Fix use of rtxn without a mdb_txn_safe wrapper

src/blockchain_db/blockchain_db.h

@@ -1883,10 +1883,11 @@ public:
   }
   virtual ~db_txn_guard()
   {
-    if (active)
     stop();
   }
   void stop()
+  {
+    if (active)
     {
       if (readonly)
         db->block_rtxn_stop();
@@ -1894,6 +1895,7 @@ public:
         db->block_wtxn_stop();
       active = false;
     }
+  }
   void abort()
   {
     if (readonly)

src/blockchain_db/lmdb/db_lmdb.cpp

@@ -465,6 +465,32 @@ void mdb_txn_safe::increment_txns(int i)
 	num_active_txns += i;
 }
 
+#define TXN_PREFIX(flags); \
+  mdb_txn_safe auto_txn; \
+  mdb_txn_safe* txn_ptr = &auto_txn; \
+  if (m_batch_active) \
+    txn_ptr = m_write_txn; \
+  else \
+  { \
+    if (auto mdb_res = lmdb_txn_begin(m_env, NULL, flags, auto_txn)) \
+      throw0(DB_ERROR(lmdb_error(std::string("Failed to create a transaction for the db in ")+__FUNCTION__+": ", mdb_res).c_str())); \
+  } \
+
+#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); \
+  if (my_rtxn) auto_txn.m_tinfo = m_tinfo.get(); \
+  else auto_txn.uncheck()
+#define TXN_POSTFIX_RDONLY()
+
+#define TXN_POSTFIX_SUCCESS() \
+  do { \
+    if (! m_batch_active) \
+      auto_txn.commit(); \
+  } while(0)
+
 void lmdb_resized(MDB_env *env, int isactive)
 {
   mdb_txn_safe::prevent_new_txns();
@@ -713,9 +739,8 @@ uint64_t BlockchainLMDB::get_estimated_batch_size(uint64_t batch_num_blocks, uin
   }
   else
   {
-    MDB_txn *rtxn;
+    {
-    mdb_txn_cursors *rcurs;
+      TXN_PREFIX_RDONLY();
-    bool my_rtxn = block_rtxn_start(&rtxn, &rcurs);
       for (uint64_t block_num = block_start; block_num <= block_stop; ++block_num)
       {
         // we have access to block weight, which will be greater or equal to block size,
@@ -727,7 +752,7 @@ uint64_t BlockchainLMDB::get_estimated_batch_size(uint64_t batch_num_blocks, uin
         // some blocks were to be skipped for being outliers.
         ++num_blocks_used;
       }
-    if (my_rtxn) block_rtxn_stop();
+    }
     avg_block_size = total_block_size / (num_blocks_used ? num_blocks_used : 1);
     MDEBUG("average block size across recent " << num_blocks_used << " blocks: " << avg_block_size);
   }
@@ -1678,32 +1703,6 @@ void BlockchainLMDB::unlock()
   check_open();
 }
 
-#define TXN_PREFIX(flags); \
-  mdb_txn_safe auto_txn; \
-  mdb_txn_safe* txn_ptr = &auto_txn; \
-  if (m_batch_active) \
-    txn_ptr = m_write_txn; \
-  else \
-  { \
-    if (auto mdb_res = lmdb_txn_begin(m_env, NULL, flags, auto_txn)) \
-      throw0(DB_ERROR(lmdb_error(std::string("Failed to create a transaction for the db in ")+__FUNCTION__+": ", mdb_res).c_str())); \
-  } \
-
-#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); \
-  if (my_rtxn) auto_txn.m_tinfo = m_tinfo.get(); \
-  else auto_txn.uncheck()
-#define TXN_POSTFIX_RDONLY()
-
-#define TXN_POSTFIX_SUCCESS() \
-  do { \
-    if (! m_batch_active) \
-      auto_txn.commit(); \
-  } while(0)
-
 
 // The below two macros are for DB access within block add/remove, whether
 // regular batch txn is in use or not. m_write_txn is used as a batch txn, even
@@ -3923,13 +3922,20 @@ void BlockchainLMDB::block_rtxn_stop() const
   LOG_PRINT_L3("BlockchainLMDB::" << __func__);
   mdb_txn_reset(m_tinfo->m_ti_rtxn);
   memset(&m_tinfo->m_ti_rflags, 0, sizeof(m_tinfo->m_ti_rflags));
+  /* cancel out the increment from rtxn_start */
+  mdb_txn_safe::increment_txns(-1);
 }
 
 bool BlockchainLMDB::block_rtxn_start() const
 {
   MDB_txn *mtxn;
   mdb_txn_cursors *mcur;
-  return block_rtxn_start(&mtxn, &mcur);
+  /* auto_txn is only used for the create gate */
+  mdb_txn_safe auto_txn;
+  bool ret = block_rtxn_start(&mtxn, &mcur);
+  if (ret)
+    auto_txn.increment_txns(1); /* remember there is an active readtxn */
+  return ret;
 }
 
 void BlockchainLMDB::block_wtxn_start()