Re: [Tarantool-patches] [PATCH v5 5/6] box/cbox: implement cbox Lua module

[Vladislav Shpilevoy <v.shpilevoy@tarantool.org>]

>>> +
>>> +	int coro_ref = luaL_ref(tarantool_L, LUA_REGISTRYINDEX);
>>> +	lua_xmove(L, args_L, lua_gettop(L) - 1);
>>
>> 2. Why didn't you use lua_remove() to get rid of the unnecessary
>> parts of the stack?
> 
> Which one you think is redundant? The object itself? If so then
> it sits as a bottom element of the stack and removing it would
> simply make code slower.

Yes, I see the problem now. First issue is that you can remove
the root argument, because it may triggers its GC. Second issue
is that you still need luaT_newthread(), because port_lua uses
tarantool_L. Both issues can be easily fixed in port_lua.

https://github.com/tarantool/tarantool/issues/5406

> Vlad this "prepare environment for calling" part of code is very
> sensitive to the arguments, I tried a various combinations in
> attempts to get rid of lua thread and none of them work. So I
> left it as is and put a FIXME, because it requires more serious
> rework.

FIXME don't work, they are never fixed after creation unless you
have a ticket. Although these times even a ticket tends to be sent
to the wishlist for years.

>>> +
>>> +	lua_pushstring(L, "__call");
>>> +	lua_pushcfunction(L, lcbox_func_call);
>>> +	lua_settable(L, -3);
>>> +
>>> +	lua_setmetatable(L, -2);
>>
>> 3. What happens when the function object is deleted by Lua GC?
>> It seems the user can't get the function again without dropping
>> it.
> 
> And that is a good question. I must confess I missed Lua GC aspect
> here. Thanks a huge! I'll rework!

Another thing to look at - on each call you perform a lookup in the
function hash. But you don't really need it. You can store the
function pointer in a hidden field. For example, in __index metatable
of the function object. Or inside the function object with _ prefix,
so at least it won't be visible for autocompletion. Or turn the
function object into cdata. Anyway you will need it to properly
implement a GC handler. And this cdata would be a struct
lbox_sym/lbox_func/whatever with the function pointer in it.