core: prevent the database from being used by multiple daemons

A boost lock is used to determine whether more than one process
wants to access the database. The boost file_lock doesn't seem
to like locking directories, so we use an arbitrary file in it.
This allows to still run two daemons if they have different
database directories (ie, LMDB/BDB, different data directories).

src/cryptonote_core/cryptonote_core.cpp

@@ -218,6 +218,39 @@ namespace cryptonote
     return m_blockchain_storage.get_alternative_blocks_count();
   }
   //-----------------------------------------------------------------------------------------------
+  bool core::lock_db_directory(const boost::filesystem::path &path)
+  {
+    // boost doesn't like locking directories...
+    const boost::filesystem::path lock_path = path / ".daemon_lock";
+
+    try
+    {
+      // ensure the file exists
+      std::ofstream(lock_path.string(), std::ios::out).close();
+
+      db_lock = boost::interprocess::file_lock(lock_path.string().c_str());
+      LOG_PRINT_L1("Locking " << lock_path.string());
+      if (!db_lock.try_lock())
+      {
+        LOG_PRINT_L0("Failed to lock " << lock_path.string());
+        return false;
+      }
+      return true;
+    }
+    catch (const std::exception &e)
+    {
+      LOG_PRINT_L0("Error trying to lock " << lock_path.string() << ": " << e.what());
+      return false;
+    }
+  }
+  //-----------------------------------------------------------------------------------------------
+  bool core::unlock_db_directory()
+  {
+    db_lock.unlock();
+    db_lock = boost::interprocess::file_lock();
+    return true;
+  }
+  //-----------------------------------------------------------------------------------------------
   bool core::init(const boost::program_options::variables_map& vm)
   {
     bool r = handle_command_line(vm);
@@ -280,6 +313,13 @@ namespace cryptonote
     folder /= db->get_db_name();
     LOG_PRINT_L0("Loading blockchain from folder " << folder.string() << " ...");
 
+    if (!lock_db_directory (folder))
+    {
+      LOG_ERROR ("Failed to lock " << folder);
+      delete db;
+      return false;
+    }
+
     const std::string filename = folder.string();
     // temporarily default to fastest:async:1000
     blockchain_db_sync_mode sync_mode = db_async;
@@ -398,6 +438,7 @@ namespace cryptonote
     {
       m_blockchain_storage.deinit();
     }
+    unlock_db_directory();
     return true;
   }
   //-----------------------------------------------------------------------------------------------

src/cryptonote_core/cryptonote_core.h

@@ -34,6 +34,7 @@
 
 #include <boost/program_options/options_description.hpp>
 #include <boost/program_options/variables_map.hpp>
+#include <boost/interprocess/sync/file_lock.hpp>
 
 #include "p2p/net_node_common.h"
 #include "cryptonote_protocol/cryptonote_protocol_handler_common.h"
@@ -172,6 +173,8 @@ namespace cryptonote
      void graceful_exit();
      bool check_fork_time();
      bool relay_txpool_transactions();
+     bool lock_db_directory(const boost::filesystem::path &path);
+     bool unlock_db_directory();
 
      static std::atomic<bool> m_fast_exit;
      bool m_test_drop_download = true;
@@ -205,6 +208,8 @@ namespace cryptonote
      time_t m_last_json_checkpoints_update;
 
      std::atomic_flag m_checkpoints_updating;
+
+     boost::interprocess::file_lock db_lock;
    };
 }