lua: luaT_tolstring: keep Lua stack size unchanged

Totktonada · Totktonada · commit fa0f10cb0b47 · 2020-11-03T19:17:36.000+03:00
I found it counter-intuitive to change a Lua stack in a function that
returns its result as the function return value. It complicates
addressing of existing Lua stack elements using negative indices
(relatively to the stack top). In fact, the amount of those temporary
stack elements is the implementation detail. A couple of dubious
examples:

- Whether the function adds an extra element when the given value is
  already a string?
- How much extra elements is added, when the __tostring metamethod
  returns several values?

The original luaL_tostring() function from Lua 5.2 also left the
temporary elements on the stack, but we're not obligated to keep
bug-to-bug compatibility in a function of the luaT namespace.
diff --git a/src/lua/utils.c b/src/lua/utils.c
@@ -1040,6 +1040,7 @@ luaT_cpcall(lua_State *L, lua_CFunction func, void *ud)
 const char *
 luaT_tolstring(lua_State *L, int idx, size_t *len)
 {
+	int top = lua_gettop(L);
 	if (!luaL_callmeta(L, idx, "__tostring")) {
 		switch (lua_type(L, idx)) {
 		case LUA_TNUMBER:
@@ -1060,7 +1061,9 @@ luaT_tolstring(lua_State *L, int idx, size_t *len)
 		}
 	}
 
-	return lua_tolstring(L, -1, len);
+	const char *res = lua_tolstring(L, -1, len);
+	lua_settop(L, top);
+	return res;
 }
 
 /* Based on ffi_meta___call() from luajit/src/lib_ffi.c. */
diff --git a/test/app-tap/module_api.c b/test/app-tap/module_api.c
@@ -1986,6 +1986,10 @@ static int table_tostring(lua_State *L) {
 static int
 test_tostring(lua_State *L)
 {
+	size_t len = 0;
+	const char *res = NULL;
+	int top;
+
 	/* original table */
 	lua_createtable(L, 0, 0);
 	/* meta-table */
@@ -2012,6 +2016,21 @@ test_tostring(lua_State *L)
 	lua_pushnil(L);
 	assert(strcmp(luaT_tolstring(L, -1, NULL), "nil") == 0);
 
+	/*
+	 * Verify a non-top string element.
+	 *
+	 * Check the length output value. Check that the Lua stack
+	 * size remains unchanged.
+	 */
+	lua_pushliteral(L, "foo");
+	lua_pushnil(L);
+	top = lua_gettop(L);
+	res = luaT_tolstring(L, -2, &len);
+	assert(strcmp(res, "foo") == 0);
+	assert(len == 3);
+	assert(lua_gettop(L) - top == 0);
+	lua_pop(L, 2);
+
 	lua_pushboolean(L, true);
 	return 1;
 }

Original file line number	Diff line number	Diff line change
`@@ -1040,6 +1040,7 @@ luaT_cpcall(lua_State L, lua_CFunction func, void ud)`
`1040`	`1040`	`const char *`
`1041`	`1041`	`luaT_tolstring(lua_State L, int idx, size_t len)`
`1042`	`1042`	`{`
	`1043`	`+ int top = lua_gettop(L);`
`1043`	`1044`	`if (!luaL_callmeta(L, idx, "__tostring")) {`
`1044`	`1045`	`switch (lua_type(L, idx)) {`
`1045`	`1046`	`case LUA_TNUMBER:`
`@@ -1060,7 +1061,9 @@ luaT_tolstring(lua_State L, int idx, size_t len)`
`1060`	`1061`	`}`
`1061`	`1062`	`}`
`1062`	`1063`
`1063`		`- return lua_tolstring(L, -1, len);`
	`1064`	`+ const char *res = lua_tolstring(L, -1, len);`
	`1065`	`+ lua_settop(L, top);`
	`1066`	`+ return res;`
`1064`	`1067`	`}`
`1065`	`1068`
`1066`	`1069`	`/* Based on ffi_meta___call() from luajit/src/lib_ffi.c. */`