Toggle SAFE syncmode on and off automatically

If monerod is started with default sync mode, set it to SAFE after
synchronization completes. Set it back to FAST if synchronization
restarts (e.g. because another peer has a longer blockchain).

If monerod is started with an explicit sync mode, none of this
automation takes effect.

src/blockchain_db/blockchain_db.h

@@ -605,6 +605,13 @@ public:
    */
   virtual void sync() = 0;
 
+  /**
+   * @brief toggle safe syncs for the DB
+   *
+   * Used to switch DBF_SAFE on or off after starting up with DBF_FAST.
+   */
+  virtual void safesyncmode(const bool onoff) = 0;
+
   /**
    * @brief Remove everything from the BlockchainDB
    *

src/blockchain_db/lmdb/db_lmdb.cpp

@@ -1318,6 +1318,11 @@ void BlockchainLMDB::sync()
   }
 }
 
+void BlockchainLMDB::safesyncmode(const bool onoff)
+{
+  mdb_env_set_flags(m_env, MDB_NOSYNC|MDB_MAPASYNC, !onoff);
+}
+
 void BlockchainLMDB::reset()
 {
   LOG_PRINT_L3("BlockchainLMDB::" << __func__);

src/blockchain_db/lmdb/db_lmdb.h

@@ -165,6 +165,8 @@ public:
 
   virtual void sync();
 
+  virtual void safesyncmode(const bool onoff);
+
   virtual void reset();
 
   virtual std::vector<std::string> get_filenames() const;

src/cryptonote_core/blockchain.cpp

@@ -126,7 +126,7 @@ static const uint64_t testnet_hard_fork_version_1_till = 624633;
 //------------------------------------------------------------------
 Blockchain::Blockchain(tx_memory_pool& tx_pool) :
   m_db(), m_tx_pool(tx_pool), m_hardfork(NULL), m_timestamps_and_difficulties_height(0), m_current_block_cumul_sz_limit(0),
-  m_enforce_dns_checkpoints(false), m_max_prepare_blocks_threads(4), m_db_blocks_per_sync(1), m_db_sync_mode(db_async), m_fast_sync(true), m_show_time_stats(false), m_sync_counter(0), m_cancel(false)
+  m_enforce_dns_checkpoints(false), m_max_prepare_blocks_threads(4), m_db_blocks_per_sync(1), m_db_sync_mode(db_async), m_db_default_sync(false), m_fast_sync(true), m_show_time_stats(false), m_sync_counter(0), m_cancel(false)
 {
   LOG_PRINT_L3("Blockchain::" << __func__);
 }
@@ -4034,12 +4034,29 @@ bool Blockchain::for_all_txpool_txes(std::function<bool(const crypto::hash&, con
 
 void Blockchain::set_user_options(uint64_t maxthreads, uint64_t blocks_per_sync, blockchain_db_sync_mode sync_mode, bool fast_sync)
 {
+  if (sync_mode == db_defaultsync)
+  {
+    m_db_default_sync = true;
+    sync_mode = db_async;
+  }
   m_db_sync_mode = sync_mode;
   m_fast_sync = fast_sync;
   m_db_blocks_per_sync = blocks_per_sync;
   m_max_prepare_blocks_threads = maxthreads;
 }
 
+void Blockchain::safesyncmode(const bool onoff)
+{
+  /* all of this is no-op'd if the user set a specific
+   * --db-sync-mode at startup.
+   */
+  if (m_db_default_sync)
+  {
+    m_db->safesyncmode(onoff);
+    m_db_sync_mode = onoff ? db_nosync : db_async;
+  }
+}
+
 HardFork::State Blockchain::get_hard_fork_state() const
 {
   return m_hardfork->get_state();

src/cryptonote_core/blockchain.h

@@ -65,6 +65,7 @@ namespace cryptonote
    */
   enum blockchain_db_sync_mode
   {
+    db_defaultsync, //!< user didn't specify, use db_async
     db_sync,  //!< handle syncing calls instead of the backing db, synchronously
     db_async, //!< handle syncing calls instead of the backing db, asynchronously
     db_nosync //!< Leave syncing up to the backing db (safest, but slowest because of disk I/O)
@@ -700,6 +701,11 @@ namespace cryptonote
     void set_user_options(uint64_t maxthreads, uint64_t blocks_per_sync,
         blockchain_db_sync_mode sync_mode, bool fast_sync);
 
+    /**
+     * @brief Put DB in safe sync mode
+     */
+    void safesyncmode(const bool onoff);
+
     /**
      * @brief set whether or not to show/print time statistics
      *
@@ -932,6 +938,7 @@ namespace cryptonote
     blockchain_db_sync_mode m_db_sync_mode;
     bool m_fast_sync;
     bool m_show_time_stats;
+    bool m_db_default_sync;
     uint64_t m_db_blocks_per_sync;
     uint64_t m_max_prepare_blocks_threads;
     uint64_t m_fake_pow_calc_time;

src/cryptonote_core/cryptonote_core.cpp

@@ -316,7 +316,7 @@ namespace cryptonote
 
     const std::string filename = folder.string();
     // default to fast:async:1
-    blockchain_db_sync_mode sync_mode = db_async;
+    blockchain_db_sync_mode sync_mode = db_defaultsync;
     uint64_t blocks_per_sync = 1;
 
     try
@@ -349,11 +349,15 @@ namespace cryptonote
           sync_mode = db_nosync;
         }
         else if(options[0] == "fast")
+        {
           db_flags = DBF_FAST;
+          sync_mode = db_async;
+        }
         else if(options[0] == "fastest")
         {
           db_flags = DBF_FASTEST;
           blocks_per_sync = 1000; // default to fastest:async:1000
+          sync_mode = db_async;
         }
         else
           db_flags = DEFAULT_FLAGS;

src/cryptonote_protocol/cryptonote_protocol_handler.inl

@@ -295,6 +295,7 @@ namespace cryptonote
       << " [Your node is " << std::abs(diff) << " blocks (" << ((abs(diff) - diff_v2) / (24 * 60 * 60 / DIFFICULTY_TARGET_V1)) + (diff_v2 / (24 * 60 * 60 / DIFFICULTY_TARGET_V2)) << " days) "
       << (0 <= diff ? std::string("behind") : std::string("ahead"))
       << "] " << ENDL << "SYNCHRONIZATION started");
+      m_core.get_blockchain_storage().safesyncmode(false);
     }
     LOG_PRINT_L1("Remote blockchain height: " << hshd.current_height << ", id: " << hshd.top_id);
     context.m_state = cryptonote_connection_context::state_synchronizing;
@@ -1473,6 +1474,7 @@ skip:
         << "**********************************************************************");
       m_core.on_synchronized();
     }
+    m_core.get_blockchain_storage().safesyncmode(true);
     return true;
   }
   //------------------------------------------------------------------------------------------------------------------------

tests/unit_tests/hardfork.cpp

@@ -47,6 +47,7 @@ public:
   virtual void open(const std::string& filename, const int db_flags = 0) { }
   virtual void close() {}
   virtual void sync() {}
+  virtual void safesyncmode(const bool onoff) {}
   virtual void reset() {}
   virtual std::vector<std::string> get_filenames() const { return std::vector<std::string>(); }
   virtual std::string get_db_name() const { return std::string(); }