JSON: Support consistent larger max. depth of 1024

games/devtest/mods/unittests/misc.lua

@@ -174,6 +174,21 @@ local function test_parse_json()
 end
 unittests.register("test_parse_json", test_parse_json)
 
+local function test_write_json()
+	-- deeply nested structures should be preserved
+	local leaf = 42
+	local data = leaf
+	for i = 1, 1000 do
+		data = {data}
+	end
+	local roundtripped = minetest.parse_json(minetest.write_json(data))
+	for i = 1, 1000 do
+		roundtripped = roundtripped[1]
+	end
+	assert(roundtripped == 42)
+end
+unittests.register("test_write_json", test_write_json)
+
 local function test_game_info()
 	local info = minetest.get_game_info()
 	local game_conf = Settings(info.path .. "/game.conf")

src/script/common/c_content.cpp

@@ -2149,11 +2149,11 @@ bool push_json_value(lua_State *L, const Json::Value &value, int nullindex)
 }
 
 // Converts Lua table --> JSON
-void read_json_value(lua_State *L, Json::Value &root, int index, u8 recursion)
+void read_json_value(lua_State *L, Json::Value &root, int index, u16 max_depth)
 {
-	if (recursion > 16) {
+	if (max_depth == 0)
-		throw SerializationError("Maximum recursion depth exceeded");
+		throw SerializationError("depth exceeds MAX_JSON_DEPTH");
-	}
+
 	int type = lua_type(L, index);
 	if (type == LUA_TBOOLEAN) {
 		root = (bool) lua_toboolean(L, index);
@@ -2164,11 +2164,13 @@ void read_json_value(lua_State *L, Json::Value &root, int index, u8 recursion)
 		const char *str = lua_tolstring(L, index, &len);
 		root = std::string(str, len);
 	} else if (type == LUA_TTABLE) {
+		// Reserve two slots for key and value.
+		lua_checkstack(L, 2);
 		lua_pushnil(L);
 		while (lua_next(L, index)) {
 			// Key is at -2 and value is at -1
 			Json::Value value;
-			read_json_value(L, value, lua_gettop(L), recursion + 1);
+			read_json_value(L, value, lua_gettop(L), max_depth - 1);
 
 			Json::ValueType roottype = root.type();
 			int keytype = lua_type(L, -1);

src/script/common/c_content.h

@@ -200,7 +200,7 @@ bool               push_json_value           (lua_State *L,
                                               const Json::Value &value,
                                               int nullindex);
 void               read_json_value           (lua_State *L, Json::Value &root,
-                                              int index, u8 recursion = 0);
+                                              int index, u16 max_depth);
 
 /*!
  * Pushes a Lua `pointed_thing` to the given Lua stack.

src/script/lua_api/l_util.cpp

@@ -93,6 +93,10 @@ int ModApiUtil::l_get_us_time(lua_State *L)
 	return 1;
 }
 
+// Maximum depth of a JSON object:
+// Reading and writing should not overflow the Lua, C, or jsoncpp stacks.
+constexpr static u16 MAX_JSON_DEPTH = 1024;
+
 // parse_json(str[, nullvalue, return_error])
 int ModApiUtil::l_parse_json(lua_State *L)
 {
@@ -129,10 +133,11 @@ int ModApiUtil::l_parse_json(lua_State *L)
 	Json::Value root;
 	{
 		Json::CharReaderBuilder builder;
+		builder.settings_["stackLimit"] = MAX_JSON_DEPTH;
 		builder.settings_["collectComments"] = false;
 		const std::unique_ptr<Json::CharReader> reader(builder.newCharReader());
 		std::string errmsg;
-		if (!reader->parse(jsonstr.begin(), jsonstr.end(), &root, &errmsg))
+		if (!reader->parse(jsonstr.data(), jsonstr.data() + jsonstr.size(), &root, &errmsg))
 			return handle_error(errmsg.c_str());
 	}
 
@@ -155,7 +160,7 @@ int ModApiUtil::l_write_json(lua_State *L)
 
 	Json::Value root;
 	try {
-		read_json_value(L, root, 1);
+		read_json_value(L, root, 1, MAX_JSON_DEPTH);
 	} catch (SerializationError &e) {
 		lua_pushnil(L);
 		lua_pushstring(L, e.what());