[PATCH v2 3/3] qapi: deprecate drive-backup

[Vladimir Sementsov-Ogievskiy]

08.06.2021 14:12, Markus Armbruster wrote:
> Vladimir Sementsov-Ogievskiy <vsementsov at virtuozzo.com> writes:
> 
> [...]
> 
>> TODO: We also need to deprecate drive-backup transaction action..
>> But union members in QAPI doesn't support 'deprecated' feature. I tried
>> to dig a bit, but failed :/ Markus, could you please help with it? At
>> least by advice?
> 
> There are two closely related things in play here: the union branch and
> the corresponding enum value.
> 
> So far, the QAPI schema language doesn't support tacking feature flags
> to either.
> 
> If an enum value is deprecated, any union branches corresponding to it
> must also be deprecated (because their use requires using the deprecated
> enum value).
> 
> The converse is not true, but I can't see a use for deprecating a union
> branch without also deprecating the enum member.
> 
> I think we can implement feature flags just for enum members, then
> document that 'deprecated' enum value implies corresponding union
> branches are also deprecated.
> 
> I have unfinished patches implementing feature flags for enum members.
> 
> Since TransactionAction is a simple union, the corresponding enum is
> implicit.  We can make it explicit by converting to a flat union.
> Simple unions need to die anyway.


Does BlockStatsSpecific from qapi/block-core.json a correct example of flat union you mean? I can make patch to convert TransactionAction to be similar if that helps (discriminator field should be called "type", yes?).


> 
> Does this make sense?
> 

Yes if it helps)

Did you also look at John's https://gitlab.com/jsnow/qemu/-/commits/hack-deprecate-union-branches/ ?

I hope you and John will send patches that you have, I'll help with reviewing (keep me in CC), and finally we'll get the feature.

-- 
Best regards,
Vladimir