Re: [tarantool-patches] [PATCH v3 4/7] lua: add non-recursive msgpack decoding functions

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

In short:

* The behaviour is what I need (I cannot skip data).
* The names are confusing, but I don't see better variants. These ones
  at least consistent with others.

I posted below relevant discussions, old variants, possible variants,
usage examples and so on.

WBR, Alexander Turenko.

On Thu, Apr 18, 2019 at 08:35:28PM +0300, Konstantin Osipov wrote:
> * Alexander Turenko <alexander.turenko@tarantool.org> [19/04/10 18:23]:
> > Contracts:
> > 
> > ```
> > msgpack.decode_array(buf.rpos, buf:size()) -> arr_len, new_rpos
> > msgpack.decode_map(buf.rpos, buf:size()) -> map_len, new_rpos
> > ```
> 
> From what they do, they actually neither decode nor skip an
> array/map. They decode array/map header only. I know that msgpuck
> uses this naming, but it is actually confusing.

Yep, names are most hard part of the patch. They were check_array/map(),
then were renamed to decode_array/map(). The relevant parts from the
previous discussion:

Vladimir:

> Not sure about the name either, because it doesn't just checks the
> msgpack - it decodes it, but can't come up with anything substantially
> better. May be, msgpack.decode_array?
>
> https://www.freelists.org/post/tarantool-patches/PATCH-v2-56-netbox-add-helpers-to-decode-msgpack-headers,1

Me:

> Re name: now I understood: decode_unchecked() is like mp_decode(),
> decode() is like mp_check() + mp_decode(). So it worth to rename it to
> decode_array(). Done.
>
> https://www.freelists.org/post/tarantool-patches/PATCH-v2-56-netbox-add-helpers-to-decode-msgpack-headers,2

All the words 'check', 'skip' and 'decode' are assumed intuitively as
something related to the whole array/map. The only thing we able to do
here is to add '_header' postfix (it would look a bit weird, IMHO).
Should I do?

> Why not just encode array/map header, but skip array/map  data in
> these functions and and call them msgpack.skip_array(),
> msgpack.skip_map() respectively? 

I need to skip an array/map header (and save length), but feed bare data
to a source constructor.

Initially I had a parameter in merger (now it would be in a source
constructor) that say how much array headers it need to skip before
actually read tuples. AFAIR, the option values were BUFFER_RAW,
BUFFER_SELECT, BUFFER_CALL, BUFFER_CHUNKED.

Then we decided to factor it our from merger and then splitted to
msgpack helpers and a net.box option.

> > +
> > +	uint32_t len = mp_decode_array(&data);
> 
> This doesn't actually skip array members, does it.

This is exactly what I need.

Don't know how to describe it in short. I receive [[tuple, tuple, ...],
[tuple, tuple, ...]] in a buffer and need to feed each [tuple, tuple,
...] part to merger.

The following sections of merger's examples should shed light on that I
hope:

https://github.com/Totktonada/tarantool-merger-examples/tree/695fc9511685033f4b4b22c0df537a12ddf2eaf6#preparing-buffers
https://github.com/Totktonada/tarantool-merger-examples/tree/695fc9511685033f4b4b22c0df537a12ddf2eaf6#multiplexing-requests

The usage example itself is here:

https://github.com/Totktonada/tarantool-merger-examples/blob/695fc9511685033f4b4b22c0df537a12ddf2eaf6/multiplexed_example/frontend.lua#L38-L46

WBR, Alexander Turenko.