Re: [Tarantool-patches] [PATCH v2 2/3] box: introduce box_return_mp() public C function

[Nikita Pettik <korablev@tarantool.org>]

On 27 Apr 23:29, Vladislav Shpilevoy wrote:
> Thanks for the question!
> 
> On 27/04/2020 17:14, Nikita Pettik wrote:
> > On 23 Apr 02:12, Vladislav Shpilevoy wrote:
> >> +
> >> +int
> >> +test_return_mp(box_function_ctx_t *ctx, const char *args, const char *args_end)
> >> +{
> >> +	(void) args;
> >> +	(void) args_end;
> >> +	char buf[512];
> >> +	char *pos = mp_encode_uint(buf, 1);
> >> +	int rc = box_return_mp(ctx, buf, pos);
> >> +	if (rc != 0)
> >> +		return rc;
> >> +
> >> +	pos = mp_encode_int(buf, -1);
> >> +	rc = box_return_mp(ctx, buf, pos);
> >> +	if (rc != 0)
> >> +		return rc;
> >> +
> >> +	pos = mp_encode_uint(buf, UINT64_MAX);
> >> +	rc = box_return_mp(ctx, buf, pos);
> >> +	if (rc != 0)
> >> +		return rc;
> >> +
> >> +	const char *str = "123456789101112131415";
> >> +	pos = mp_encode_str(buf, str, strlen(str));
> >> +	rc = box_return_mp(ctx, buf, pos);
> >> +	if (rc != 0)
> >> +		return rc;
> >> +
> >> +	pos = mp_encode_array(buf, 1);
> >> +	pos = mp_encode_uint(pos, 2);
> >> +	box_tuple_t *tuple = box_tuple_new(box_tuple_format_default(),
> >> +					   buf, pos);
> >> +	if (tuple == NULL)
> >> +		return -1;
> >> +	rc = box_return_tuple(ctx, tuple);
> > 
> > Probably I'm missing something (since I've never used C API) but
> > when I do:
> > 
> > res = net:connect(box.cfg.listen):call(name)
> > print(type(res[5]))
> > 
> > I get table. But shouldn't it be tuple (cdata), since it (last member)
> > is explicitly wrapped into tuple)? I guess it is due to certain
> > convention but failed to find it in docs. Could you please clarify?
> 
> That is not about C functions. It is about tuple data type lost during
> its marshaling over network. You will get the same result, if you
> write that in a Lua stored function. We solve that problem by introducing
> MP_EXT to be able to restore types on the client side for uuids, errors,
> decimals. But not for tuples. So they turn into tables when
> serialized-deserialized from MessagePack during sending over the network.

Ok, thanks. Just in case - patch LGTM.
 
> Exception is IPROTO_SELECT and other DML/DQL mapped onto exact IPROTO_*
> commands, because client always knows they return tuples, and can create
> them on the client side instead of tables.