[Tarantool-patches] [PATCH] error: add format string usage to form a CustomError message

[Leonid Vasiliev]

Hi! Thank you for the review.
I updated the commit message:

error: add format string usage to compose a CustomError message

For the CustomError the ability to create a message
using a format string was added.

Closes #4903

@TarantoolBot document
Title: Add format string usage to compose a CustomError message
When an error created using the separate members mode
(box.error(code, errtext[, errtext ...])) in the case of
ClientError error creation, a pre-defined format is used
(corresponding to the error code) (nothing has changed), in
the case of CustomError error creation, a format string can
be used to compose a message.

ClientError(nothing has changed):
```Lua
box.error(code, reason args)
```

Example for ER_CREATE_SPACE:
```Lua
box.error(9, "my_space", "reason")
```
Result:
```Lua
error: 'Failed to create space ''my_space'': reason'
```

CustomError:
```Lua
box.error(type, reason format string, reason args)
```
Example:
```Lua
box.error("MyCustomType", "The error reason: %s", "some error reason")
```
Result:
```Lua
error: 'The error reason: some error reason'
```

On 30.05.2020 01:05, Vladislav Shpilevoy wrote:
> Hi! Thanks for the patch!
> 
> On 28/05/2020 10:31, Leonid Vasiliev wrote:
>> For the CustomError the ability to create a message
>> using a format string was added.
>>
>> Closes #4903
>>
>> @TarantoolBot document
>> Title: Add format string usage to form a CustomError message
>> When creating a ClientError error the predefined format
>> (corresponding with the error code) is used.
>> When creating a CustomError error a format string can be
>> used to form the message.
>>
>> ClientError:
>> ```Lua
>> box.error(code, reason args)
>> ```
>>
>> CustomError:
>> ```Lua
>> box.error(type, reason format string, reason args)
>> ```
> 
> It would be good to provide an example here. And to state, that
> client error case is not changed. Otherwise from the text above
> it looks like you changed it to be 'the predefined format is used'.
> But I will leave it to the doc team to ping you after this is
> pushed.
> 
> Talking of the client error formatting correctness, it looks like
> currently the documentation says:
> https://www.tarantool.io/en/doc/2.3/reference/reference_lua/box_error/#lua-function.box.error
> 
> 	When called with a Lua-table argument, the code and reason have any user-desired values. The result will be those values.
> 
> 	Parameters:
> 
> 	reason (string) – description of an error, defined by user
> 	code (integer) – numeric code for this error, defined by user
> 
> So in the site it is said, that 'reason' is defined by user, not be
> a predefined internal format. The predefined format is used in
> box.error(code, ...) syntax.
> 
>> ---
>> https://github.com/tarantool/tarantool/issues/4903
>> https://github.com/tarantool/tarantool/tree/lvasiliev/gh-4903-format-string-for-CustomError
>> @ChangeLog Add format string usage to form a CustomError message