Re: [qemu-s390x] [RFC] error: auto propagated local_err

[Vladimir Sementsov-Ogievskiy]

19.09.2019 12:17, Kevin Wolf wrote:
> Am 18.09.2019 um 19:10 hat Eric Blake geschrieben:
>> On 9/18/19 8:02 AM, Vladimir Sementsov-Ogievskiy wrote:
>>> + */
>>> +#define MAKE_ERRP_SAFE(errp) \
>>> +g_auto(ErrorPropagationStruct) (__auto_errp_prop) = {.errp = (errp)}; \
>>> +if ((errp) == NULL || *(errp) == error_abort || *(errp) == error_fatal) { \
>>> +    (errp) = &__auto_errp_prop.local_err; \
>>> +}
>>
>> Not written to take a trailing semicolon in the caller.
>>
>> You could even set __auto_errp_prop unconditionally rather than trying
>> to reuse incoming errp (the difference being that error_propagate() gets
>> called more frequently).
> 
> I think this difference is actually a problem.
> 
> When debugging things, I hate error_propagate(). It means that the Error
> (specifically its fields src/func/line) points to the outermost
> error_propagate() rather than the place where the error really happened.
> It also makes error_abort completely useless because at the point where
> the process gets aborted, the interesting information is already lost.
> 
> So I'd really like to restrict the use of error_propagate() to places
> where it's absolutely necessary. Unless, of course, you can fix these
> practical problems that error_propagate() causes for debugging.
> 
> In fact, in the context of Greg's series, I think we really only need to
> support hints for error_fatal, which are cases that users are supposed
> to see. We should exclude error_abort in MAKE_ERRP_SAFE() because these
> are things that are never supposed to happen. A good stack trace is more
> important there than adding a hint to the message.
> 

Interesting, that to handle error_append_hint problem, we don't need to
create local_err in case of errp==NULL either..

So, possibly, we need the following steps:

1. implement MAKE_ERRP_SAFE_FOR_HINT (which only leave "*(errp) == error_fatal" 
in the if condition
2. rebase Greg's series on it, to fix hints for fatal errors
3. implement MAKE_ERRP_SAFE_FOR_DEREFERENCE (which only leave "(errp) == NULL" 
in the if condition
4. convert all (almost all) local_err usage to use 
MAKE_ERRP_SAFE_FOR_DEREFERENCE, which will
    fix problem with error_abort (and also drop a lot of calls of 
error_propagate)
5. merely convert "void func(.., errp)" to "int func(.., errp)" and drop 
MAKE_ERRP_SAFE_FOR_DEREFERENCE()
    magic, together with dereferencing.


-- 
Best regards,
Vladimir