Re: [Qemu-devel] [PATCH for-2.5] error: Document chained error handling

[Markus Armbruster]

Eric Blake <address@hidden> writes:

> On 11/17/2015 06:48 AM, Markus Armbruster wrote:
>
>>> ---
>>> based on feedback of my qapi series v5 7/46; doc only, so might
>>> be worth having in 2.5
>>> ---
>
>>> + *
>>> + * In a situation where cleanup must happen even if a first step fails,
>>> + * but the cleanup may also set an error, the first error to occur will
>>> + * take priority when combined by:
>>> + *     Error *err = NULL;
>>> + *     action1(arg, errp);
>>> + *     action2(arg, &err);
>>> + *     error_propagate(errp, err);
>
> Your proposal covers this idiom.
>
>>> + * or by:
>>> + *     Error *err = NULL;
>>> + *     action1(arg, &err);
>>> + *     action2(arg, err ? NULL : &err);
>>> + *     error_propagate(errp, err);
>
> This idiom doesn't appear in the current code base, so not documenting
> it is okay...
>
>>> + * although the second form is required if any further error handling
>>> + * will inspect err to see if all earlier locations succeeded.
>
> ...if we instead document how to check if either error happened, but
> your version also does that.
>
>>>   */
>>>
>>>  #ifndef ERROR_H
>> 
>> Yet another one:
>> 
>>     *     Error *err = NULL, *local_err = NULL;
>>     *     action1(arg, &err);
>>     *     action2(arg, &local_err)
>>     *     error_propagate(&err, err);
>
> This line should be error_propagate(&err, local_err);

Yes.

>>     *     error_propagate(errp, err);
>> 
>> Less clever.
>> 
>> Can we find a single, recommended way to do this?  See below.
>> 
>> Not mentioned: the obvious
>> 
>>     action1(arg, errp);
>>     action2(arg, errp);
>> 
>> is wrong, because a non-null Error ** argument must point to a null.  It
>> isn't when errp is non-null, and action1() sets an error.  It actually
>> fails an assertion in error_setv() when action2() sets an error other
>> than with error_propagate().
>
> Indeed, pointing out what we must NOT do is worthwhile.
>
>> 
>> The existing how-to comment doesn't spell this out.  It always shows the
>> required err = NULL, though.  You can derive "must point to null" from
>> the function comments of error_setg() and error_propagate().
>> 
>> I agree the how-to comment could use a section on accumulating errors.
>> Your patch adds one on "accumulate and pass to caller".  Here's my
>> attempt:
>> 
>> diff --git a/include/qapi/error.h b/include/qapi/error.h
>> index 4d42cdc..b2362a5 100644
>> --- a/include/qapi/error.h
>> +++ b/include/qapi/error.h
>> @@ -76,6 +76,23 @@
>>   * But when all you do with the error is pass it on, please use
>>   *     foo(arg, errp);
>>   * for readability.
>> + *
>> + * Receive and accumulate multiple errors (first one wins):
>> + *     Error *err = NULL, *local_err = NULL;
>> + *     foo(arg, &err);
>> + *     bar(arg, &local_err);
>> + *     error_propagate(&err, local_err);
>> + *     if (err) {
>> + *         handle the error...
>> + *     }
>> + *
>> + * Do *not* "optimize" this to
>> + *     foo(arg, &err);
>> + *     bar(arg, &err); // WRONG!
>> + *     if (err) {
>> + *         handle the error...
>> + *     }
>> + * because this may pass a non-null err to bar().
>
> I like that.
>
>>   */
>>  
>>  #ifndef ERROR_H
>> 
>> Leaves replacing &err by errp when the value of err isn't needed to the
>> reader.  I think that's okay given we've shown it already above.
>> 
>> What do you think?
>
> I agree; knowing when it is safe to replace &err by errp is already
> sufficiently covered in existing text, and limiting this example to a
> single point is better.

I'll post it as a formal patch, with your R-by.  Thanks!