Re: [Tarantool-patches] [PATCH 2.X 5/7] module api: luaT_temp_luastate & luaT_release_temp_luastate

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

On 29.09.2020 07:17, Alexander Turenko wrote:
> On Tue, Sep 29, 2020 at 12:21:18AM +0200, Vladislav Shpilevoy wrote:
>> Thanks for the patch!
>>
>> I strongly don't like this export. It seems to be too internal.
> 
> I have no point to start discussion. Can you clarify your feeling?

I don't like that we give access to fiber's Lua state. If a user will
leave anything on it, it can lead to anything as well, UB I guess. But
I didn't check.

>> But I have no better ideas than propose to implement your own
>> lua_State cache in the merger module. It does not seem to be too
>> hard, and I don't think it would occupy much memory.
>>
>> Just a simple list of lua_State objects, created by luaT_newthread
>> on demand, and put back into the list when unused. What is wrong
>> with that?
> 
> When we able to simplify modules, it worth to do so (when there are no
> certain objections). Write once, use many.

What are applications for that except the merger? Potentially.

>> Adding Igor as Lua master to help with this.
>>
>> See 2 comments below.
>>
>>> diff --git a/src/lua/utils.h b/src/lua/utils.h
>>> index 9b1fe7e57..da0140076 100644
>>> --- a/src/lua/utils.h
>>> +++ b/src/lua/utils.h
>>> @@ -554,6 +554,41 @@ luaL_iscallable(lua_State *L, int idx);
>>>  struct lua_State *
>>>  luaT_newthread(struct lua_State *L);
>>>  
>>> +/**
>>> + * Get a temporary Lua state.
>>> + *
>>> + * Use case: a function does not accept a Lua state as an argument
>>> + * to allow using from C code, but uses a Lua value, which is
>>> + * referenced in LUA_REGISTRYINDEX. A temporary Lua stack is needed
>>> + * to get and process the value.
>>> + *
>>> + * The resulting Lua state has a separate Lua stack, but the same
>>> + * globals and registry as `tarantool_L` (and all Lua states in
>>
>> 1. Users don't know what is tarantool_L.
> 
> Note: It is <luaT_state>() in the module API.

Nice, didn't know that either. But still the comment is invalid.