`setmetatable' cannot change protected objects

lauxlib.c

@@ -1,5 +1,5 @@
 /*
-** $Id: lauxlib.c,v 1.84 2002/08/30 20:00:59 roberto Exp roberto $
+** $Id: lauxlib.c,v 1.85 2002/09/05 19:45:42 roberto Exp roberto $
 ** Auxiliary functions for building Lua libraries
 ** See Copyright Notice in lua.h
 */
@@ -147,7 +147,7 @@ LUALIB_API lua_Number luaL_opt_number (lua_State *L, int narg, lua_Number def) {
 }
 
 
-LUALIB_API int luaL_callmeta (lua_State *L, int obj, const char *event) {
+LUALIB_API int luaL_getmetafield (lua_State *L, int obj, const char *event) {
   if (!lua_getmetatable(L, obj))  /* no metatable? */
     return 0;
   lua_pushstring(L, event);
@@ -156,6 +156,13 @@ LUALIB_API int luaL_callmeta (lua_State *L, int obj, const char *event) {
     lua_pop(L, 2);  /* remove metatable and metafield */
     return 0;
   }
+  return 1;
+}
+
+
+LUALIB_API int luaL_callmeta (lua_State *L, int obj, const char *event) {
+  if (!luaL_getmetafield(L, obj, event))  /* no metafield? */
+    return 0;
   lua_pushvalue(L, obj);
   lua_call(L, 1, 1);
   return 1;

lauxlib.h

@@ -1,5 +1,5 @@
 /*
-** $Id: lauxlib.h,v 1.52 2002/08/08 20:08:41 roberto Exp roberto $
+** $Id: lauxlib.h,v 1.53 2002/08/30 20:00:59 roberto Exp roberto $
 ** Auxiliary functions for building Lua libraries
 ** See Copyright Notice in lua.h
 */
@@ -30,6 +30,7 @@ typedef struct luaL_reg {
 LUALIB_API void luaL_openlib (lua_State *L, const luaL_reg *l, int nup);
 LUALIB_API void luaL_opennamedlib (lua_State *L, const char *libname,
                                    const luaL_reg *l, int nup);
+LUALIB_API int luaL_getmetafield (lua_State *L, int obj, const char *e);
 LUALIB_API int luaL_callmeta (lua_State *L, int obj, const char *e);
 LUALIB_API int luaL_typerror (lua_State *L, int narg, const char *tname);
 LUALIB_API int luaL_argerror (lua_State *L, int numarg, const char *extramsg);

lbaselib.c

@@ -1,5 +1,5 @@
 /*
-** $Id: lbaselib.c,v 1.97 2002/08/08 20:08:41 roberto Exp roberto $
+** $Id: lbaselib.c,v 1.98 2002/09/05 19:45:42 roberto Exp roberto $
 ** Basic library
 ** See Copyright Notice in lua.h
 */
@@ -109,14 +109,8 @@ static int luaB_getmetatable (lua_State *L) {
     lua_pushnil(L);
     return 1;  /* no metatable */
   }
-  else {
-    lua_pushliteral(L, "__metatable");
-    lua_rawget(L, -2);
-    if (lua_isnil(L, -1))
-      lua_pop(L, 1);
-    /* otherwise returns metatable.__metatable */
-  }
-  return 1;
+  luaL_getmetafield(L, 1, "__metatable");
+  return 1;  /* returns either __metatable field (if present) or metatable */
 }
 
 
@@ -125,6 +119,8 @@ static int luaB_setmetatable (lua_State *L) {
   luaL_check_type(L, 1, LUA_TTABLE);
   luaL_arg_check(L, t == LUA_TNIL || t == LUA_TTABLE, 2,
                     "nil or table expected");
+  if (luaL_getmetafield(L, 1, "__metatable"))
+    luaL_error(L, "cannot change a protected metatable");
   lua_settop(L, 2);
   lua_setmetatable(L, 1);
   return 1;