Re: [PATCH v2 1/5] qapi: Enable enum member introspection to show more than name

[Kevin Wolf]

Am 09.10.2021 um 14:09 hat Markus Armbruster geschrieben:
> 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.
> 
> Signed-off-by: Markus Armbruster <armbru@redhat.com>
> ---
>  qapi/introspect.json       | 21 +++++++++++++++++++--
>  scripts/qapi/introspect.py | 18 ++++++++++++++----
>  2 files changed, 33 insertions(+), 6 deletions(-)

docs/devel/qapi-code-gen.rst still says:

  The SchemaInfo for an enumeration type has meta-type "enum" and
  variant member "values".  The values are listed in no particular
  order; clients must search the entire enum when learning whether a
  particular value is supported.

  Example: the SchemaInfo for MyEnum from section `Enumeration types`_ ::

      { "name": "MyEnum", "meta-type": "enum",
        "values": [ "value1", "value2", "value3" ] }

It probably needs an update.

Kevin