Re: [Tarantool-patches] [PATCH 3/4] fiber: keep reference to userdata if fiber created once

[Vladislav Shpilevoy via Tarantool-patches <tarantool-patches@dev.tarantool.org>]

Hi! Thanks for the patch!

See 5 comments below.

> diff --git a/src/lua/fiber.c b/src/lua/fiber.c
> index 5575f2079..268ddf9cc 100644
> --- a/src/lua/fiber.c
> +++ b/src/lua/fiber.c
> @@ -87,27 +87,31 @@ luaL_testcancel(struct lua_State *L)
>  static const char *fiberlib_name = "fiber";
>  
>  /**
> - * @pre: stack top contains a table
> - * @post: sets table field specified by name of the table on top
> - * of the stack to a weak kv table and pops that weak table.
> + * Trigger invoked when the fiber has stopped execution of its
> + * current request. Only purpose - delete storage.lua.ref and
> + * storage.lua.storage_ref keeping a reference of Lua
> + * fiber and fiber.storage objects. Unlike Lua stack,
> + * Lua fiber storage may be created not only for fibers born from
> + * Lua land. For example, an IProto request may execute a Lua
> + * function, which can create the storage. Trigger guarantees,
> + * that even for non-Lua fibers the Lua storage is destroyed.
>   */
> -static void
> -lbox_create_weak_table(struct lua_State *L, const char *name)
> +static int
> +lbox_fiber_on_stop(struct trigger *trigger, void *event)
>  {
> -	lua_newtable(L);
> -	/* and a metatable */
> -	lua_newtable(L);
> -	/* weak keys and values */
> -	lua_pushstring(L, "kv");
> -	/* pops 'kv' */
> -	lua_setfield(L, -2, "__mode");
> -	/* pops the metatable */
> -	lua_setmetatable(L, -2);
> -	/* assigns and pops table */
> -	lua_setfield(L, -2, name);
> -	/* gets memoize back. */
> -	lua_getfield(L, -1, name);
> -	assert(! lua_isnil(L, -1));
> +	struct fiber *f = (struct fiber *) event;

1. In new code we do not use whitespaces after unary operators,
including type casts. But here you don't need a cast. void * is
implicitly compatible with any other pointer type in C.

> +	int storage_ref = f->storage.lua.storage_ref;
> +	if (storage_ref > 0) {
> +		luaL_unref(tarantool_L, LUA_REGISTRYINDEX, storage_ref);
> +		f->storage.lua.storage_ref = LUA_NOREF;
> +	}
> +	int ref = f->storage.lua.ref;
> +	assert(ref > 0);
> +	luaL_unref(tarantool_L, LUA_REGISTRYINDEX, ref);
> +	f->storage.lua.ref = LUA_NOREF;
> +	trigger_clear(trigger);
> +	free(trigger);
> +	return 0;
>  }
>  
>  /**
> @@ -116,42 +120,26 @@ lbox_create_weak_table(struct lua_State *L, const char *name)
>  static void
>  lbox_pushfiber(struct lua_State *L, struct fiber *f)
>  {
> -	/*
> -	 * Use 'memoize'  pattern and keep a single userdata for
> -	 * the given fiber. This is important to not run __gc
> -	 * twice for a copy of an attached fiber -- __gc should
> -	 * not remove attached fiber's coro prematurely.
> -	 */
> -	luaL_getmetatable(L, fiberlib_name);
> -	lua_getfield(L, -1, "memoize");
> -	if (lua_isnil(L, -1)) {
> -		/* first access - instantiate memoize */
> -		/* pop the nil */
> -		lua_pop(L, 1);
> -		/* create memoize table */
> -		lbox_create_weak_table(L, "memoize");
> -	}
> -	/* Find out whether the fiber is  already in the memoize table. */
> -	uint64_t fid = f->fid;
> -	luaL_pushuint64(L, fid);
> -	lua_gettable(L, -2);
> -	if (lua_isnil(L, -1)) {
> -		/* no userdata for fiber created so far */
> -		/* pop the nil */
> -		lua_pop(L, 1);
> -		/* push the key back */
> -		luaL_pushuint64(L, fid);
> +	int ref = f->storage.lua.ref;
> +	if (ref <= 0) {
> +		struct trigger *t = (struct trigger *)malloc(sizeof(*t));

2. No need to cast, in C it works as is.

> +		if (t == NULL) {
> +			diag_set(OutOfMemory, sizeof(*t), "malloc", "t");
> +			luaT_error(L);
> +		}
> +		trigger_create(t, lbox_fiber_on_stop, NULL, (trigger_f0) free);

3. Should not be a whitespace after the cast.

> +		trigger_add(&f->on_stop, t);
> +
> +		uint64_t fid = f->fid;
>  		/* create a new userdata */
>  		uint64_t *ptr = lua_newuserdata(L, sizeof(*ptr));
>  		*ptr = fid;

4. Did you consider pushing the fiber's pointer instead of its ID?
And keep the fiber struct from deletion until it has no refs in
Lua anymore. That would eliminate lookup in fiber hash on each attempt
to access it. Also might make it simpler to return stuff like fiber.csw
in Lua. I remember there was a problem with the fiber being deleted by
the time you try to access its members. This would solve it.

>  		luaL_getmetatable(L, fiberlib_name);
>  		lua_setmetatable(L, -2);
> -		/* memoize it */
> -		lua_settable(L, -3);
> -		luaL_pushuint64(L, fid);
> -		/* get it back */
> -		lua_gettable(L, -2);
> +		ref = luaL_ref(L, LUA_REGISTRYINDEX);
> +		f->storage.lua.ref = ref;
>  	}
> +	lua_rawgeti(L, LUA_REGISTRYINDEX, ref);
>  }
> @@ -703,8 +669,6 @@ lbox_fiber_storage(struct lua_State *L)
>  			diag_set(OutOfMemory, sizeof(*t), "malloc", "t");
>  			return luaT_error(L);
>  		}
> -		trigger_create(t, lbox_fiber_on_stop, NULL, (trigger_f0) free);

5. The trigger 't' now is not used and leaks.

> -		trigger_add(&f->on_stop, t);
>  		lua_newtable(L); /* create local storage on demand */
>  		storage_ref = luaL_ref(L, LUA_REGISTRYINDEX);
>  		f->storage.lua.storage_ref = storage_ref;
>