[PATCH v4 1/5] qapi: Enable enum member introspection to show more than name

[John Snow]

On Mon, Oct 25, 2021 at 12:24 AM Markus Armbruster <armbru at redhat.com>
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.  In my testing, output of
>    qemu-system-x86_64's query-qmp-schema grows by 11% (18.5KiB).
>
>    We can deprecate @values now and drop it later.  This will break
>    outmoded clients.  Well-behaved clients such as libvirt are
>    expected to break cleanly.
>
> 2. Like 1, but omit "boring" elements of @member, and empty @member.
>
>    @values does not become redundant.  @members augments it.  Somewhat
>    cumbersome, but output of query-qmp-schema grows only as we make
>    enum members non-boring.
>
>    There is nothing to deprecate here.
>
> 3. Versioned query-qmp-schema.
>
>    query-qmp-schema provides either @values or @members.  The QMP
>    client can select which version it wants.  There is no redundant
>    output.
>
>    We can deprecate old versions and eventually drop them.  This will
>    break outmoded clients.  Breaking cleanly is easier than for 1.
>
>    While 1 and 2 operate within the common rules for compatible
>    evolution apply (section "Compatibility considerations" in
>    docs/devel/qapi-code-gen.rst), 3 bypasses them.  Attractive when
>    operating within the rules is just too awkward.  Not the case here.
>
> This commit implements 1.  Libvirt developers prefer it.
>
> Deprecate @values in favour of @members.  Since query-qmp-schema
> compatibility is pretty fundamental for management applications, an
> extended grace period is advised.
>
> Signed-off-by: Markus Armbruster <armbru at redhat.com>
> Reviewed-by: Eric Blake <eblake at redhat.com>
> Tested-by: Peter Krempa <pkrempa at redhat.com>
> Acked-by: Peter Krempa <pkrempa at redhat.com>
>

Reviewed-by: John Snow <jsnow at redhat.com>
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