Increase LMDB maxreaders if large number of threads in use

src/blockchain_db/lmdb/db_lmdb.cpp

@@ -34,6 +34,7 @@
 #include <cstring>  // memcpy
 #include <random>
 
+#include "common/util.h"
 #include "cryptonote_basic/cryptonote_format_utils.h"
 #include "crypto/crypto.h"
 #include "profile_tools.h"
@@ -1129,6 +1130,11 @@ void BlockchainLMDB::open(const std::string& filename, const int db_flags)
   if ((result = mdb_env_set_maxdbs(m_env, 20)))
     throw0(DB_ERROR(lmdb_error("Failed to set max number of dbs: ", result).c_str()));
 
+  int threads = tools::get_max_concurrency();
+  if (threads > 110 &&	/* maxreaders default is 126, leave some slots for other read processes */
+    (result = mdb_env_set_maxreaders(m_env, threads+16)))
+    throw0(DB_ERROR(lmdb_error("Failed to set max number of readers: ", result).c_str()));
+
   size_t mapsize = DEFAULT_MAPSIZE;
 
   if (db_flags & DBF_FAST)