StackableDB optionally take shared ownership of the underlying DB

Summary:
Allow StackableDB optionally takes a shared_ptr on construction and thus hold shared ownership of the underlying DB.
Closes https://github.com/facebook/rocksdb/pull/3423

Differential Revision: D6824163

Pulled By: yiwu-arbug

fbshipit-source-id: dbdc30c42e007533a987ef413785e192340f03eb

include/rocksdb/utilities/stackable_db.h

@@ -4,6 +4,7 @@
 
 #pragma once
 #include <map>
+#include <memory>
 #include <string>
 #include "rocksdb/db.h"
 
@@ -18,11 +19,20 @@ namespace rocksdb {
 // This class contains APIs to stack rocksdb wrappers.Eg. Stack TTL over base d
 class StackableDB : public DB {
  public:
-  // StackableDB is the owner of db now!
+  // StackableDB take sole ownership of the underlying db.
   explicit StackableDB(DB* db) : db_(db) {}
 
+  // StackableDB take shared ownership of the underlying db.
+  explicit StackableDB(std::shared_ptr<DB> db)
+      : db_(db.get()), shared_db_ptr_(db) {}
+
   ~StackableDB() {
-    delete db_;
+    if (shared_db_ptr_ == nullptr) {
+      delete db_;
+    } else {
+      assert(shared_db_ptr_.get() == db_);
+    }
+    db_ = nullptr;
   }
 
   virtual Status Close() override { return db_->Close(); }
@@ -375,6 +385,7 @@ class StackableDB : public DB {
 
  protected:
   DB* db_;
+  std::shared_ptr<DB> shared_db_ptr_;
 };
 
 } //  namespace rocksdb