Parts of LMDB impl of BlockchainDB done and working

The rest should just be tedious copypasta and modification.

src/cryptonote_core/BlockchainDB_impl/db_lmdb.cpp

@@ -27,15 +27,21 @@
 
 #include "db_lmdb.h"
 
+#include <boost/filesystem.hpp>
+#include <memory>  // std::unique_ptr
+#include <cstring>  // memcpy
+
+#include "cryptonote_core/cryptonote_format_utils.h"
+
 namespace cryptonote
 {
 
-#define CN_LMDB_DBI_OPEN( txn, flags, dbi ) \
-  if (mdb_dbi_open(txn, NULL, flags, &dbi)) \
-  { \
-    throw DB_OPEN_FAILURE( "Failed to open db handle for ##dbi" ); \
-  }
-
+const char* LMDB_BLOCKS = "blocks";
+const char* LMDB_BLOCK_HASHES = "block_hashes";
+const char* LMDB_BLOCK_SIZES = "block_sizes";
+const char* LMDB_BLOCK_DIFFS = "block_diffs";
+const char* LMDB_BLOCK_COINS = "block_coins";
+const char* LMDB_TXS = "txs";
 
 void BlockchainLMDB::add_block( const block& blk
               , const size_t& block_size
@@ -43,34 +49,121 @@ void BlockchainLMDB::add_block( const block& blk
               , const uint64_t& coins_generated
               )
 {
+  check_open();
+
+  crypto::hash h = get_block_hash(blk);
+  MDB_val val_h;
+  val_h.mv_size = sizeof(crypto::hash);
+  val_h.mv_data = &h;
+
+  MDB_val unused;
+  if (mdb_get(*m_write_txn, m_block_hashes, &val_h, &unused) == 0)
+  {
+      LOG_PRINT_L1("Attempting to add block that's already in the db");
+      throw BLOCK_EXISTS("Attempting to add block that's already in the db");
   }
 
-void BlockchainLMDB::remove_block(const crypto::hash& blk_hash)
+  if (m_height > 0)
   {
+    MDB_val parent_key;
+    crypto::hash parent = blk.prev_id;
+    parent_key.mv_size = sizeof(crypto::hash);
+    parent_key.mv_data = &parent;
+
+    MDB_val parent_h;
+    if (mdb_get(*m_write_txn, m_block_hashes, &parent_key, &parent_h))
+    {
+      LOG_PRINT_L0("Failed to get top block hash to check for new block's parent");
+      throw DB_ERROR("Failed to get top block hash to check for new block's parent");
+    }
+
+    uint64_t parent_height = *(uint64_t *)parent_h.mv_data;
+    if (parent_height != m_height - 1)
+    {
+      LOG_PRINT_L0("Top block is not new block's parent");
+      throw BLOCK_PARENT_DNE("Top block is not new block's parent");
+    }
+  }
+
+  MDB_val key;
+  key.mv_size = sizeof(uint64_t);
+  key.mv_data = &m_height;
+
+  auto bd = block_to_blob(blk);
+
+  // const-correctness be trolling, yo
+  std::unique_ptr<char[]> bd_cpy(new char[bd.size()]);
+  memcpy(bd_cpy.get(), bd.data(), bd.size());
+
+  MDB_val blob;
+  blob.mv_size = bd.size();
+  blob.mv_data = bd_cpy.get();
+  if (mdb_put(*m_write_txn, m_blocks, &key, &blob, 0))
+  {
+    LOG_PRINT_L0("Failed to add block blob to db transaction");
+    throw DB_ERROR("Failed to add block blob to db transaction");
+  }
+
+  size_t size_cpy = block_size;
+  MDB_val sz;
+  sz.mv_size = sizeof(block_size);
+  sz.mv_data = &size_cpy;
+  if (mdb_put(*m_write_txn, m_block_sizes, &key, &sz, 0))
+  {
+    LOG_PRINT_L0("Failed to add block size to db transaction");
+    throw DB_ERROR("Failed to add block size to db transaction");
+  }
+
+  if (mdb_put(*m_write_txn, m_block_hashes, &val_h, &key, 0))
+  {
+    LOG_PRINT_L0("Failed to add block size to db transaction");
+    throw DB_ERROR("Failed to add block size to db transaction");
+  }
+
+}
+
+void BlockchainLMDB::remove_block()
+{
+  check_open();
 }
 
 void BlockchainLMDB::add_transaction_data(const crypto::hash& blk_hash, const transaction& tx)
 {
+  check_open();
 }
 
 void BlockchainLMDB::remove_transaction_data(const crypto::hash& tx_hash)
 {
+  check_open();
 }
 
 void BlockchainLMDB::add_output(const crypto::hash& tx_hash, const tx_out& tx_output, const uint64_t& local_index)
 {
+  check_open();
 }
 
 void BlockchainLMDB::remove_output(const tx_out& tx_output)
 {
+  check_open();
 }
 
 void BlockchainLMDB::add_spent_key(const crypto::key_image& k_image)
 {
+  check_open();
 }
 
 void BlockchainLMDB::remove_spent_key(const crypto::key_image& k_image)
 {
+  check_open();
+}
+
+void BlockchainLMDB::check_open()
+{
+  if (!m_open)
+  {
+    LOG_PRINT_L0("DB operation attempted on a not-open DB instance");
+    throw DB_ERROR("DB operation attempted on a not-open DB instance");
+  }
 }
 
 BlockchainLMDB::~BlockchainLMDB()
@@ -79,41 +172,113 @@ BlockchainLMDB::~BlockchainLMDB()
 
 BlockchainLMDB::BlockchainLMDB()
 {
+  // initialize folder to something "safe" just in case
+  // someone accidentally misuses this class...
+  m_folder = "thishsouldnotexistbecauseitisgibberish";
+  m_open = false;
 }
 
 void BlockchainLMDB::open(const std::string& filename)
 {
+
+  if (m_open)
+  {
+    LOG_PRINT_L0("Attempted to open db, but it's already open");
+    throw DB_OPEN_FAILURE("Attempted to open db, but it's already open");
+  }
+
+  boost::filesystem::path direc(filename);
+  if (boost::filesystem::exists(direc))
+  {
+    if (!boost::filesystem::is_directory(direc))
+    {
+      LOG_PRINT_L0("LMDB needs a directory path, but a file was passed");
+      throw DB_OPEN_FAILURE("LMDB needs a directory path, but a file was passed");
+    }
+  }
+  else
+  {
+    if (!boost::filesystem::create_directory(direc))
+    {
+      LOG_PRINT_L0("Failed to create directory " << filename);
+      throw DB_OPEN_FAILURE(std::string("Failed to create directory ").append(filename).c_str());
+    }
+  }
+
+  m_folder = filename;
+
+  // set up lmdb environment
   if (mdb_env_create(&m_env))
   {
-    throw DB_ERROR("Failed to create lmdb environment");
-  }
-  if (mdb_env_open(m_env, filename.c_str(), 0, 0664))
-  {
-    throw DB_ERROR("Failed to open lmdb environment");
+    LOG_PRINT_L0("Failed to create lmdb environment");
+    throw DB_OPEN_FAILURE("Failed to create lmdb environment");
   }
   if (mdb_env_set_maxdbs(m_env, 15))
   {
-    throw DB_ERROR("Failed to set max number of dbs");
+    LOG_PRINT_L0("Failed to set max number of dbs");
+    throw DB_OPEN_FAILURE("Failed to set max number of dbs");
   }
-  MDB_txn* txn;
-  if (mdb_txn_begin(m_env, NULL, 0, &txn))
+  if (mdb_env_open(m_env, filename.c_str(), 0, 0664))
   {
+    LOG_PRINT_L0("Failed to open lmdb environment");
+    throw DB_OPEN_FAILURE("Failed to open lmdb environment");
+  }
+
+  // get a read/write MDB_txn
+  txn_safe txn;
+  if (mdb_txn_begin(m_env, NULL, 0, txn))
+  {
+    LOG_PRINT_L0("Failed to create a transaction for the db");
     throw DB_ERROR("Failed to create a transaction for the db");
   }
 
-  CN_LMDB_DBI_OPEN(txn, MDB_CREATE | MDB_DUPSORT, m_blocks)
-  CN_LMDB_DBI_OPEN(txn, MDB_CREATE | MDB_DUPSORT, m_block_txs)
-  CN_LMDB_DBI_OPEN(txn, MDB_CREATE | MDB_DUPSORT, m_block_hashes)
-  CN_LMDB_DBI_OPEN(txn, MDB_CREATE | MDB_DUPSORT, m_block_sizes)
-  CN_LMDB_DBI_OPEN(txn, MDB_CREATE | MDB_DUPSORT, m_block_diffs)
-  CN_LMDB_DBI_OPEN(txn, MDB_CREATE | MDB_DUPSORT, m_block_coins)
-  CN_LMDB_DBI_OPEN(txn, MDB_CREATE | MDB_DUPSORT, m_txs)
-
-  if (mdb_txn_commit(txn))
+  // open necessary databases, and set properties as needed
+  // uses macros to avoid having to change things too many places
+  if (mdb_dbi_open(txn, LMDB_BLOCKS, MDB_INTEGERKEY | MDB_CREATE, &m_blocks))
   {
-    throw DB_OPEN_FAILURE("Failed to commit db open transaction");
+    LOG_PRINT_L0("Failed to open db handle for m_blocks");
+    throw DB_OPEN_FAILURE( "Failed to open db handle for m_blocks" );
   }
 
+  if (mdb_dbi_open(txn, LMDB_BLOCK_HASHES, MDB_INTEGERKEY | MDB_CREATE, &m_block_hashes))
+  {
+    LOG_PRINT_L0("Failed to open db handle for m_block_hashes");
+    throw DB_OPEN_FAILURE( "Failed to open db handle for m_block_hashes" );
+  }
+  if (mdb_dbi_open(txn, LMDB_BLOCK_SIZES, MDB_INTEGERKEY | MDB_CREATE, &m_block_sizes))
+  {
+    LOG_PRINT_L0("Failed to open db handle for m_block_sizes");
+    throw DB_OPEN_FAILURE( "Failed to open db handle for m_block_sizes" );
+  }
+  if (mdb_dbi_open(txn, LMDB_BLOCK_DIFFS, MDB_INTEGERKEY | MDB_CREATE, &m_block_diffs))
+  {
+    LOG_PRINT_L0("Failed to open db handle for m_block_diffs");
+    throw DB_OPEN_FAILURE( "Failed to open db handle for m_block_diffs" );
+  }
+  if (mdb_dbi_open(txn, LMDB_BLOCK_COINS, MDB_INTEGERKEY | MDB_CREATE, &m_block_coins))
+  {
+    LOG_PRINT_L0("Failed to open db handle for m_block_coins");
+    throw DB_OPEN_FAILURE( "Failed to open db handle for m_block_coins" );
+  }
+  if (mdb_dbi_open(txn, LMDB_TXS, MDB_CREATE, &m_txs))
+  {
+    LOG_PRINT_L0("Failed to open db handle for m_txs");
+    throw DB_OPEN_FAILURE( "Failed to open db handle for m_txs" );
+  }
+
+  // get and keep current height
+  MDB_stat db_stats;
+  if (mdb_stat(txn, m_blocks, &db_stats))
+  {
+    LOG_PRINT_L0("Failed to query m_blocks");
+    throw DB_ERROR("Failed to query m_blocks");
+  }
+  m_height = db_stats.ms_entries;
+
+  // commit the transaction
+  txn.commit();
+
+  m_open = true;
   // from here, init should be finished
 }
 
@@ -136,184 +301,340 @@ void BlockchainLMDB::reset()
 
 std::vector<std::string> BlockchainLMDB::get_filenames()
 {
-  std::vector<std::string> retval;
-  return retval;
+  std::vector<std::string> filenames;
+
+  boost::filesystem::path datafile(m_folder);
+  datafile /= "data.mdb";
+  boost::filesystem::path lockfile(m_folder);
+  lockfile /= "lock.mdb";
+
+  filenames.push_back(datafile.string());
+  filenames.push_back(lockfile.string());
+
+  return filenames;
 }
 
 
 bool BlockchainLMDB::lock()
 {
+  check_open();
   return false;
 }
 
 void BlockchainLMDB::unlock()
 {
+  check_open();
 }
 
 
 bool BlockchainLMDB::block_exists(const crypto::hash& h)
 {
+  check_open();
+
+  txn_safe txn;
+  if (mdb_txn_begin(m_env, NULL, MDB_RDONLY, txn))
+  {
+    LOG_PRINT_L0("Failed to create a transaction for the db");
+    throw DB_ERROR("Failed to create a transaction for the db");
+  }
+
+  crypto::hash key_cpy = h;
+  MDB_val key;
+  key.mv_size = sizeof(crypto::hash);
+  key.mv_data = &key_cpy;
+
+  MDB_val result;
+  auto get_result = mdb_get(txn, m_block_hashes, &key, &result);
+  if (get_result == MDB_NOTFOUND)
+  {
+    LOG_PRINT_L1("Block with hash " << epee::string_tools::pod_to_hex(h) << "not found in db");
     return false;
   }
+  else if (get_result)
+  {
+    LOG_PRINT_L0("DB error attempting to fetch block index from hash");
+    throw DB_ERROR("DB error attempting to fetch block index from hash");
+  }
+
+  return true;
+}
 
 block BlockchainLMDB::get_block(const crypto::hash& h)
 {
-  block b;
-  return b;
+  check_open();
+
+  txn_safe txn;
+  if (mdb_txn_begin(m_env, NULL, MDB_RDONLY, txn))
+  {
+    LOG_PRINT_L0("Failed to create a transaction for the db");
+    throw DB_ERROR("Failed to create a transaction for the db");
+  }
+
+  crypto::hash key_cpy = h;
+  MDB_val key;
+  key.mv_size = sizeof(crypto::hash);
+  key.mv_data = &key_cpy;
+
+  MDB_val result;
+  auto get_result = mdb_get(txn, m_block_hashes, &key, &result);
+  if (get_result == MDB_NOTFOUND)
+  {
+    LOG_PRINT_L1("Attempted to retrieve non-existent block");
+    throw BLOCK_DNE("Attempted to retrieve non-existent block");
+  }
+  else if (get_result)
+  {
+    LOG_PRINT_L0("Error attempting to retrieve a block index from the db");
+    throw DB_ERROR("Error attempting to retrieve a block index from the db");
+  }
+
+  txn.commit();
+
+  uint64_t index = *(uint64_t*)result.mv_data;
+
+  return get_block_from_height(index);
 }
 
 uint64_t BlockchainLMDB::get_block_height(const crypto::hash& h)
 {
+  check_open();
   return 0;
 }
 
 block_header BlockchainLMDB::get_block_header(const crypto::hash& h)
 {
+  check_open();
   block_header bh;
   return bh;
 }
 
 block BlockchainLMDB::get_block_from_height(const uint64_t& height)
 {
+  check_open();
+
+  txn_safe txn;
+  if (mdb_txn_begin(m_env, NULL, MDB_RDONLY, txn))
+  {
+    LOG_PRINT_L0("Failed to create a transaction for the db");
+    throw DB_ERROR("Failed to create a transaction for the db");
+  }
+
+  uint64_t height_cpy = height;
+  MDB_val key;
+  key.mv_size = sizeof(uint64_t);
+  key.mv_data = &height_cpy;
+  MDB_val result;
+  auto get_result = mdb_get(txn, m_blocks, &key, &result);
+  if (get_result == MDB_NOTFOUND)
+  {
+    LOG_PRINT_L0("Attempted to get block from height " << height << ", but no such block exists");
+    throw DB_ERROR("Attempt to get block from height failed -- block not in db");
+  }
+  else if (get_result)
+  {
+    LOG_PRINT_L0("Error attempting to retrieve a block from the db");
+    throw DB_ERROR("Error attempting to retrieve a block from the db");
+  }
+
+  txn.commit();
+
+  blobdata bd;
+  bd.assign(reinterpret_cast<char*>(result.mv_data), result.mv_size);
+
   block b;
+  if (!parse_and_validate_block_from_blob(bd, b))
+  {
+    LOG_PRINT_L0("Failed to parse block from blob retrieved from the db");
+    throw DB_ERROR("Failed to parse block from blob retrieved from the db");
+  }
+
   return b;
 }
 
 uint64_t BlockchainLMDB::get_block_timestamp(const uint64_t& height) 
 {
+  check_open();
   return 0;
 }
 
 uint64_t BlockchainLMDB::get_top_block_timestamp()
 {
+  check_open();
   return 0;
 }
 
 size_t BlockchainLMDB::get_block_size(const uint64_t& height)
 {
+  check_open();
   return 0;
 }
 
 difficulty_type BlockchainLMDB::get_block_cumulative_difficulty(const uint64_t& height)
 {
+  check_open();
   return 0;
 }
 
 difficulty_type BlockchainLMDB::get_block_difficulty(const uint64_t& height)
 {
+  check_open();
   return 0;
 }
 
 uint64_t BlockchainLMDB::get_block_already_generated_coins(const uint64_t& height)
 {
+  check_open();
   return 0;
 }
 
 crypto::hash BlockchainLMDB::get_block_hash_from_height(const uint64_t& height)
 {
+  check_open();
   crypto::hash h;
   return h;
 }
 
 std::vector<block> BlockchainLMDB::get_blocks_range(const uint64_t& h1, const uint64_t& h2)
 {
+  check_open();
   std::vector<block> v;
   return v;
 }
 
 std::vector<crypto::hash> BlockchainLMDB::get_hashes_range(const uint64_t& h1, const uint64_t& h2)
 {
+  check_open();
   std::vector<crypto::hash> v;
   return v;
 }
 
 crypto::hash BlockchainLMDB::top_block_hash()
 {
+  check_open();
   crypto::hash h;
   return h;
 }
 
 block BlockchainLMDB::get_top_block()
 {
+  check_open();
   block b;
   return b;
 }
 
 uint64_t BlockchainLMDB::height()
 {
+  check_open();
   return 0;
 }
 
 
 bool BlockchainLMDB::tx_exists(const crypto::hash& h)
 {
+  check_open();
   return false;
 }
 
 uint64_t BlockchainLMDB::get_tx_unlock_time(const crypto::hash& h)
 {
+  check_open();
   return 0;
 }
 
 transaction BlockchainLMDB::get_tx(const crypto::hash& h)
 {
+  check_open();
   transaction t;
   return t;
 }
 
 uint64_t BlockchainLMDB::get_tx_count()
 {
+  check_open();
   return 0;
 }
 
 std::vector<transaction> BlockchainLMDB::get_tx_list(const std::vector<crypto::hash>& hlist)
 {
+  check_open();
   std::vector<transaction> v;
   return v;
 }
 
 uint64_t BlockchainLMDB::get_tx_block_height(const crypto::hash& h)
 {
+  check_open();
   return 0;
 }
 
 uint64_t BlockchainLMDB::get_random_output(const uint64_t& amount)
 {
+  check_open();
   return 0;
 }
 
 uint64_t BlockchainLMDB::get_num_outputs(const uint64_t& amount)
 {
+  check_open();
   return 0;
 }
 
 crypto::public_key BlockchainLMDB::get_output_key(const uint64_t& amount, const uint64_t& index)
 {
+  check_open();
   crypto::public_key pk;
   return pk;
 }
 
 tx_out BlockchainLMDB::get_output(const crypto::hash& h, const uint64_t& index)
 {
+  check_open();
   tx_out o;
   return o;
 }
 
 tx_out_index BlockchainLMDB::get_output_tx_and_index(const uint64_t& amount, const uint64_t& index)
 {
+  check_open();
   tx_out_index i;
   return i;
 }
 
 std::vector<uint64_t> BlockchainLMDB::get_tx_output_indices(const crypto::hash& h)
 {
+  check_open();
   std::vector<uint64_t> v;
   return v;
 }
 
 bool BlockchainLMDB::has_key_image(const crypto::key_image& img)
 {
+  check_open();
   return false;
 }
 
+uint64_t BlockchainLMDB::add_block( const block& blk
+                                  , const size_t& block_size
+                                  , const difficulty_type& cumulative_difficulty
+                                  , const uint64_t& coins_generated
+                                  , const std::vector<transaction>& txs
+                                  )
+{
+  check_open();
+  txn_safe txn;
+  if (mdb_txn_begin(m_env, NULL, 0, txn))
+  {
+    LOG_PRINT_L0("Failed to create a transaction for the db");
+    throw DB_ERROR("Failed to create a transaction for the db");
+  }
+  m_write_txn = &txn;
+
+  BlockchainDB::add_block(blk, block_size, cumulative_difficulty, coins_generated, txs);
+
+  txn.commit();
+
+  m_height++;
+  return m_height - 1;
+}
+
 }  // namespace cryptonote

src/cryptonote_core/BlockchainDB_impl/db_lmdb.h

@@ -32,6 +32,47 @@
 namespace cryptonote
 {
 
+struct txn_safe
+{
+  txn_safe() : m_txn(NULL) { }
+  ~txn_safe()
+  {
+    if(m_txn != NULL)
+    {
+      mdb_txn_abort(m_txn);
+    }
+  }
+
+  void commit(std::string message = "")
+  {
+    if (message.size() == 0)
+    {
+      message = "Failed to commit a transaction to the db";
+    }
+
+    if (mdb_txn_commit(m_txn))
+    {
+      m_txn = NULL;
+      LOG_PRINT_L0(message);
+      throw DB_ERROR(message.c_str());
+    }
+    m_txn = NULL;
+  }
+
+  operator MDB_txn*()
+  {
+    return m_txn;
+  }
+
+  operator MDB_txn**()
+  {
+    return &m_txn;
+  }
+
+  MDB_txn* m_txn;
+};
+
+
 class BlockchainLMDB : public BlockchainDB
 {
 public:
@@ -114,6 +155,13 @@ public:
 
   virtual bool has_key_image(const crypto::key_image& img);
 
+  virtual uint64_t add_block( const block& blk
+                            , const size_t& block_size
+                            , const difficulty_type& cumulative_difficulty
+                            , const uint64_t& coins_generated
+                            , const std::vector<transaction>& txs
+                            );
+
 private:
   virtual void add_block( const block& blk
                 , const size_t& block_size
@@ -121,7 +169,7 @@ private:
                 , const uint64_t& coins_generated
                 );
 
-  virtual void remove_block(const crypto::hash& blk_hash);
+  virtual void remove_block();
 
   virtual void add_transaction_data(const crypto::hash& blk_hash, const transaction& tx);
 
@@ -135,10 +183,11 @@ private:
 
   virtual void remove_spent_key(const crypto::key_image& k_image);
 
+  void check_open();
+
   MDB_env* m_env;
 
   MDB_dbi m_blocks;
-  MDB_dbi m_block_txs;
   MDB_dbi m_block_hashes;
   MDB_dbi m_block_sizes;
   MDB_dbi m_block_diffs;
@@ -148,6 +197,11 @@ private:
 
   MDB_dbi m_spent;
   MDB_dbi m_utxo;
+
+  bool m_open;
+  uint64_t m_height;
+  std::string m_folder;
+  txn_safe* m_write_txn;
 };
 
 }  // namespace cryptonote

src/cryptonote_core/blockchain_db.cpp

@@ -67,36 +67,18 @@ uint64_t BlockchainDB::add_block( const block& blk
                                 , const uint64_t& coins_generated
                                 , const std::vector<transaction>& txs
                                 )
-{
-  try
 {
   // call out to subclass implementation to add the block & metadata
   add_block(blk, block_size, cumulative_difficulty, coins_generated);
 
   crypto::hash blk_hash = get_block_hash(blk);
   // call out to add the transactions
+
+  add_transaction(blk_hash, blk.miner_tx);
   for (const transaction& tx : txs)
   {
     add_transaction(blk_hash, tx);
   }
-  }
-  // in case any of the add_block process goes awry, undo
-  catch (const std::exception& e)
-  {
-    LOG_ERROR("Error adding block to db: " << e.what());
-    try
-    {
-      pop_block();
-    }
-    // if undoing goes wrong as well, we need to throw, as blockchain
-    // will be in a bad state
-    catch (const std::exception& e)
-    {
-      LOG_ERROR("Error undoing partially added block: " << e.what());
-      throw;
-    }
-    throw;
-  }
 
   return height();
 }
@@ -105,6 +87,9 @@ void BlockchainDB::pop_block(block& blk, std::vector<transaction>& txs)
 {
   blk = get_top_block();
 
+  remove_block();
+  
+  remove_transaction(get_transaction_hash(blk.miner_tx));
   for (const auto& h : blk.tx_hashes)
   {
     txs.push_back(get_tx(h));

src/cryptonote_core/blockchain_db.h

@@ -332,8 +332,8 @@ private:
                 , const uint64_t& coins_generated
                 ) = 0;
 
-  // tells the subclass to remove data about a block
-  virtual void remove_block(const crypto::hash& blk_hash) = 0;
+  // tells the subclass to remove data about the top block
+  virtual void remove_block() = 0;
 
   // tells the subclass to store the transaction and its metadata
   virtual void add_transaction_data(const crypto::hash& blk_hash, const transaction& tx) = 0;
@@ -403,7 +403,9 @@ public:
 
 
   // adds a block with the given metadata to the top of the blockchain, returns the new height
-  uint64_t add_block( const block& blk
+  // NOTE: subclass implementations of this (or the functions it calls) need
+  // to handle undoing any partially-added blocks in the event of a failure.
+  virtual uint64_t add_block( const block& blk
                             , const size_t& block_size
                             , const difficulty_type& cumulative_difficulty
                             , const uint64_t& coins_generated