Kevin Wolf <kwolf@xxxxxxxxxx> writes:
> 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@xxxxxxxxxx>
>> ---
>> 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.
It sure does. Thanks!
