From 84a8b2da140c20be3b0c4042160524d226634d0f Mon Sep 17 00:00:00 2001
From: Howard Chu <hyc@symas.com>
Date: Wed, 31 Jan 2018 18:18:01 +0000
Subject: [PATCH] Don't create readtxn until after txn_safe gate check

---
 src/blockchain_db/lmdb/db_lmdb.cpp | 11 +++++++++--
 src/blockchain_db/lmdb/db_lmdb.h   |  1 +
 2 files changed, 10 insertions(+), 2 deletions(-)

diff --git a/src/blockchain_db/lmdb/db_lmdb.cpp b/src/blockchain_db/lmdb/db_lmdb.cpp
index d19399bec..59daf1bf0 100644
--- a/src/blockchain_db/lmdb/db_lmdb.cpp
+++ b/src/blockchain_db/lmdb/db_lmdb.cpp
@@ -338,6 +338,12 @@ mdb_txn_safe::~mdb_txn_safe()
   num_active_txns--;
 }
 
+void mdb_txn_safe::uncheck()
+{
+  num_active_txns--;
+  m_check = false;
+}
+
 void mdb_txn_safe::commit(std::string message)
 {
   if (message.size() == 0)
@@ -1439,9 +1445,10 @@ void BlockchainLMDB::unlock()
 #define TXN_PREFIX_RDONLY() \
   MDB_txn *m_txn; \
   mdb_txn_cursors *m_cursors; \
+  mdb_txn_safe auto_txn; \
   bool my_rtxn = block_rtxn_start(&m_txn, &m_cursors); \
-  mdb_txn_safe auto_txn(my_rtxn); \
-  if (my_rtxn) auto_txn.m_tinfo = m_tinfo.get()
+  if (my_rtxn) auto_txn.m_tinfo = m_tinfo.get(); \
+  else auto_txn.uncheck()
 #define TXN_POSTFIX_RDONLY()
 
 #define TXN_POSTFIX_SUCCESS() \
diff --git a/src/blockchain_db/lmdb/db_lmdb.h b/src/blockchain_db/lmdb/db_lmdb.h
index ecd14f11b..e4c1fef84 100644
--- a/src/blockchain_db/lmdb/db_lmdb.h
+++ b/src/blockchain_db/lmdb/db_lmdb.h
@@ -112,6 +112,7 @@ struct mdb_txn_safe
   // BlockchainLMDB destructor to call mdb_txn_safe destructor, as that's too late
   // to properly abort, since mdb_env_close would have been called earlier.
   void abort();
+  void uncheck();
 
   operator MDB_txn*()
   {