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

[Vladislav Shpilevoy]

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.

>> 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.

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)
 {