Make Optimistic Tx database stackable

Summary:
This change models Optimistic Tx db after Pessimistic TX db. The motivation for this change is to make the ptr polymorphic so it can be held by the same raw or smart ptr.

Currently, due to the inheritance of the Opt Tx db not being rooted in the manner of Pess Tx from a single DB root it is more difficult to write clean code and have clear ownership of the database in cases when options dictate instantiate of plan DB, Pess Tx DB or Opt tx db.
Closes facebook#3566

Differential Revision: D7184502

Pulled By: yiwu-arbug

fbshipit-source-id: 31d06efafd79497bb0c230e971857dba3bd962c3

include/rocksdb/utilities/optimistic_transaction_db.h

@@ -11,6 +11,7 @@
 
 #include "rocksdb/comparator.h"
 #include "rocksdb/db.h"
+#include "rocksdb/utilities/stackable_db.h"
 
 namespace rocksdb {
 
@@ -30,7 +31,7 @@ struct OptimisticTransactionOptions {
   const Comparator* cmp = BytewiseComparator();
 };
 
-class OptimisticTransactionDB {
+class OptimisticTransactionDB : public StackableDB {
  public:
   // Open an OptimisticTransactionDB similar to DB::Open().
   static Status Open(const Options& options, const std::string& dbname,
@@ -57,18 +58,12 @@ class OptimisticTransactionDB {
           OptimisticTransactionOptions(),
       Transaction* old_txn = nullptr) = 0;
 
-  // Return the underlying Database that was opened
-  virtual DB* GetBaseDB() = 0;
+  OptimisticTransactionDB(const OptimisticTransactionDB&) = delete;
+  void operator=(const OptimisticTransactionDB&) = delete;
 
  protected:
   // To Create an OptimisticTransactionDB, call Open()
-  explicit OptimisticTransactionDB(DB* /*db*/) {}
-  OptimisticTransactionDB() {}
-
- private:
-  // No copying allowed
-  OptimisticTransactionDB(const OptimisticTransactionDB&);
-  void operator=(const OptimisticTransactionDB&);
+  explicit OptimisticTransactionDB(DB* db) : StackableDB(db) {}
 };
 
 }  // namespace rocksdb

utilities/transactions/optimistic_transaction_db_impl.h

@@ -15,23 +15,23 @@ namespace rocksdb {
 class OptimisticTransactionDBImpl : public OptimisticTransactionDB {
  public:
   explicit OptimisticTransactionDBImpl(DB* db, bool take_ownership = true)
-      : OptimisticTransactionDB(db), db_(db), db_owner_(take_ownership) {}
+      : OptimisticTransactionDB(db), db_owner_(take_ownership) {}
 
   ~OptimisticTransactionDBImpl() {
+    // Prevent this stackable from destroying
+    // base db
     if (!db_owner_) {
-      db_.release();
+      db_ = nullptr;
     }
   }
 
   Transaction* BeginTransaction(const WriteOptions& write_options,
                                 const OptimisticTransactionOptions& txn_options,
                                 Transaction* old_txn) override;
 
-  DB* GetBaseDB() override { return db_.get(); }
-
  private:
-  std::unique_ptr<DB> db_;
-  bool db_owner_;
+
+   bool db_owner_;
 
   void ReinitializeTransaction(Transaction* txn,
                                const WriteOptions& write_options,