On 8/15/19 9:44 AM, Maxim Levitsky wrote:

>>>> Does the idea of a union type with a default value for the discriminator
>>>> help?  Maybe we have a discriminator which defaults to 'auto', and add a
>>>> union branch 'auto':'any'.  During creation, if the "driver":"auto"
>>>> branch is selected (usually implicitly by omitting "driver", but also
>>>> possible explicitly), the creation attempt is rejected as invalid
>>>> regardless of the contents of the remaining 'any'.  But during amend
>>>> usage, if the 'auto' branch is selected, we then add in the proper
>>>> "driver":"xyz" and reparse the QAPI object to determine if the remaining
>>>> fields in 'any' still meet the specification for the required driver branch.
>>>>
>>>> This idea may still require some tweaks to the QAPI generator, but it's
>>>> the best I can come up with for a way to parse an arbitrary JSON object
>>>> with unknown validation, then reparse it again after adding more
>>>> information that would constrain the parse differently.
>>>
>>> Feels like this would be a lot of code just to allow the client to omit
>>> passing a value that it knows anyway. If this were a human interface, I
>>> could understand the desire to make commands less verbose, but for QMP I
>>> honestly don't see the point when it's not trivial.
>>
>> Seconded.
> 
> 
> But what about my suggestion of adding something like:
> 
> { 'union': 'BlockdevAmendOptions',
> 
>   'base': {
>       'node-name':         'str' },
> 
>   'discriminator': { 'get_block_driver(node-name)' } ,

Not worth it. It makes the QAPI generator more complex (to invoke
arbitrary code instead of a fixed name) just to avoid a little bit of
complexity in the caller (which is assumed to be a computer, and thus
shouldn't have a hard time providing a sane 'driver' unconditionally).
An HMP wrapper around the QMP command can do whatever magic it needs to
omit driver, but making driver mandatory for QMP is just fine.

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org