsoleteFiles(deletion_state, false, true);
+  }
   state->mu->Unlock();
   state->db->PurgeObsoleteFiles(deletion_state);
   delete state;

db/version_set.cc

@@ -591,13 +591,15 @@ void Version::Ref() {
   ++refs_;
 }
 
-void Version::Unref() {
+bool Version::Unref() {
   assert(this != &vset_->dummy_versions_);
   assert(refs_ >= 1);
   --refs_;
   if (refs_ == 0) {
     delete this;
+    return true;
   }
+  return false;
 }
 
 bool Version::OverlapInLevel(int level,

db/version_set.h

@@ -89,7 +89,9 @@ class Version {
   // Reference count management (so Versions do not disappear out from
   // under live iterators)
   void Ref();
-  void Unref();
+  // Decrease reference count. Delete the object if no reference left
+  // and return true. Otherwise, return false.
+  bool Unref();
 
   void GetOverlappingInputs(
       int level,