daemon: allow printing N blocks from the end of the chain

It's a very common usage (for my anyway) and avoids the need to
get the current height, paste, subtract one, etc

src/daemon/command_parser_executor.cpp

@@ -154,6 +154,16 @@ bool t_command_parser_executor::print_blockchain_info(const std::vector<std::str
   }
   uint64_t start_index = 0;
   uint64_t end_index = 0;
+  if (args[0][0] == '-')
+  {
+    int64_t nblocks;
+    if(!epee::string_tools::get_xtype_from_string(nblocks, args[0]))
+    {
+      std::cout << "wrong number of blocks" << std::endl;
+      return false;
+    }
+    return m_executor.print_blockchain_info(nblocks, (uint64_t)-nblocks);
+  }
   if(!epee::string_tools::get_xtype_from_string(start_index, args[0]))
   {
     std::cout << "wrong starter block index parameter" << std::endl;

src/daemon/rpc_command_executor.cpp

@@ -743,17 +743,46 @@ bool t_rpc_command_executor::print_net_stats()
   return true;
 }
 
-bool t_rpc_command_executor::print_blockchain_info(uint64_t start_block_index, uint64_t end_block_index) {
+bool t_rpc_command_executor::print_blockchain_info(int64_t start_block_index, uint64_t end_block_index) {
   cryptonote::COMMAND_RPC_GET_BLOCK_HEADERS_RANGE::request req;
   cryptonote::COMMAND_RPC_GET_BLOCK_HEADERS_RANGE::response res;
   epee::json_rpc::error error_resp;
+  std::string fail_message = "Problem fetching info";
+
+  // negative: relative to the end
+  if (start_block_index < 0)
+  {
+    cryptonote::COMMAND_RPC_GET_INFO::request ireq;
+    cryptonote::COMMAND_RPC_GET_INFO::response ires;
+    if (m_is_rpc)
+    {
+      if (!m_rpc_client->rpc_request(ireq, ires, "/getinfo", fail_message.c_str()))
+      {
+        return true;
+      }
+    }
+    else
+    {
+      if (!m_rpc_server->on_get_info(ireq, ires) || ires.status != CORE_RPC_STATUS_OK)
+      {
+        tools::fail_msg_writer() << make_error(fail_message, ires.status);
+        return true;
+      }
+    }
+    if (start_block_index < 0 && (uint64_t)-start_block_index >= ires.height)
+    {
+      tools::fail_msg_writer() << "start offset is larger than blockchain height";
+      return true;
+    }
+    start_block_index = ires.height + start_block_index;
+    end_block_index = start_block_index + end_block_index - 1;
+  }
 
   req.start_height = start_block_index;
   req.end_height = end_block_index;
   req.fill_pow_hash = false;
 
-  std::string fail_message = "Unsuccessful";
-
+  fail_message = "Failed calling getblockheadersrange";
   if (m_is_rpc)
   {
     if (!m_rpc_client->json_rpc_request(req, res, "getblockheadersrange", fail_message.c_str()))

src/daemon/rpc_command_executor.h

@@ -85,7 +85,7 @@ public:
 
   bool print_connections();
 
-  bool print_blockchain_info(uint64_t start_block_index, uint64_t end_block_index);
+  bool print_blockchain_info(int64_t start_block_index, uint64_t end_block_index);
 
   bool set_log_level(int8_t level);