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/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