BlockchainLMDB: Add support for batch transactions

src/cryptonote_core/BlockchainDB_impl/db_lmdb.cpp

@@ -600,15 +600,23 @@ void BlockchainLMDB::check_open() const
 BlockchainLMDB::~BlockchainLMDB()
 {
   LOG_PRINT_L3("BlockchainLMDB::" << __func__);
+
+  // batch transaction shouldn't be active at this point. If it is, consider it aborted.
+  if (m_batch_active)
+    batch_abort();
 }
 
-BlockchainLMDB::BlockchainLMDB()
+BlockchainLMDB::BlockchainLMDB(bool batch_transactions)
 {
   LOG_PRINT_L3("BlockchainLMDB::" << __func__);
   // initialize folder to something "safe" just in case
   // someone accidentally misuses this class...
   m_folder = "thishsouldnotexistbecauseitisgibberish";
   m_open = false;
+
+  m_batch_transactions = batch_transactions;
+  m_write_txn = nullptr;
+  m_batch_active = false;
   m_height = 0;
 }
 
@@ -720,6 +728,13 @@ void BlockchainLMDB::create(const std::string& filename)
 void BlockchainLMDB::close()
 {
   LOG_PRINT_L3("BlockchainLMDB::" << __func__);
+  if (m_batch_active)
+  {
+    LOG_PRINT_L3("close() first calling batch_abort() due to active batch transaction");
+    batch_abort();
+  }
+  this->sync();
+
   // FIXME: not yet thread safe!!!  Use with care.
   mdb_env_close(m_env);
 }
@@ -727,7 +742,13 @@ void BlockchainLMDB::close()
 void BlockchainLMDB::sync()
 {
   LOG_PRINT_L3("BlockchainLMDB::" << __func__);
-  // LMDB documentation leads me to believe this is unnecessary
+
+  // Does nothing unless LMDB environment was opened with MDB_NOSYNC or in part
+  // MDB_NOMETASYNC. Force flush to be synchronous.
+  if (auto result = mdb_env_sync(m_env, true))
+  {
+    throw0(DB_ERROR(std::string("Failed to sync database").append(mdb_strerror(result)).c_str()));
+  }
 }
 
 void BlockchainLMDB::reset()
@@ -867,12 +888,17 @@ uint64_t BlockchainLMDB::get_block_timestamp(const uint64_t& height) const
   check_open();
 
   txn_safe txn;
-  if (mdb_txn_begin(m_env, NULL, MDB_RDONLY, txn))
+  txn_safe* txn_ptr = &txn;
-    throw0(DB_ERROR("Failed to create a transaction for the db"));
+  if (m_batch_active)
-
+    txn_ptr = m_write_txn;
+  else
+  {
+    if (mdb_txn_begin(m_env, NULL, MDB_RDONLY, txn))
+      throw0(DB_ERROR("Failed to create a transaction for the db"));
+  }
   MDB_val_copy<uint64_t> key(height);
   MDB_val result;
-  auto get_result = mdb_get(txn, m_block_timestamps, &key, &result);
+  auto get_result = mdb_get(*txn_ptr, m_block_timestamps, &key, &result);
   if (get_result == MDB_NOTFOUND)
   {
     throw0(DB_ERROR(std::string("Attempt to get timestamp from height ").append(boost::lexical_cast<std::string>(height)).append(" failed -- timestamp not in db").c_str()));
@@ -880,7 +906,8 @@ uint64_t BlockchainLMDB::get_block_timestamp(const uint64_t& height) const
   else if (get_result)
     throw0(DB_ERROR("Error attempting to retrieve a timestamp from the db"));
 
-  txn.commit();
+  if (! m_batch_active)
+    txn.commit();
   return *(const uint64_t*)result.mv_data;
 }
 
@@ -904,12 +931,18 @@ size_t BlockchainLMDB::get_block_size(const uint64_t& height) const
   check_open();
 
   txn_safe txn;
-  if (mdb_txn_begin(m_env, NULL, MDB_RDONLY, txn))
+  txn_safe* txn_ptr = &txn;
-    throw0(DB_ERROR("Failed to create a transaction for the db"));
+  if (m_batch_active)
+    txn_ptr = m_write_txn;
+  else
+  {
+    if (mdb_txn_begin(m_env, NULL, MDB_RDONLY, txn))
+      throw0(DB_ERROR("Failed to create a transaction for the db"));
+  }
 
   MDB_val_copy<uint64_t> key(height);
   MDB_val result;
-  auto get_result = mdb_get(txn, m_block_sizes, &key, &result);
+  auto get_result = mdb_get(*txn_ptr, m_block_sizes, &key, &result);
   if (get_result == MDB_NOTFOUND)
   {
     throw0(DB_ERROR(std::string("Attempt to get block size from height ").append(boost::lexical_cast<std::string>(height)).append(" failed -- block size not in db").c_str()));
@@ -917,7 +950,8 @@ size_t BlockchainLMDB::get_block_size(const uint64_t& height) const
   else if (get_result)
     throw0(DB_ERROR("Error attempting to retrieve a block size from the db"));
 
-  txn.commit();
+  if (! m_batch_active)
+    txn.commit();
   return *(const size_t*)result.mv_data;
 }
 
@@ -927,12 +961,17 @@ difficulty_type BlockchainLMDB::get_block_cumulative_difficulty(const uint64_t&
   check_open();
 
   txn_safe txn;
-  if (mdb_txn_begin(m_env, NULL, MDB_RDONLY, txn))
+  txn_safe* txn_ptr = &txn;
-    throw0(DB_ERROR("Failed to create a transaction for the db"));
+  if (m_batch_active)
-
+    txn_ptr = m_write_txn;
+  else
+  {
+    if (mdb_txn_begin(m_env, NULL, MDB_RDONLY, txn))
+      throw0(DB_ERROR("Failed to create a transaction for the db"));
+  }
   MDB_val_copy<uint64_t> key(height);
   MDB_val result;
-  auto get_result = mdb_get(txn, m_block_diffs, &key, &result);
+  auto get_result = mdb_get(*txn_ptr, m_block_diffs, &key, &result);
   if (get_result == MDB_NOTFOUND)
   {
     throw0(DB_ERROR(std::string("Attempt to get cumulative difficulty from height ").append(boost::lexical_cast<std::string>(height)).append(" failed -- difficulty not in db").c_str()));
@@ -940,7 +979,8 @@ difficulty_type BlockchainLMDB::get_block_cumulative_difficulty(const uint64_t&
   else if (get_result)
     throw0(DB_ERROR("Error attempting to retrieve a cumulative difficulty from the db"));
 
-  txn.commit();
+  if (! m_batch_active)
+    txn.commit();
   return *(difficulty_type*)result.mv_data;
 }
 
@@ -967,12 +1007,18 @@ uint64_t BlockchainLMDB::get_block_already_generated_coins(const uint64_t& heigh
   check_open();
 
   txn_safe txn;
-  if (mdb_txn_begin(m_env, NULL, MDB_RDONLY, txn))
+  txn_safe* txn_ptr = &txn;
-    throw0(DB_ERROR("Failed to create a transaction for the db"));
+  if (m_batch_active)
+    txn_ptr = m_write_txn;
+  else
+  {
+    if (mdb_txn_begin(m_env, NULL, MDB_RDONLY, txn))
+      throw0(DB_ERROR("Failed to create a transaction for the db"));
+  }
 
   MDB_val_copy<uint64_t> key(height);
   MDB_val result;
-  auto get_result = mdb_get(txn, m_block_coins, &key, &result);
+  auto get_result = mdb_get(*txn_ptr, m_block_coins, &key, &result);
   if (get_result == MDB_NOTFOUND)
   {
     throw0(DB_ERROR(std::string("Attempt to get generated coins from height ").append(boost::lexical_cast<std::string>(height)).append(" failed -- block size not in db").c_str()));
@@ -980,7 +1026,8 @@ uint64_t BlockchainLMDB::get_block_already_generated_coins(const uint64_t& heigh
   else if (get_result)
     throw0(DB_ERROR("Error attempting to retrieve a total generated coins from the db"));
 
-  txn.commit();
+  if (! m_batch_active)
+    txn.commit();
   return *(const uint64_t*)result.mv_data;
 }
 
@@ -990,20 +1037,28 @@ crypto::hash BlockchainLMDB::get_block_hash_from_height(const uint64_t& height)
   check_open();
 
   txn_safe txn;
-  if (mdb_txn_begin(m_env, NULL, MDB_RDONLY, txn))
+  txn_safe* txn_ptr = &txn;
-    throw0(DB_ERROR("Failed to create a transaction for the db"));
+  if (m_batch_active)
+    txn_ptr = m_write_txn;
+  else
+  {
+    if (mdb_txn_begin(m_env, NULL, MDB_RDONLY, txn))
+      throw0(DB_ERROR("Failed to create a transaction for the db"));
+  }
 
   MDB_val_copy<uint64_t> key(height);
   MDB_val result;
-  auto get_result = mdb_get(txn, m_block_hashes, &key, &result);
+  auto get_result = mdb_get(*txn_ptr, m_block_hashes, &key, &result);
   if (get_result == MDB_NOTFOUND)
   {
     throw0(BLOCK_DNE(std::string("Attempt to get hash from height ").append(boost::lexical_cast<std::string>(height)).append(" failed -- hash not in db").c_str()));
   }
   else if (get_result)
-    throw0(DB_ERROR("Error attempting to retrieve a block hash from the db"));
+    throw0(DB_ERROR(std::string("Error attempting to retrieve a block hash from the db: ").
+          append(mdb_strerror(get_result)).c_str()));
 
-  txn.commit();
+  if (! m_batch_active)
+    txn.commit();
   return *(crypto::hash*)result.mv_data;
 }
 
@@ -1075,6 +1130,10 @@ bool BlockchainLMDB::tx_exists(const crypto::hash& h) const
   LOG_PRINT_L3("BlockchainLMDB::" << __func__);
   check_open();
 
+  if (m_batch_active)
+  {
+    LOG_PRINT_L0("WARNING: active batch transaction while creating a read-only txn in tx_exists()");
+  }
   txn_safe txn;
   if (mdb_txn_begin(m_env, NULL, MDB_RDONLY, txn))
     throw0(DB_ERROR("Failed to create a transaction for the db"));
@@ -1120,12 +1179,18 @@ transaction BlockchainLMDB::get_tx(const crypto::hash& h) const
   check_open();
 
   txn_safe txn;
-  if (mdb_txn_begin(m_env, NULL, MDB_RDONLY, txn))
+  txn_safe* txn_ptr = &txn;
-    throw0(DB_ERROR("Failed to create a transaction for the db"));
+  if (m_batch_active)
+    txn_ptr = m_write_txn;
+  else
+  {
+    if (mdb_txn_begin(m_env, NULL, MDB_RDONLY, txn))
+      throw0(DB_ERROR("Failed to create a transaction for the db"));
+  }
 
   MDB_val_copy<crypto::hash> key(h);
   MDB_val result;
-  auto get_result = mdb_get(txn, m_txs, &key, &result);
+  auto get_result = mdb_get(*txn_ptr, m_txs, &key, &result);
   if (get_result == MDB_NOTFOUND)
     throw1(TX_DNE(std::string("tx with hash ").append(epee::string_tools::pod_to_hex(h)).append(" not found in db").c_str()));
   else if (get_result)
@@ -1137,6 +1202,8 @@ transaction BlockchainLMDB::get_tx(const crypto::hash& h) const
   transaction tx;
   if (!parse_and_validate_tx_from_blob(bd, tx))
     throw0(DB_ERROR("Failed to parse tx from blob retrieved from the db"));
+  if (! m_batch_active)
+    txn.commit();
 
   return tx;
 }
@@ -1178,13 +1245,27 @@ uint64_t BlockchainLMDB::get_tx_block_height(const crypto::hash& h) const
   LOG_PRINT_L3("BlockchainLMDB::" << __func__);
   check_open();
 
+  // If m_batch_active is set, a batch transaction exists beyond this class,
+  // such as a batch import with verification enabled, or possibly (later) a
+  // batch network sync.
+  //
+  // A regular network sync without batching would be expected to open a new
+  // read transaction here, as validation is done prior to the write for block
+  // and tx data.
+
   txn_safe txn;
-  if (mdb_txn_begin(m_env, NULL, MDB_RDONLY, txn))
+  txn_safe* txn_ptr = &txn;
-    throw0(DB_ERROR("Failed to create a transaction for the db"));
+  if (m_batch_active)
+    txn_ptr = m_write_txn;
+  else
+  {
+    if (mdb_txn_begin(m_env, NULL, MDB_RDONLY, txn))
+      throw0(DB_ERROR("Failed to create a transaction for the db"));
+  }
 
   MDB_val_copy<crypto::hash> key(h);
   MDB_val result;
-  auto get_result = mdb_get(txn, m_tx_heights, &key, &result);
+  auto get_result = mdb_get(*txn_ptr, m_tx_heights, &key, &result);
   if (get_result == MDB_NOTFOUND)
   {
     throw1(TX_DNE(std::string("tx height with hash ").append(epee::string_tools::pod_to_hex(h)).append(" not found in db").c_str()));
@@ -1192,6 +1273,9 @@ uint64_t BlockchainLMDB::get_tx_block_height(const crypto::hash& h) const
   else if (get_result)
     throw0(DB_ERROR("DB error attempting to fetch tx height from hash"));
 
+  if (! m_batch_active)
+    txn.commit();
+
   return *(const uint64_t*)result.mv_data;
 }
 
@@ -1334,13 +1418,18 @@ tx_out_index BlockchainLMDB::get_output_tx_and_index_from_global(const uint64_t&
   check_open();
 
   txn_safe txn;
-  if (mdb_txn_begin(m_env, NULL, MDB_RDONLY, txn))
+  txn_safe* txn_ptr = &txn;
-    throw0(DB_ERROR("Failed to create a transaction for the db"));
+  if (m_batch_active)
-
+    txn_ptr = m_write_txn;
+  else
+  {
+    if (mdb_txn_begin(m_env, NULL, MDB_RDONLY, txn))
+      throw0(DB_ERROR("Failed to create a transaction for the db"));
+  }
   MDB_val_copy<uint64_t> k(index);
   MDB_val v;
 
-  auto get_result = mdb_get(txn, m_output_txs, &k, &v);
+  auto get_result = mdb_get(*txn_ptr, m_output_txs, &k, &v);
   if (get_result == MDB_NOTFOUND)
     throw1(OUTPUT_DNE("output with given index not in db"));
   else if (get_result)
@@ -1348,11 +1437,13 @@ tx_out_index BlockchainLMDB::get_output_tx_and_index_from_global(const uint64_t&
 
   crypto::hash tx_hash = *(crypto::hash*)v.mv_data;
 
-  get_result = mdb_get(txn, m_output_indices, &k, &v);
+  get_result = mdb_get(*txn_ptr, m_output_indices, &k, &v);
   if (get_result == MDB_NOTFOUND)
     throw1(OUTPUT_DNE("output with given index not in db"));
   else if (get_result)
     throw0(DB_ERROR("DB error attempting to fetch output tx index"));
+  if (! m_batch_active)
+    txn.commit();
 
   return tx_out_index(tx_hash, *(const uint64_t *)v.mv_data);
 }
@@ -1363,10 +1454,15 @@ tx_out_index BlockchainLMDB::get_output_tx_and_index(const uint64_t& amount, con
   check_open();
 
   txn_safe txn;
-  if (mdb_txn_begin(m_env, NULL, MDB_RDONLY, txn))
+  txn_safe* txn_ptr = &txn;
-    throw0(DB_ERROR("Failed to create a transaction for the db"));
+  if (m_batch_active)
-
+    txn_ptr = m_write_txn;
-  lmdb_cur cur(txn, m_output_amounts);
+  else
+  {
+    if (mdb_txn_begin(m_env, NULL, MDB_RDONLY, txn))
+      throw0(DB_ERROR("Failed to create a transaction for the db"));
+  }
+  lmdb_cur cur(*txn_ptr, m_output_amounts);
 
   MDB_val_copy<uint64_t> k(amount);
   MDB_val v;
@@ -1395,7 +1491,8 @@ tx_out_index BlockchainLMDB::get_output_tx_and_index(const uint64_t& amount, con
 
   cur.close();
 
-  txn.commit();
+  if (! m_batch_active)
+    txn.commit();
 
   return get_output_tx_and_index_from_global(glob_index);
 }
@@ -1538,6 +1635,85 @@ bool BlockchainLMDB::has_key_image(const crypto::key_image& img) const
   return false;
 }
 
+void BlockchainLMDB::batch_start()
+{
+  LOG_PRINT_L3("BlockchainLMDB::" << __func__);
+  if (! m_batch_transactions)
+    throw0(DB_ERROR("batch transactions not enabled"));
+  if (m_batch_active)
+    throw0(DB_ERROR("batch transaction already in progress"));
+  if (m_write_txn)
+    throw0(DB_ERROR("batch transaction attempted, but m_write_txn already in use"));
+  check_open();
+  // NOTE: need to make sure it's destroyed properly when done
+  if (mdb_txn_begin(m_env, NULL, 0, m_write_batch_txn))
+    throw0(DB_ERROR("Failed to create a transaction for the db"));
+  // indicates this transaction is for batch transactions, but not whether it's
+  // active
+  m_write_batch_txn.m_batch_txn = true;
+  m_write_txn = &m_write_batch_txn;
+  m_batch_active = true;
+  LOG_PRINT_L3("batch transaction: begin");
+}
+
+void BlockchainLMDB::batch_commit()
+{
+  LOG_PRINT_L3("BlockchainLMDB::" << __func__);
+  if (! m_batch_transactions)
+    throw0(DB_ERROR("batch transactions not enabled"));
+  if (! m_batch_active)
+    throw0(DB_ERROR("batch transaction not in progress"));
+  check_open();
+  LOG_PRINT_L3("batch transaction: committing...");
+  m_write_txn->commit();
+  LOG_PRINT_L3("batch transaction: committed");
+
+  if (mdb_txn_begin(m_env, NULL, 0, m_write_batch_txn))
+    throw0(DB_ERROR("Failed to create a transaction for the db"));
+  if (! m_write_batch_txn.m_batch_txn)
+    throw0(DB_ERROR("m_write_batch_txn not marked as a batch transaction"));
+  m_write_txn = &m_write_batch_txn;
+}
+
+void BlockchainLMDB::batch_stop()
+{
+  LOG_PRINT_L3("BlockchainLMDB::" << __func__);
+  if (! m_batch_transactions)
+    throw0(DB_ERROR("batch transactions not enabled"));
+  if (! m_batch_active)
+    throw0(DB_ERROR("batch transaction not in progress"));
+  check_open();
+  LOG_PRINT_L3("batch transaction: committing...");
+  m_write_txn->commit();
+  // for destruction of batch transaction
+  m_write_txn = nullptr;
+  m_batch_active = false;
+  LOG_PRINT_L3("batch transaction: end");
+}
+
+void BlockchainLMDB::batch_abort()
+{
+  LOG_PRINT_L3("BlockchainLMDB::" << __func__);
+  if (! m_batch_transactions)
+    throw0(DB_ERROR("batch transactions not enabled"));
+  if (! m_batch_active)
+    throw0(DB_ERROR("batch transaction not in progress"));
+  check_open();
+  // for destruction of batch transaction
+  m_write_txn = nullptr;
+  // explicitly call in case mdb_env_close() (BlockchainLMDB::close()) called before BlockchainLMDB destructor called.
+  m_write_batch_txn.abort();
+  m_batch_active = false;
+  LOG_PRINT_L3("batch transaction: aborted");
+}
+
+void BlockchainLMDB::set_batch_transactions(bool batch_transactions)
+{
+  LOG_PRINT_L3("BlockchainLMDB::" << __func__);
+  m_batch_transactions = batch_transactions;
+  LOG_PRINT_L3("batch transactions " << (m_batch_transactions ? "enabled" : "disabled"));
+}
+
 uint64_t BlockchainLMDB::add_block( const block& blk
                                   , const size_t& block_size
                                   , const difficulty_type& cumulative_difficulty
@@ -1547,23 +1723,31 @@ uint64_t BlockchainLMDB::add_block( const block& blk
 {
   LOG_PRINT_L3("BlockchainLMDB::" << __func__);
   check_open();
+
   txn_safe txn;
-  if (mdb_txn_begin(m_env, NULL, 0, txn))
+  if (! m_batch_active)
-    throw0(DB_ERROR("Failed to create a transaction for the db"));
+  {
-  m_write_txn = &txn;
+    if (mdb_txn_begin(m_env, NULL, 0, txn))
+      throw0(DB_ERROR("Failed to create a transaction for the db"));
+    m_write_txn = &txn;
+  }
 
   uint64_t num_outputs = m_num_outputs;
   try
   {
     BlockchainDB::add_block(blk, block_size, cumulative_difficulty, coins_generated, txs);
-    m_write_txn = NULL;
+    if (! m_batch_active)
+    {
+      m_write_txn = NULL;
 
-    txn.commit();
+      txn.commit();
+    }
   }
   catch (...)
   {
     m_num_outputs = num_outputs;
-    m_write_txn = NULL;
+    if (! m_batch_active)
+      m_write_txn = NULL;
     throw;
   }
 
@@ -1574,17 +1758,23 @@ void BlockchainLMDB::pop_block(block& blk, std::vector<transaction>& txs)
 {
   LOG_PRINT_L3("BlockchainLMDB::" << __func__);
   txn_safe txn;
-  if (mdb_txn_begin(m_env, NULL, 0, txn))
+  if (! m_batch_active)
-    throw0(DB_ERROR("Failed to create a transaction for the db"));
+  {
-  m_write_txn = &txn;
+    if (mdb_txn_begin(m_env, NULL, 0, txn))
+      throw0(DB_ERROR("Failed to create a transaction for the db"));
+    m_write_txn = &txn;
+  }
 
   uint64_t num_outputs = m_num_outputs;
   try
   {
     BlockchainDB::pop_block(blk, txs);
-    m_write_txn = NULL;
+    if (! m_batch_active)
+    {
+      m_write_txn = NULL;
 
-    txn.commit();
+      txn.commit();
+    }
   }
   catch (...)
   {

src/cryptonote_core/BlockchainDB_impl/db_lmdb.h

@@ -38,8 +38,23 @@ struct txn_safe
   txn_safe() : m_txn(NULL) { }
   ~txn_safe()
   {
-    if(m_txn != NULL)
+    LOG_PRINT_L3("txn_safe: destructor");
+    if (m_txn != NULL)
     {
+      if (m_batch_txn) // this is a batch txn and should have been handled before this point for safety
+      {
+        LOG_PRINT_L0("WARNING: txn_safe: m_txn is a batch txn and it's not NULL in destructor - calling mdb_txn_abort()");
+      }
+      else
+      {
+        // Example of when this occurs: a lookup fails, so a read-only txn is
+        // aborted through this destructor. However, successful read-only txns
+        // ideally should have been committed when done and not end up here.
+        //
+        // NOTE: not sure if this is ever reached for a non-batch write
+        // transaction, but it's probably not ideal if it did.
+        LOG_PRINT_L3("txn_safe: m_txn not NULL in destructor - calling mdb_txn_abort()");
+      }
       mdb_txn_abort(m_txn);
     }
   }
@@ -60,6 +75,24 @@ struct txn_safe
     m_txn = NULL;
   }
 
+  // This should only be needed for batch transaction which must be ensured to
+  // be aborted before mdb_env_close, not after. So we can't rely on
+  // BlockchainLMDB destructor to call txn_safe destructor, as that's too late
+  // to properly abort, since mdb_env_close would have been called earlier.
+  void abort()
+  {
+    LOG_PRINT_L3("txn_safe: abort()");
+    if(m_txn != NULL)
+    {
+      mdb_txn_abort(m_txn);
+      m_txn = NULL;
+    }
+    else
+    {
+      LOG_PRINT_L0("WARNING: txn_safe: abort() called, but m_txn is NULL");
+    }
+  }
+
   operator MDB_txn*()
   {
     return m_txn;
@@ -71,13 +104,14 @@ struct txn_safe
   }
 
   MDB_txn* m_txn;
+  bool m_batch_txn = false;
 };
 
 
 class BlockchainLMDB : public BlockchainDB
 {
 public:
-  BlockchainLMDB();
+  BlockchainLMDB(bool batch_transactions=false);
   ~BlockchainLMDB();
 
   virtual void open(const std::string& filename);
@@ -177,6 +211,12 @@ public:
                             , const std::vector<transaction>& txs
                             );
 
+  virtual void set_batch_transactions(bool batch_transactions);
+  virtual void batch_start();
+  virtual void batch_commit();
+  virtual void batch_stop();
+  virtual void batch_abort();
+
   virtual void pop_block(block& blk, std::vector<transaction>& txs);
 
 private:
@@ -264,7 +304,11 @@ private:
   uint64_t m_height;
   uint64_t m_num_outputs;
   std::string m_folder;
-  txn_safe* m_write_txn;
+  txn_safe* m_write_txn; // may point to either a short-lived txn or a batch txn
+  txn_safe m_write_batch_txn; // persist batch txn outside of BlockchainLMDB
+
+  bool m_batch_transactions; // support for batch transactions
+  bool m_batch_active; // whether batch transaction is in progress
 };
 
 }  // namespace cryptonote