Re: [Tarantool-patches] [PATCH 0/6] Extending error functionality

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

We discussed several questions around the API and the implementation
yesterday with Leonid and Vlad. Vlad summarized the API points in the
issue (see [1]). I want to summarize implementation details we
more-or-less agreed on.

We have two places, where encoding and decoding is performed to/from
msgpack: they are src/lua/{msgpack.c,msgpackffi.lua}.

I propose to add two mappings to msgpack.c:

 | /*
 |  * Encode an object on @an idx slot on the Lua stack into
 |  * @a buf, promote @a buf pointer on written bytes amount.
 |  *
 |  * Don't write over @an end pointer (inclusive).
 |  */
 | typedef void (*encode_ext_f)(struct lua_State *L, int idx, char **buf,
 |                              char *end);
 |
 | /*
 |  * Decode msgpack from @buf, push it onto the Lua stack.
 |  *
 |  * Promote @a buf on amount of read bytes.
 |  *
 |  * Don't read over @an end (inclusive).
 |  *
 |  */
 | typedef void (*decode_ext_f)(struct lua_State *L, char **buf, char *end);
 |
 | /* cdata type id -> encode function */
 | static mh_encode_ext;
 |
 | /* msgpack extension type -> decode function */
 | static mh_decode_ext;
 |
 | /* Register encode function for a cdata type. */
 | void
 | register_encode_ext(int ctypeid, encode_ext_f f);
 |
 | /* Register decode function for an extension type. */
 | void
 | register_decode_ext(int mp_ext_type, decode_ext_f f);

So box will add its own encode / decode functions for box.error objects
on box initialization (at tarantool loading, not at box.cfg()). The
problem we'll solve here is that src/lua/msgpack.c should not be
statically linked with src/box.

The similar change should be made for msgpackffi.lua. It, however,
already has `encode_ext_cdata` and `ext_decoder` tables. So it seems we
just need to add registration functions.

Note: Don't sure whether encode_ext / decode_ext should operate directly on
the Lua stack or we should separate those functions into get_from_stack+encode
and decode+push_to_stack. (Vlad?)

The functions that will be exposed by box should encode any box.error
object into msgpack and decode any msgpacked box.error into box.error
object.  Build*Error() constructors should be called for this with
appropriate arguments. So the decode function will contain the switch by
an error types and call necessary Build*Error() functions. Maybe also
will replace the error message, where needed.

This way looks hacky, but at least it concentrates hacking in one place.
When we'll remove C++ classes for errors, we can refactor this part.

[1]: https://github.com/tarantool/tarantool/issues/4398#issuecomment-606937494

WBR, Alexander Turenko.