Re: [PATCH v7 21/21] nbd: assert that Error** is not NULL in nbd_iter_channel_error

[Vladimir Sementsov-Ogievskiy]

05.12.2019 20:49, Eric Blake wrote:
> On 12/5/19 11:39 AM, Vladimir Sementsov-Ogievskiy wrote:
>> 05.12.2019 20:14, Eric Blake wrote:
>>> On 12/5/19 9:20 AM, Vladimir Sementsov-Ogievskiy wrote:
>>>> The local_err parameter is not here to return information about
>>>> nbd_iter_channel_error failure. Instead it's assumed to be filled when
>>>> passed to the function. This is already stressed by its name
>>>> (local_err, instead of classic errp). Stress it additionally by
>>>> assertion.
>>>>
> 
>>>
>>> Would it be better to assert(!local_err || *local_err)?  The assertion as 
>>> written is too strict without ERRP_AUTO_PROPAGATE, but you get away with it 
>>> because none of the grandparents pass NULL; but is appropriate as written 
>>> for after after the macro conversion so then we wonder if churn on the 
>>> macro is worth it.
>>
>> We don't have any grandparents, this function is always called on local_err. 
>> And it's argument named local_err to stress it.
> 
> Then the commit message should state that. How about:
> 
> All callers of nbd_iter_channel_error() pass the address of a local_err 
> variable, and only call this function if an error has already occurred, using 
> this function

> to append details to that error. 

Hmm, not to append details but to report the error to the magical receiving 
loop, which doesn't yet know about the error

> This is already implied by its name (local_err instead of the classic errp), 
> but it is worth additionally stressing this by adding an assertion to make it 
> part of the function contract.


So, how about simply s/to append details to that error/to report that error/ ?

> 
>> The function is an API to report error, and it wants filled local_err object.
>>
>> It will crash anyway if local_err is NULL, as it dereferences it.
>>
>> I just want to place an assertion at start of functions like this,
>> which will be easily recognizable by coccinelle.
> 
> With an improved commit message, the assertion makes sense, so
> 
> Reviewed-by: Eric Blake <address@hidden>
> 
>>
>> ---
>>
>> We can improve the API, to support local_err==NULL, for the case when 
>> original request was called with
>> errp==NULL, but for this we'll need more changes, like, pass errp to 
>> NBD_FOREACH_REPLY_CHUNK and save
>> it into iter object...
>>
>> But how to detect it in code? Something like
>>
>>
>> --- a/block/nbd.c
>> +++ b/block/nbd.c
>> @@ -1059,8 +1059,10 @@ static int 
>> nbd_co_receive_blockstatus_reply(BDRVNBDState *s,
>>            case NBD_REPLY_TYPE_BLOCK_STATUS:
>>                if (received) {
>>                    nbd_channel_error(s, -EINVAL);
>> -                error_setg(&local_err, "Several BLOCK_STATUS chunks in 
>> reply");
>> -                nbd_iter_channel_error(&iter, -EINVAL, &local_err);
>> +                if (errp) {
>> +                    error_setg(&local_err, "Several BLOCK_STATUS chunks in 
>> reply");
>> +                }
>> +                nbd_iter_channel_error(&iter, -EINVAL, errp ? &local_err : 
>> NULL);
> 
> No, that's not worth it.
> 


-- 
Best regards,
Vladimir