Re: [Tarantool-patches] [PATCH 4/5] error: provide MP_ERROR extension serializer

[Nikita Pettik <korablev@tarantool.org>]

On 14 May 23:28, Vladislav Shpilevoy wrote:
> Hi! Thanks for the review!
> 
> >>> /**
> >>>  * MP_ERROR extension string serializer.
> >>>  * There are two applications for string serialization - into a
> >>>  * buffer, and into a file. Structure of both is exactly the same
> >>>  * except for the copying/writing itself. To avoid code
> >>>  * duplication the code is templated and expects some macros to do
> >>>  * the actual output.
> >>>  */
> >> This is basically a C template. The same as bps tree, rb tree, mhash. But
> >> located in the same source file which needs it. Since this is not a common
> >> enough feature to move it to a separate place.
> >>
> >> I include self with pre-declared macros MP_ERROR_PRINT_DEFINITION. When it
> >> is defined, the file skips everything except the 3 functions in the end:
> >> mp_print_error_one(), mp_print_error_stack(), mp_print_error().
> > 
> > A-ha, it is what I missed. But personally I would anyway move these
> > macros/functions to a separate file. It would definitely cut extra
> > questions and would make code cleaner (IMHO) (I bet anybody who
> > sees '#include __FILE__' for the first time is like 'WTF?!').
> 
> But! How cool the self inclusion is 😎. If someone would wrap these things
> into neat macros, it could be a useful tool to replace C++ templates, which
> are the only real problem on the way of getting rid of C++ from the code base.
> For example, tuple comparators heavily depend on templates, and their move
> to separate files is hardly possible - they depend on tuple_compare.cc
> internals too much.
> 
> Jokes aside, self inclusion solves one important issue - non templated
> dependencies. If I move the templates to a different file, it needs to include
> "mp_error.h", at least for MP_ERROR_* enum constants. Strictly speaking. Also
> it needs MessagePack headers, and can't use any mp_error.cc internal functions
> even though these templates *are* internal too.

Ok, as you wish.
 
> >> The skip uses mere #ifdef MP_ERROR_PRINT_DEFINITION to understand, that
> >> currently the file is being included into self to customize the error
> >> printers. So all is skipped except them.
> >>
> >> I include the file twice, but with different parameters. Like if I would
> >> customize one template with two parameter sets in C++. First inclusion
> >> turns
> >>
> >>     mp_print_error_one(), mp_print_error_stack(), mp_print_error()
> >>
> >> into
> >>
> >>     mp_snprint_error_one(), mp_snprint_error_stack(), mp_snprint_error()
> >>
> >> and snprintf() calls inside. The second inclusion turns the functions into
> >>
> >>     mp_fprint_error_one(), mp_fprint_error_stack(), mp_fprint_error()
> >>
> >> and fprintf() calls inside.
> >>
> >> I tried to extend the comment.
> > 
> > You gave cool explanation right in this message. Why not copy it to
> > the source file?
> 
> Because it is too common. Can't put it in just one place, since the mentioned
> things are spread on the file. I tried to split it in parts and put them
> where they belong. In addition to the comment I added previously.

Ok, thanks. LGTM.

> diff --git a/src/box/mp_error.cc b/src/box/mp_error.cc
> index a4a4ccaca..23a74d0a9 100644
> --- a/src/box/mp_error.cc
> +++ b/src/box/mp_error.cc
> @@ -28,6 +28,11 @@
>   * THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
>   * SUCH DAMAGE.
>   */
> +/**
> + * When this macros is defined, it means the file included self in
> + * order to customize some templates. Skip everything except them
> + * then.
> + */
>  #ifndef MP_ERROR_PRINT_DEFINITION
>  
>  #include "box/mp_error.h"
> @@ -556,6 +561,11 @@ error_unpack_unsafe(const char **data)
>  	return err;
>  }
>  
> +/**
> + * Include this file into self with a few template parameters
> + * to create mp_snprint_error() and mp_fprint_error() functions
> + * and their helpers from a printer template.
> + */
>  #define MP_ERROR_PRINT_DEFINITION
>  #define MP_PRINT_FUNC snprintf
>  #define MP_PRINT_SUFFIX snprint
> @@ -675,6 +685,11 @@ mp_print_error_stack(MP_PRINT_ARGS_DECL, const char **data, int depth)
>  	return total;
>  }
>  
> +/**
> + * The main printer template. Depending on template parameters it
> + * is turned into mp_snprint_error() with snprintf() semantics or
> + * into mp_fprint_error() with fprintf() semantics.
> + */
>  int
>  mp_print_error(MP_PRINT_ARGS_DECL, const char **data, int depth)
>  {