Re: [Tarantool-patches] [PATCH v2 3/3] lua: expose temporary Lua state for iproto calls

[Igor Munkin <imun@tarantool.org>]

Sasha,

Thanks for the patch! It LGTM except the nits I mentioned below.

On 18.06.20, Alexander Turenko wrote:
> There is code that may save some time and resources for creating a new
> Lua state when it is present in the fiber storage of a current fiber.

Typo: s/present/presented/.

> There are not so much of them: running a Lua trigger and construction of
> a next tuple in a merge source.

<snipped>

> This patch fills fiber->storage.lua.stack for background fibers that
> serve a Lua call or eval: we already have this state and nothing prevent

Typo: s/prevent/prevents/.

> us from exposing it via the fiber storage.
> 
> Follows up #4954
> ---
>  src/box/lua/call.c   | 27 +++++++++++++++++++++++++++
>  src/lib/core/fiber.h | 14 ++++++++++----
>  2 files changed, 37 insertions(+), 4 deletions(-)
> 
> diff --git a/src/box/lua/call.c b/src/box/lua/call.c
> index 6588ec2fa..ccdef6662 100644
> --- a/src/box/lua/call.c
> +++ b/src/box/lua/call.c
> @@ -537,12 +537,39 @@ box_process_lua(lua_CFunction handler, struct execute_lua_ctx *ctx,
>  	port_lua_create(ret, L);
>  	((struct port_lua *) ret)->ref = coro_ref;
>  
> +	/*
> +	 * A code that need a temporary fiber-local Lua state may
> +	 * save some time and resources for creating a new state
> +	 * and use this one.
> +	 */

Could you please provide an example for the fiber calling this function
with non-NULL fiber-local Lua state? Are those conditions below are
strictly required by the current implementation?

> +	bool has_lua_stack = fiber()->storage.lua.stack != NULL;
> +	if (!has_lua_stack)
> +		fiber()->storage.lua.stack = L;
> +
>  	lua_pushcfunction(L, handler);
>  	lua_pushlightuserdata(L, ctx);
>  	if (luaT_call(L, 1, LUA_MULTRET) != 0) {
> +		if (!has_lua_stack)
> +			fiber()->storage.lua.stack = NULL;
>  		port_lua_destroy(ret);
>  		return -1;
>  	}
> +
> +	/*
> +	 * Since this field is optional we're not obligated to
> +	 * keep it until the Lua state will be unreferenced in
> +	 * port_lua_destroy().
> +	 *
> +	 * There is no much sense to keep it beyond the Lua call,
> +	 * so let's zap now.
> +	 *
> +	 * But: keep the stack if it was present before the call,
> +	 * because it would be counter-intuitive if the existing
> +	 * state pointer would be zapped after this function call.
> +	 */
> +	if (!has_lua_stack)
> +		fiber()->storage.lua.stack = NULL;
> +
>  	return 0;
>  }
>  

<snipped>

> -- 
> 2.25.0
> 

-- 
Best regards,
IM