blockchain_db: fix db txn ending too early

The db txn in add_block ending caused the entire overarching
batch txn to stop.
Also add a new guard class so a db txn can be stopped in the
face of exceptions.

Also use a read only db txn in init when the db itself is
read only, and do not save the max tx size in that case.

1 files changed, 2 insertions, 6 deletions

diff --git a/src/cryptonote_basic/hardfork.cpp b/src/cryptonote_basic/hardfork.cpp
index 89bca2f09..61240ea37 100644
--- a/src/cryptonote_basic/hardfork.cpp
+++ b/src/cryptonote_basic/hardfork.cpp
@@ -266,11 +266,9 @@ bool HardFork::reorganize_from_chain_height(uint64_t height)
 bool HardFork::rescan_from_block_height(uint64_t height)
 {
   CRITICAL_REGION_LOCAL(lock);
-  db.block_txn_start(true);
-  if (height >= db.height()) {
-    db.block_txn_stop();
+  db_rtxn_guard rtxn_guard(&db);
+  if (height >= db.height())
     return false;
-  }
 
   versions.clear();
 
@@ -293,8 +291,6 @@ bool HardFork::rescan_from_block_height(uint64_t height)
     current_fork_index = voted;
   }
 
-  db.block_txn_stop();
-
   return true;
 }