Fixing ref-counting memtables

Author: igorcanadi

db/memtablelist.cc

@@ -76,17 +76,16 @@ void MemTableListVersion::AddIterators(const ReadOptions& options,
   }
 }
 
+// caller is responsible for referencing m
 void MemTableListVersion::Add(MemTable* m) {
   assert(refs_ == 1);  // only when refs_ == 1 is MemTableListVersion mutable
-  m->Ref();
   memlist_.push_front(m);
   ++size_;
 }
 
+// caller is responsible for unreferencing m
 void MemTableListVersion::Remove(MemTable* m) {
   assert(refs_ == 1);  // only when refs_ == 1 is MemTableListVersion mutable
-  MemTable* x __attribute__((unused)) = m->Unref();
-  assert(x == nullptr);  // it still needs to be alive!
   memlist_.remove(m);
   --size_;
 }
@@ -232,6 +231,11 @@ Status MemTableList::InstallMemtableFlushResults(
 void MemTableList::Add(MemTable* m) {
   assert(current_->size_ >= num_flush_not_started_);
   InstallNewVersion();
+  // this method is used to move mutable memtable into an immutable list.
+  // since mutable memtable is already refcounted by the DBImpl,
+  // and when moving to the imutable list we don't unref it,
+  // we don't have to ref the memtable here. we just take over the
+  // reference from the DBImpl.
   current_->Add(m);
   m->MarkImmutable();
   num_flush_not_started_++;

db/memtablelist.h

@@ -40,12 +40,12 @@ class MemTableListVersion {
   void AddIterators(const ReadOptions& options,
                     std::vector<Iterator*>* iterator_list);
 
+ private:
   // REQUIRE: m is mutable memtable
   void Add(MemTable* m);
   // REQUIRE: m is mutable memtable
   void Remove(MemTable* m);
 
- private:
   friend class MemTableList;
   std::list<MemTable*> memlist_;
   int size_ = 0;