Add shutdown hook interface to Lua API

Scripts can call minetest.register_on_shutdown() to register a
shutdown hook.

Document that minetest.register_on_shutdown() callbacks may not be run

If the server crashes, it is unlikely that callbacks registered using
minetest.register_on_shutdown() will be called.

builtin/misc_register.lua

@@ -303,6 +303,7 @@ end
 
 minetest.registered_on_chat_messages, minetest.register_on_chat_message = make_registration()
 minetest.registered_globalsteps, minetest.register_globalstep = make_registration()
+minetest.registered_on_shutdown, minetest.register_on_shutdown = make_registration()
 minetest.registered_on_punchnodes, minetest.register_on_punchnode = make_registration()
 minetest.registered_on_placenodes, minetest.register_on_placenode = make_registration()
 minetest.registered_on_dignodes, minetest.register_on_dignode = make_registration()

doc/lua_api.txt

@@ -792,6 +792,11 @@ minetest.register_craft(recipe)
 Global callback registration functions: (Call these only at load time)
 minetest.register_globalstep(func(dtime))
 ^ Called every server step, usually interval of 0.05s
+minetest.register_on_shutdown(func())
+^ Called before server shutdown
+^ WARNING: If the server terminates abnormally (i.e. crashes), the registered
+           callbacks WILL LIKELY NOT BE RUN.  Data should be saved at
+           semi-frequent intervals as well as on server shutdown.
 minetest.register_on_placenode(func(pos, newnode, placer, oldnode))
 ^ Called when a node has been placed
 ^ Deprecated: Use on_construct or after_place_node in node definition instead

src/scriptapi.cpp

@@ -5576,6 +5576,19 @@ bool scriptapi_on_chat_message(lua_State *L, const std::string &name,
 	return ate;
 }
 
+void scriptapi_on_shutdown(lua_State *L)
+{
+	realitycheck(L);
+	assert(lua_checkstack(L, 20));
+	StackUnroller stack_unroller(L);
+
+	// Get registered shutdown hooks
+	lua_getglobal(L, "minetest");
+	lua_getfield(L, -1, "registered_on_shutdown");
+	// Call callbacks
+	scriptapi_run_callbacks(L, 0, RUN_CALLBACKS_MODE_FIRST);
+}
+
 void scriptapi_on_newplayer(lua_State *L, ServerActiveObject *player)
 {
 	realitycheck(L);

src/scriptapi.h

@@ -55,6 +55,9 @@ void scriptapi_environment_step(lua_State *L, float dtime);
 void scriptapi_environment_on_generated(lua_State *L, v3s16 minp, v3s16 maxp,
 		u32 blockseed);
 
+/* server */
+void scriptapi_on_shutdown(lua_State *L);
+
 /* misc */
 void scriptapi_on_newplayer(lua_State *L, ServerActiveObject *player);
 void scriptapi_on_dieplayer(lua_State *L, ServerActiveObject *player);

src/server.cpp

@@ -1113,6 +1113,11 @@ Server::~Server()
 		}
 	}
 
+	/*
+		Execute script shutdown hooks
+	*/
+	scriptapi_on_shutdown(m_lua);
+	
 	{
 		JMutexAutoLock envlock(m_env_mutex);