[tarantool-patches] Re: [PATCH] box: add tuple:size function

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

On 17/10/2018 18:29, Konstantin Osipov wrote:
> * Alexander Turenko <alexander.turenko@tarantool.org> [18/10/17 10:45]:
>> Are tuple.bsize and box_tuple_bsize() subjects to change or it is only
>> about the Lua part?
> 
> tuple.bsize is used internally, so I don't think you should change
> it. But it's better to rename it to msgpack_size or something like
> that to avoid ambiguity.
> 
> box_tuple_bsize() should be ok to change.
> 

box_tuple_bsize() is already documented in module.h as
returning only tuple data.

Also, some people can use it right now to allocate a
buffer of a correct size before calling box_tuple_to_buf.
I understand, that box_tuple_to_buf(tuple, NULL, 0) returns
bsize as well, but some people could miss it, or just use
box_tuple_bsize because it looks better when you write like
this:

     size = box_tuple_bsize(tuple)
     buf = alloc(size)
     box_tuple_to_buf(tuple, buf, size)

than like this:

     size = box_tuple_to_buf(tuple, NULL, 0) // <- difference
     buf = alloc(size)
     box_tuple_to_buf(tuple, buf, size)

Even if we close eyes on the fact, that a user of
the first way will allocate more data than needed,
imagine, that then he does something like this:

     send(sockfd, buf, size)

Now, he send some garbage uninitialized data of 14
bytes at the end of buf.

I understand, that you likely not believe me that
the first way looks more intuitive, but look at
our own code: box_tuple_bsize() is used now in

*box/lua/tuple.c in tuple_to_mpstream();
* box/lua/tuple.lua via FFI;

to allocate a buffer before copying tuple data
directly or via tuple_to_buf. We can fix our code,
but can not fix user's one.

So here we can either

* fix documentation of the site without breaking anything
* or break behavior of public API destroying an ability to
   learn tuple data size in a natural way;
* add new method and again fix the site documentation.

I described it just for the record that you have chosen
the second, most destructive way.