BlockchainLMDB: Deal with DB exceptions at block level with particularity

Add another DB error exception type to distinguish failed txn setup from
general use of txn.

This keeps the error handling flow the same as before the block-level
txn setup changes that moved control up a layer to BlockchainDB.

src/blockchain_db/blockchain_db.h

@@ -175,6 +175,14 @@ class DB_ERROR : public DB_EXCEPTION
     DB_ERROR(const char* s) : DB_EXCEPTION(s) { }
 };
 
+// For distinguishing errors trying to set up a DB txn from other errors
+class DB_ERROR_TXN_START : public DB_EXCEPTION
+{
+  public:
+    DB_ERROR_TXN_START() : DB_EXCEPTION("DB Error in starting txn") { }
+    DB_ERROR_TXN_START(const char* s) : DB_EXCEPTION(s) { }
+};
+
 class DB_OPEN_FAILURE : public DB_EXCEPTION
 {
   public:

src/blockchain_db/lmdb/db_lmdb.cpp

@@ -2199,8 +2199,15 @@ void BlockchainLMDB::set_batch_transactions(bool batch_transactions)
 void BlockchainLMDB::block_txn_start()
 {
   LOG_PRINT_L3("BlockchainLMDB::" << __func__);
+  // Distinguish the exceptions here from exceptions that would be thrown while
+  // using the txn and committing it.
+  //
+  // If an exception is thrown in this setup, we don't want the caller to catch
+  // it and proceed as if there were an existing write txn, such as trying to
+  // call block_txn_abort(). It also indicates a serious issue which will
+  // probably be thrown up another layer.
   if (! m_batch_active && m_write_txn)
-    throw0(DB_ERROR((std::string("Attempted to start new write txn when write txn already exists in ")+__FUNCTION__).c_str()));
+    throw0(DB_ERROR_TXN_START((std::string("Attempted to start new write txn when write txn already exists in ")+__FUNCTION__).c_str()));
   if (! m_batch_active)
   {
     m_write_txn = new mdb_txn_safe();
@@ -2208,7 +2215,7 @@ void BlockchainLMDB::block_txn_start()
     {
       delete m_write_txn;
       m_write_txn = nullptr;
-      throw0(DB_ERROR(lmdb_error("Failed to create a transaction for the db: ", mdb_res).c_str()));
+      throw0(DB_ERROR_TXN_START(lmdb_error("Failed to create a transaction for the db: ", mdb_res).c_str()));
     }
   }
 }
@@ -2270,6 +2277,10 @@ uint64_t BlockchainLMDB::add_block(const block& blk, const size_t& block_size, c
   {
     BlockchainDB::add_block(blk, block_size, cumulative_difficulty, coins_generated, txs);
   }
+  catch (DB_ERROR_TXN_START& e)
+  {
+    throw;
+  }
   catch (...)
   {
     m_num_outputs = num_outputs;