Re: [Tarantool-patches] [PATCH v2 2/3] merger: clean fiber-local Lua stack after next()

[Alexander Turenko <alexander.turenko@tarantool.org>]

> > Now those functions may be called only from Lua and if the fiber-local
> > Lua state is present it is the same as one that is passed to a Lua/C
> 
> Typo: s/present/presented/.

Cited from [1]:

 | "As presented" (verb) connotes deliberate placement. "As present"
 | (adjective) just means it's there.

'It is there' meanging fits better here, IMHO.

Isn't I miss something about grammar here?

[1]: https://www.instructionalsolutions.com/blog/bid/102954/Tables-in-Report-Writing-Presented-or-Present

> > The merge_source_next() implementations do not leave any garbage on a
> > Lua stack at success path, but may left something when an error occurs
> > (say, when a Lua iterator generator returns more then two values). I
> 
> Typo: s/then/than/.

Thanks! Fixed.

> > @@ -206,14 +216,10 @@ luaT_temp_luastate(int *coro_ref)
> >   * It is the other half of `luaT_temp_luastate()`.
> >   */
> >  static void
> > -luaT_release_temp_luastate(int coro_ref)
> > +luaT_release_temp_luastate(struct lua_State *L, int coro_ref, int top)
> >  {
> > -	/*
> > -	 * FIXME: The reusable fiber-local Lua state is not
> > -	 * unreferenced here (coro_ref == LUA_REFNIL), but
> > -	 * it must be truncated to its past top to prevent
> > -	 * stack overflow.
> > -	 */
> > +	if (top >= 0)
> > +		lua_settop(L, top);
> >  	luaL_unref(tarantool_L, LUA_REGISTRYINDEX, coro_ref);
> 
> Minor: You can just either restore top value for fiber-local Lua state
> or unreference Lua coroutine without restoring a pointer to its stack
> top slot. As a result you need to preserve the top value only for the
> first case (i.e. when the coro_ref is LUA_NOREF) and ignore the value
> for all other cases.

Are you propose the following?

 | if (top >= 0)
 |         lua_settop(L, top);
 | else
 |         luaL_unref(tarantool_L, LUA_REGISTRYINDEX, coro_ref);

When I look into the code locally, it does not seem to be logical: why
decision whether we should unreference a state should be made based on
`top` value?

Now the code seem to be logical for me: if `top` was saved, then we drop
it. If reference was saved, we unreference it.

Vlad even proposed to drop `top >= 0`, which works in fact, but Lua
Reference Manual does not guarantee it (it was discussed within this
mailing thread).

> > +static int
> > +lbox_check_merge_source_call_next(struct lua_State *L)
> > +{
> > +	assert(lua_gettop(L) == 1);
> > +
> > +	/*
> > +	 * Ensure that there is reusable temporary Lua stack.
> > +	 *
> > +	 * Note: It may be the same as L (and usually do).
> 
> Minor: It would be nice to mention (at least for inquisitive persons) a
> case when <temporary_L> differs from the given <L> for Lua-born fibers.

No-no, it is always so for a Lua born fiber. It seems I should reword
the comment in a more strict way and explain why I use <temporary_L>
explicitly despite the fact that it is always the same as <L>.

The new comment:

 | /*
 |  * Ensure that there is a reusable temporary Lua stack.
 |  *
 |  * Note: It is the same as <L> for a Lua born fiber (at
 |  * least at the moment of writing), but it is the
 |  * implementation detail and the test looks more clean
 |  * when we don't lean on this fact.
 |  */