[PATCH RFC 1/5] qapi: Enable enum member introspection to show more than name

[Peter Krempa]

On Wed, Sep 15, 2021 at 21:24:21 +0200, Markus Armbruster wrote:
> The next commit will add feature flags to enum members.  There's a
> problem, though: query-qmp-schema shows an enum type's members as an
> array of member names (SchemaInfoEnum member @values).  If it showed
> an array of objects with a name member, we could simply add more
> members to these objects.  Since it's just strings, we can't.
> 
> I can see three ways to correct this design mistake:
> 
> 1. Do it the way we should have done it, plus compatibility goo.
> 
>    We want a ['SchemaInfoEnumMember'] member in SchemaInfoEnum.  Since
>    changing @values would be a compatibility break, add a new member
>    @members instead.
> 
>    @values is now redundant.  We should be able to get rid of it
>    eventually.
> 
>    In my testing, output of qemu-system-x86_64's query-qmp-schema
>    grows by 11% (18.5KiB).

I prefer this one. While the schema output grows, nobody is really
reading it manually.

The implementation in libvirt is very straightforward:

https://gitlab.com/pipo.sk/libvirt/-/commit/24f1709cfae72cd765d02dc2124d6c954554ea55
https://gitlab.com/pipo.sk/libvirt/-/commit/5909db9d4994327b3f64d5c329edd4b175495bfe

> 2. Like 1, but omit "boring" elements of @member, and empty @member.
> 
>    @values does not become redundant.  Output of query-qmp-schema
>    grows only as we make enum members non-boring.

This has 2 drawbacks:
- we would never get rid of either of those
- clients would have to check both, one for whether the member is
  present and second whether it's non-boring.

IMO it's simpler for clients just to prefer the new approach when
present as the old is simply a subset.

> 3. Versioned query-qmp-schema.
> 
>    query-qmp-schema provides either @values or @members.  The QMP
>    client can select which version it wants.

At least for libvirt this poses a chicken & egg problem. We'd have to
query the schema to see that it has the switch to do the selection and
then probe with the modern one.