blockchain: add --reorg-notify

This will trigger if a reorg is seen. This may be used to do things
like stop automated withdrawals on large reorgs.

%s is replaced by the height at the split point
%h is replaced by the height of the new chain
%n is replaced by the number of new blocks after the reorg

src/cryptonote_core/blockchain.cpp

@@ -1073,6 +1073,11 @@ bool Blockchain::switch_to_alternative_blockchain(std::list<blocks_ext_by_hash::
 
   m_hardfork->reorganize_from_chain_height(split_height);
 
+  std::shared_ptr<tools::Notify> reorg_notify = m_reorg_notify;
+  if (reorg_notify)
+    reorg_notify->notify("%s", std::to_string(split_height).c_str(), "%h", std::to_string(m_db->height()).c_str(),
+        "%n", std::to_string(m_db->height() - split_height).c_str(), NULL);
+
   MGINFO_GREEN("REORGANIZE SUCCESS! on height: " << split_height << ", new blockchain size: " << m_db->height());
   return true;
 }

src/cryptonote_core/blockchain.h

@@ -731,10 +731,17 @@ namespace cryptonote
     /**
      * @brief sets a block notify object to call for every new block
      *
-     * @param notify the notify object to cal at every new block
+     * @param notify the notify object to call at every new block
      */
     void set_block_notify(const std::shared_ptr<tools::Notify> &notify) { m_block_notify = notify; }
 
+    /**
+     * @brief sets a reorg notify object to call for every reorg
+     *
+     * @param notify the notify object to call at every reorg
+     */
+    void set_reorg_notify(const std::shared_ptr<tools::Notify> &notify) { m_reorg_notify = notify; }
+
     /**
      * @brief Put DB in safe sync mode
      */
@@ -1077,6 +1084,7 @@ namespace cryptonote
     bool m_btc_valid;
 
     std::shared_ptr<tools::Notify> m_block_notify;
+    std::shared_ptr<tools::Notify> m_reorg_notify;
 
     /**
      * @brief collects the keys for all outputs being "spent" as an input

src/cryptonote_core/cryptonote_core.cpp

@@ -185,6 +185,13 @@ namespace cryptonote
   , "Prune blockchain"
   , false
   };
+  static const command_line::arg_descriptor<std::string> arg_reorg_notify = {
+    "reorg-notify"
+  , "Run a program for each reorg, '%s' will be replaced by the split height, "
+    "'%h' will be replaced by the new blockchain height, and '%n' will be "
+    "replaced by the number of new blocks in the new chain"
+  , ""
+  };
 
   //-----------------------------------------------------------------------------------------------
   core::core(i_cryptonote_protocol* pprotocol):
@@ -300,6 +307,7 @@ namespace cryptonote
     command_line::add_arg(desc, arg_pad_transactions);
     command_line::add_arg(desc, arg_block_notify);
     command_line::add_arg(desc, arg_prune_blockchain);
+    command_line::add_arg(desc, arg_reorg_notify);
 
     miner::init_options(desc);
     BlockchainDB::init_options(desc);
@@ -582,6 +590,16 @@ namespace cryptonote
       MERROR("Failed to parse block notify spec");
     }
 
+    try
+    {
+      if (!command_line::is_arg_defaulted(vm, arg_reorg_notify))
+        m_blockchain_storage.set_reorg_notify(std::shared_ptr<tools::Notify>(new tools::Notify(command_line::get_arg(vm, arg_reorg_notify).c_str())));
+    }
+    catch (const std::exception &e)
+    {
+      MERROR("Failed to parse reorg notify spec");
+    }
+
     const std::pair<uint8_t, uint64_t> regtest_hard_forks[3] = {std::make_pair(1, 0), std::make_pair(Blockchain::get_hard_fork_heights(MAINNET).back().version, 1), std::make_pair(0, 0)};
     const cryptonote::test_options regtest_test_options = {
       regtest_hard_forks