查看文件scripting.c中的源代码

/* This function installs metamethods in the global table _G that prevent
 * the creation of globals accidentally.
 *
 * It should be the last to be called in the scripting engine initialization
 * sequence, because it may interact with creation of globals. */
void scriptingEnableGlobalsProtection(lua_State *lua) {
    char *s[32];
    sds code = sdsempty();
    int j = 0;

    /* strict.lua from: http://metalua.luaforge.net/src/lib/strict.lua.html.
     * Modified to be adapted to Redis. */
    s[j++]="local mt = {}\n";
    s[j++]="setmetatable(_G, mt)\n";
    s[j++]="mt.__newindex = function (t, n, v)\n";
    s[j++]="  if debug.getinfo(2) then\n";
    s[j++]="    local w = debug.getinfo(2, \"S\").what\n";
    s[j++]="    if w ~= \"main\" and w ~= \"C\" then\n";
    s[j++]="      error(\"Script attempted to create global variable '\"..tostring(n)..\"'\", 2)\n";
    s[j++]="    end\n";
    s[j++]="  end\n";
    s[j++]="  rawset(t, n, v)\n";
    s[j++]="end\n";
    s[j++]="mt.__index = function (t, n)\n";
    s[j++]="  if debug.getinfo(2) and debug.getinfo(2, \"S\").what ~= \"C\" then\n";
    s[j++]="    error(\"Script attempted to access unexisting global variable '\"..tostring(n)..\"'\", 2)\n";
    s[j++]="  end\n";
    s[j++]="  return rawget(t, n)\n";
    s[j++]="end\n";
    s[j++]=NULL;

    for (j = 0; s[j] != NULL; j++) code = sdscatlen(code,s[j],strlen(s[j]));
    luaL_loadbuffer(lua,code,sdslen(code),"@enable_strict_lua");
    lua_pcall(lua,0,0,0);
    sdsfree(code);
}

的doc字符串scriptingEnableGlobalsProtection表示意图是将常见错误通知脚本作者（不使用local）。

看来这不是安全功能，所以我们有两个解决方案：

可以删除此保护：

local mt = setmetatable(_G, nil)
-- define global functions / variables
function alex() return 3.1415 end
-- return globals protection mechanizm
setmetatable(_G, mt)

或使用rawset：

local function alex() return 3.1415 end
rawset(_G, "alex", alex)