[Tarantool-patches] [PATCH rfc] lua: expose luaL_serializer to Lua

[Olga Arkhangelskaia <arkholga@tarantool.org>]

There are several modules in Lua allowing to transform data from one format to
another (yaml, msgpack, json). They are configured via cfg method. The options
are parts of struct luaL_serializer object in C. That method used to be
implemented via additional table on Lua stack and overriden __call metamethod.
Such implementation had hudge shortcoming: any field could be
changed/added via raw modification. And such change was not visible for
tarantool.
To leave the only one source of information/configuration possibility cfg table
was changed to userdata with index, newindex, pairs, call and serialize
metamethods. And the interface was remained unchanged.
Closes #4521
---
Branch: https://github.com/tarantool/tarantool/tree/OKriw/gh-4521-lua-serializers-should-use-FFI-to-check-and-set-options
 src/lua/utils.c | 193 +++++++++++++++++++++++++++++++++++++++---------
 1 file changed, 160 insertions(+), 33 deletions(-)

diff --git a/src/lua/utils.c b/src/lua/utils.c
index de14c778c..7bb6966b7 100644
--- a/src/lua/utils.c
+++ b/src/lua/utils.c
@@ -311,34 +311,171 @@ luaL_serializer_parse_options(struct lua_State *L,
 }
 
 /**
- * @brief serializer.cfg{} Lua binding for serializers.
- * serializer.cfg is a table that contains current configuration values from
- * luaL_serializer structure. serializer.cfg has overriden __call() method
- * to change configuration keys in internal userdata (like box.cfg{}).
- * Please note that direct change in serializer.cfg.key will not affect
- * internal state of userdata. Changes via cfg() are reflected in
- * both Lua cfg table, and C serializer structure.
+ * @brief serializer.cfg() Lua binding for serializers.
+ * serializer.cfg is a userdata that contains pointer to struct luaL_serializer.
+ * It has __call metamethod, that directly updates  internal state of userdata.
  * @param L lua stack
  * @return 0
  */
 static int
 luaL_serializer_cfg(struct lua_State *L)
 {
-	/* Serializer.cfg */
-	luaL_checktype(L, 1, LUA_TTABLE);
 	/* Updated parameters. */
 	luaL_checktype(L, 2, LUA_TTABLE);
 	struct luaL_serializer *cfg = luaL_checkserializer(L);
 	for (int i = 0; OPTIONS[i].name != NULL; ++i) {
-		if (luaL_serializer_parse_option(L, i, cfg) == NULL)
+		if(luaL_serializer_parse_option(L, i, cfg) == NULL)
 			lua_pop(L, 1);
-		else
-			lua_setfield(L, 1, OPTIONS[i].name);
-	}
+        }
 	trigger_run(&cfg->on_update, cfg);
 	return 0;
 }
 
+/**
+ * @brief __serialize metamethod for serializer.cfg.
+ * Converts userdata to table for console output.
+ * @param L lua stack
+ * @return 0
+ */
+static int
+luaL_serialize_info(lua_State *L)
+{
+    struct luaL_serializer *cfg =  lua_touserdata(L, lua_upvalueindex(1));
+    lua_newtable(L);
+    for(int i = 0; OPTIONS[i].name != NULL; i++){
+        lua_pushstring(L, OPTIONS[i].name);
+        int *pval = (int *) ((char *) cfg + OPTIONS[i].offset);
+        switch (OPTIONS[i].type) {
+        case LUA_TBOOLEAN:
+            lua_pushboolean(L, *pval);
+            break;
+        case LUA_TNUMBER:
+            lua_pushnumber(L, *pval);
+            break;
+        }
+        lua_settable(L, -3);
+    }
+	return 1;
+}
+
+static int
+luaL_serializer_name_to_pos(const char *name)
+{
+	if (name == NULL)
+		return -1;
+	for(int i = 0; OPTIONS[i].name != NULL; i++) {
+		if(strcmp(OPTIONS[i].name, name) == 0) {
+			return i;
+		}
+	}
+	return -2;
+}
+
+/**
+ * @brief __index metamethod for serializer.cfg.
+ * @param L lua stack
+ * @return 0
+ */
+static int
+luaL_serializer_cfg_index(lua_State *L)
+{
+	struct luaL_serializer *cfg = luaL_checkserializer(L);
+	const char* name = lua_tostring(L, 2);
+	int i = luaL_serializer_name_to_pos(name);
+	if (i > -1) {
+		int *pval = (int *) ((char *) cfg + OPTIONS[i].offset);
+		switch (OPTIONS[i].type){
+		case LUA_TBOOLEAN:
+			lua_pushboolean(L, *pval);
+			break;
+		case LUA_TNUMBER:
+			lua_pushnumber(L, *pval);
+			break;
+		}
+	} else  if (strcmp(name, "__serialize") == 0) {
+			lua_pushlightuserdata(L, cfg);
+			lua_pushcclosure(L, luaL_serialize_info, 1);
+			return 1;
+	} else {
+			lua_pushnil(L);
+			return 1;
+	}
+	return 1;
+}
+
+/**
+ * @brief __newindex metamethod for serializer.cfg.
+ * protects serializer.cfg fro, row modification.
+ * @param L lua stack
+ * @return 0
+ */
+static int
+luaL_serializer_cfg_newindex(lua_State *L)
+{
+	luaL_error(L, "incorrect way of setting configuration, please use cfg()");
+	printf("__newindex \n");
+	return 1;
+}
+
+/**
+ * @brief lua_next for userdata.
+ * @param L lua stack
+ * @return 0
+ */
+static int
+luaL_serializer_next(lua_State *L)
+{
+	luaL_checktype(L, 1, LUA_TLIGHTUSERDATA);
+	struct luaL_serializer *cfg = (struct luaL_serializer*)lua_touserdata(L, 1);
+	const char *name = lua_tostring(L, 2);
+	int j = luaL_serializer_name_to_pos(name);
+	if (j > -2 && OPTIONS[j+1].name != NULL) {
+		lua_pushstring(L, OPTIONS[j+1].name);
+		int *pval = (int *) ((char *) cfg + OPTIONS[j+1].offset);
+		switch (OPTIONS[j+1].type){
+		case LUA_TBOOLEAN:
+			lua_pushboolean(L, *pval);
+			break;
+		case LUA_TNUMBER:
+			lua_pushnumber(L, *pval);
+			break;
+		}
+		return 2;
+	} else
+		lua_pushnil(L);
+	return 1;
+}
+
+/**
+ * @brief __pairs metamethod for serializer.cfg.
+ * Traverses userdata like a table.
+ * @param L lua stack
+ * @return 0
+ */
+static int
+luaL_serializer_cfg_pairs(lua_State *L)
+{
+	struct luaL_serializer *cfg = luaL_checkserializer(L);
+	lua_pushcfunction(L, luaL_serializer_next);
+	lua_pushlightuserdata(L, cfg);
+	lua_pushstring(L, NULL);
+	return 3;
+}
+
+/**
+ * To get configure/change/get setting of serializer
+ * we directly write to struct luaL_serializer using
+ * userdata and  metatable desctibed here.
+ */
+static const luaL_Reg serializermeta[] = {
+        {"__serialize", luaL_serialize_info},
+        {"__index",luaL_serializer_cfg_index},
+        {"__newindex",luaL_serializer_cfg_newindex},
+        {"__call", luaL_serializer_cfg},
+	{"__pairs", luaL_serializer_cfg_pairs},
+		{NULL, NULL},
+	};
+
 /**
  * @brief serializer.new() Lua binding.
  * @param L stack
@@ -369,28 +506,17 @@ luaL_newserializer(struct lua_State *L, const char *modname, const luaL_Reg *reg
 		lua_setfield(L, -3, reg->name);
 	}
 
-	/* Add cfg{} */
+	/* Add cfg userdata with pointer */
+	lua_newuserdata(L, sizeof(*serializer));
+	/* Add cfg methods */
 	lua_newtable(L); /* cfg */
-	lua_newtable(L); /* metatable */
-	lua_pushvalue(L, -3); /* luaL_serializer */
-	lua_pushcclosure(L, luaL_serializer_cfg, 1);
-	lua_setfield(L, -2, "__call");
-	lua_setmetatable(L, -2);
-	/* Save configuration values to serializer.cfg */
-	for (int i = 0; OPTIONS[i].name != NULL; i++) {
-		int *pval = (int *) ((char *) serializer + OPTIONS[i].offset);
-		switch (OPTIONS[i].type) {
-		case LUA_TBOOLEAN:
-			lua_pushboolean(L, *pval);
-			break;
-		case LUA_TNUMBER:
-			lua_pushinteger(L, *pval);
-			break;
-		default:
-			unreachable();
-		}
-		lua_setfield(L, -2, OPTIONS[i].name);
+	const luaL_Reg *ser_info = serializermeta;
+	for (; ser_info->name != NULL; ser_info++) {
+		lua_pushvalue(L, -3);
+		lua_pushcclosure(L, ser_info->func, 1);
+		lua_setfield(L, -2, ser_info->name);
 	}
+	lua_setmetatable(L, -2);
 	lua_setfield(L, -3, "cfg");
 
 	lua_pop(L, 1);  /* remove upvalues */
@@ -421,6 +547,7 @@ lua_gettable_wrapper(lua_State *L)
 	return 1;
 }
 
+
 static void
 lua_field_inspect_ucdata(struct lua_State *L, struct luaL_serializer *cfg,
 			int idx, struct luaL_field *field)
-- 
2.20.1 (Apple Git-117)