Re: [Tarantool-patches] [PATCH v2 1/5] error: Add a Lua backtrace to error

[lvasiliev <lvasiliev@tarantool.org>]

On 16.04.2020 12:30, Alexander Turenko wrote:
> On Thu, Apr 16, 2020 at 11:58:03AM +0300, lvasiliev wrote:
>> Hi! Thanks for the feedback.
>>
>> On 16.04.2020 4:11, Alexander Turenko wrote:
>>>>>> 2) What to do with stacked errors? Currently only the first
>>>>>> error in the stack gets a traceback, because luaT_pusherror() is
>>>>>> called only on the top error in the stack. Consider this test:
>>>>>>
>>>>>>        box.cfg{}
>>>>>>        lua_code = [[function(tuple)
>>>>>>                        local json = require('json')
>>>>>>                        return json.encode(tuple)
>>>>>>                     end]]
>>>>>>        box.schema.func.create('runtimeerror', {body = lua_code, is_deterministic = true, is_sandboxed = true})
>>>>>>        s = box.schema.space.create('withdata')
>>>>>>        pk = s:create_index('pk')
>>>>>>        idx = s:create_index('idx', {func = box.func.runtimeerror.id, parts = {{1, 'string'}}})
>>>>>>
>>>>>>        function test_func() return pcall(s.insert, s, {1}) end
>>>>>>        box.error.cfg{traceback_enable = true}
>>>>>>        ok, err = test_func()
>>>>>>
>>>>>>
>>>>>>        tarantool> err:unpack()
>>>>>>        ---
>>>>>>        - traceback: "stack traceback:\n\t[C]: at 0x010014d1b0\n\t[C]: in function 'test_func'\n\t[string
>>>>>>            \"ok, err = test_func()\"]:1: in main chunk\n\t[C]: in function 'pcall'\n\tbuiltin/box/console.lua:382:
>>>>>>            in function 'eval'\n\tbuiltin/box/console.lua:676: in function 'repl'\n\tbuiltin/box/console.lua:725:
>>>>>>            in function <builtin/box/console.lua:713>"
>>>>>>        ... <snipped>
>>>
>>> BTW, can we call :split('\n') for .traceback field in at least
>>> __serialize? The cited output is hard to read. Alternative: place two
>>> newlines in row somewhere to force yaml serializer to use multiline
>>> string format.
>>
>> Traceback is absent in __serialize, because it will change the error view
>> for old client. If the client matches result with some pattern it, will be
>> broken.
> 
> It is better to keep __serialize on track with newly added fields. I
> would not bother with possibility that someone may call __serialize
> manually or capture console output to compare against a sample.
> Extending a map should be okay from backward-compatibility point of
> view.
> 
Now __serialize is used for transfer old style error too.
>>
>>>>>>
>>>>>>        tarantool> err.prev:unpack()
>>>>>>        ---
>>>>>>        - type: LuajitError
>>>>>>          message: '[string "return function(tuple)..."]:2: attempt to call global ''require''
>>>>>>            (a nil value)'
>>>>>>        ... <snipped>
>>>>>>
>>>>>> The second error does not have a traceback at all.
>>>>> (I added Turenko to To)
>>>>> I have two variants:
>>>>> - Leave as is and to document such behavior
>>>>> - Add the same traceback to all errors in the stack
>>>>> Alexander what do you think?
>>>
>>> The first approach look inconsistent. A user may want to get a cause of
>>> a topmost error and pass it somewhere. The function, where the error
>>> will be processed (say, serialized), don't know whether a traceback
>>> should be grabbed from some other error object (and how to find it).
>>>
>> Not quite, you either have a traceback or not. Don't try to get it from
>> another error.
> 
> You propose to introduce some kind of 'full' and 'partial' errors. It is
> hard to document, because there is no rationale for this. When something
> is introduced, it should be for the sake of something.
No, the error without traceback is not 'partial'. If
global error_is_traceback_enabled is false - all errors haven't a
traceback. If error creates with traceback=false, it hasn't a traceback.
> 
> A kind of 'the API is complex, but, I see, it is highly flexible' or
> 'here I should take care of this peculiar, but OTOH some cases may be
> processed much faster due to this'.
> 
You are hyperbolizing. It can be regarded as "Technical debt".
If you insist, I can remove this patch.