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

[Markus Armbruster]

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 this make sense?