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

[Cyrill Gorcunov <gorcunov@gmail.com>]

On Fri, Oct 09, 2020 at 11:31:12PM +0200, Vladislav Shpilevoy wrote:
> >>> +
> >>> +	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

Thanks for making an issue but i've a serious doubt about this "easily
fixed" statement.

> > 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.

We've a ticket now. Moreover the use of newthread is definitely not
a blocker for the functionality. We've been using it in _func space
for years. As to me -- unitifaction of "call" procedure looks like
a separate task which is of course related to the cbox but we started
implementing cbox code not to eliminate "thread" call in first place
or optimize a code flow. But I agree we've to do something.

> >>> +
> >>> +	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.

I though about it as well but must confess I don't like this approach:
it means the memory allocated by our cbox code then passed to luajit
as a variable and then we always "trust" this object. I'll think about
more. Still the mark about GC handler is correct, thanks!