Re: [PATCH 4/9] qapi: Tools for sets of special feature flags in generated code

[Markus Armbruster <armbru@redhat.com>]

John Snow <jsnow@redhat.com> writes:

> On Mon, Oct 25, 2021 at 1:25 AM Markus Armbruster <armbru@redhat.com> wrote:
>
>> New enum QapiSpecialFeature enumerates the special feature flags.
>>
>> New helper gen_special_features() returns code to represent a
>> collection of special feature flags as a bitset.
>>
>> The next few commits will put them to use.
>>
>> Signed-off-by: Markus Armbruster <armbru@redhat.com>
>> ---
>>  include/qapi/util.h    |  4 ++++
>>  scripts/qapi/gen.py    | 13 +++++++++++++
>>  scripts/qapi/schema.py |  3 +++
>>  3 files changed, 20 insertions(+)
>>
>> diff --git a/include/qapi/util.h b/include/qapi/util.h
>> index 257c600f99..7a8d5c7d72 100644
>> --- a/include/qapi/util.h
>> +++ b/include/qapi/util.h
>> @@ -11,6 +11,10 @@
>>  #ifndef QAPI_UTIL_H
>>  #define QAPI_UTIL_H
>>
>> +typedef enum {
>> +    QAPI_DEPRECATED,
>> +} QapiSpecialFeature;
>> +
>>  /* QEnumLookup flags */
>>  #define QAPI_ENUM_DEPRECATED 1
>>
>> diff --git a/scripts/qapi/gen.py b/scripts/qapi/gen.py
>> index 2ec1e7b3b6..9d07b88cf6 100644
>> --- a/scripts/qapi/gen.py
>> +++ b/scripts/qapi/gen.py
>> @@ -29,6 +29,7 @@
>>      mcgen,
>>  )
>>  from .schema import (
>> +    QAPISchemaFeature,
>>      QAPISchemaIfCond,
>>      QAPISchemaModule,
>>      QAPISchemaObjectType,
>> @@ -37,6 +38,18 @@
>>  from .source import QAPISourceInfo
>>
>>
>> +def gen_special_features(features: QAPISchemaFeature):
>> +    ret = ''
>> +    sep = ''
>> +
>> +    for feat in features:
>> +        if feat.is_special():
>> +            ret += ('%s1u << QAPI_%s' % (sep, feat.name.upper()))
>>
>
> Building the constant name here "feels" fragile, but I'll trust that the
> test suite and/or the compiler will catch us if we accidentally goof up
> this mapping. In the interest of simplicity, then, "sure, why not."

It relies on .is_special() remaining consistent with enum
QapiSpecialFeature.  The C compiler should catch any screwups.

>
>> +            sep = ' | '
>> +
>>
> +    return ret or '0'
>> +
>>
>
> Subjectively more pythonic:
>
> special_features = [f"1u << QAPI_{feat.name.upper()}" for feat in features
> if feat.is_special()]
> ret = ' | '.join(special_features)
> return ret or '0'
>
> A bit more dense, but more functional. Up to you, but I find join() easier
> to read and reason about for the presence of separators.

Sold!

>> +
>>  class QAPIGen:
>>      def __init__(self, fname: str):
>>          self.fname = fname
>> diff --git a/scripts/qapi/schema.py b/scripts/qapi/schema.py
>> index 6d5f46509a..55f82d7389 100644
>> --- a/scripts/qapi/schema.py
>> +++ b/scripts/qapi/schema.py
>> @@ -725,6 +725,9 @@ def connect_doc(self, doc):
>>  class QAPISchemaFeature(QAPISchemaMember):
>>      role = 'feature'
>>
>> +    def is_special(self):
>> +        return self.name in ('deprecated')
>> +
>>
>
> alrighty.
>
> (Briefly wondered: is it worth naming special features as a property of the
> class, but with only two names, it's probably fine enough to leave it
> embedded in the method logic. Only a style thing and doesn't have any
> actual impact that I can imagine, so ... nevermind.)

Let's keep it simple.

>>  class QAPISchemaObjectTypeMember(QAPISchemaMember):
>>      def __init__(self, name, info, typ, optional, ifcond=None,
>> features=None):
>> --
>> 2.31.1
>>
>>
> Well, either way:
>
> Reviewed-by: John Snow <jsnow@redhat.com>

Thanks!