Re: [PATCH 6/6] qapi: expose all schema features to code

[Markus Armbruster <armbru@redhat.com>]

Daniel P. Berrangé <berrange@redhat.com> writes:

> This removed the QapiFeatures enum and auto-generates an enum which
> exposes all features defined by the schema to code.
>
> The 'deprecated' and 'unstable' features still have a little bit of
> special handling, being force defined to be the 1st + 2nd features
> in the enum, regardless of whether they're used in the schema. This
> is because QAPI common code references these features.
>
> Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
> ---
>  include/qapi/util.h      |   5 --
>  meson.build              |   1 +
>  scripts/qapi/features.py | 134 +++++++++++++++++++++++++++++++++++++++
>  scripts/qapi/main.py     |   2 +
>  scripts/qapi/schema.py   |   5 +-
>  scripts/qapi/types.py    |   4 +-
>  6 files changed, 144 insertions(+), 7 deletions(-)
>  create mode 100644 scripts/qapi/features.py
>
> diff --git a/include/qapi/util.h b/include/qapi/util.h
> index a693cac9ea..9e390486c0 100644
> --- a/include/qapi/util.h
> +++ b/include/qapi/util.h
> @@ -11,11 +11,6 @@
>  #ifndef QAPI_UTIL_H
>  #define QAPI_UTIL_H
>  
> -typedef enum {
> -    QAPI_FEATURE_DEPRECATED,
> -    QAPI_FEATURE_UNSTABLE,
> -} QapiFeature;
> -
>  typedef struct QEnumLookup {
>      const char *const *array;
>      const uint64_t *const features;
> diff --git a/meson.build b/meson.build
> index 97f63aa86c..40002c59f5 100644
> --- a/meson.build
> +++ b/meson.build
> @@ -3268,6 +3268,7 @@ qapi_gen_depends = [ meson.current_source_dir() / 'scripts/qapi/__init__.py',
>                       meson.current_source_dir() / 'scripts/qapi/schema.py',
>                       meson.current_source_dir() / 'scripts/qapi/source.py',
>                       meson.current_source_dir() / 'scripts/qapi/types.py',
> +                     meson.current_source_dir() / 'scripts/qapi/features.py',
>                       meson.current_source_dir() / 'scripts/qapi/visit.py',
>                       meson.current_source_dir() / 'scripts/qapi-gen.py'
>  ]
> diff --git a/scripts/qapi/features.py b/scripts/qapi/features.py
> new file mode 100644
> index 0000000000..9b77be6310
> --- /dev/null
> +++ b/scripts/qapi/features.py
> @@ -0,0 +1,134 @@
> +"""
> +QAPI types generator
> +
> +Copyright 2024 Red Hat
> +
> +This work is licensed under the terms of the GNU GPL, version 2.
> +# See the COPYING file in the top-level directory.
> +"""
> +
> +from typing import List, Optional
> +
> +from .common import c_enum_const, mcgen, c_name
> +from .gen import QAPISchemaMonolithicCVisitor
> +from .schema import (
> +    QAPISchema,
> +    QAPISchemaAlternatives,
> +    QAPISchemaBranches,
> +    QAPISchemaEntity,
> +    QAPISchemaEnumMember,
> +    QAPISchemaFeature,
> +    QAPISchemaIfCond,
> +    QAPISchemaObjectType,
> +    QAPISchemaObjectTypeMember,
> +    QAPISchemaType,
> +    QAPISchemaVariants,
> +)
> +from .source import QAPISourceInfo
> +
> +
> +class QAPISchemaGenFeatureVisitor(QAPISchemaMonolithicCVisitor):
> +
> +    def __init__(self, prefix: str):
> +        super().__init__(
> +            prefix, 'qapi-features',
> +            ' * Schema-defined QAPI features',
> +            __doc__)
> +
> +        self.features = {}
> +
> +    def visit_end(self) -> None:
> +        features = []
> +
> +        # We always want special features to have the same
> +        # enum value across all schemas, since they're
> +        # referenced from common code. Put them at the
> +        # start of the list, regardless of whether they
> +        # are actually referenced in the schema
> +        for name in QAPISchemaFeature.SPECIAL_NAMES:
> +            features.append(name)
> +
> +        features.extend(sorted(self.features.keys()))
> +
> +        if len(features) > 64:
> +            raise Exception("Maximum of 64 schema features is permitted")

This is just one notch above assert len(features) > 64, so nope :)

Backends are not supposed to diagnose schema errors or constraint
violations.  That's the frontend's job.

Perhaps the simplest way to check this in the frontend is to build a
feature set in QAPISchema: initialize in .__init__(), update in
._make_features(), fail right there when asked to make a 64th.

> +
> +        self._genh.add("typedef enum {\n")
> +        for name in features:
> +            self._genh.add(f"    {c_enum_const(self._prefix + 'QAPI_FEATURE', name)},\n")

This duplicates part of gen.gen_features().  Suggest to factor out the
common part as gen_feature().

Note for later: enum QapiFeature is defined in generated
qapi-features.h.

> +
> +        self._genh.add("} " + c_name(self._prefix + 'QapiFeature') + ";\n")
> +
> +    def _record(self, features: List[QAPISchemaFeature]):
> +        for f in features:
> +            # Special features are handled separately
> +            if f.name in QAPISchemaFeature.SPECIAL_NAMES:
> +                continue
> +            self.features[f.name] = True
> +
> +    def visit_enum_type(self,
> +                        name: str,
> +                        info: Optional[QAPISourceInfo],
> +                        ifcond: QAPISchemaIfCond,
> +                        features: List[QAPISchemaFeature],
> +                        members: List[QAPISchemaEnumMember],
> +                        prefix: Optional[str]) -> None:
> +        self._record(features)
> +
> +    def visit_object_type_flat(self,
> +                               name: str,
> +                               info: Optional[QAPISourceInfo],
> +                               ifcond: QAPISchemaIfCond,
> +                               features: List[QAPISchemaFeature],
> +                               members: List[QAPISchemaObjectTypeMember],
> +                               branches: Optional[QAPISchemaBranches]) -> None:
> +        self._record(features)
> +
> +    def visit_object_type(self,
> +                          name: str,
> +                          info: Optional[QAPISourceInfo],
> +                          ifcond: QAPISchemaIfCond,
> +                          features: List[QAPISchemaFeature],
> +                          base: Optional[QAPISchemaObjectType],
> +                          members: List[QAPISchemaObjectTypeMember],
> +                          branches: Optional[QAPISchemaBranches]) -> None:
> +        self._record(features)
> +
> +    def visit_alternate_type(self,
> +                             name: str,
> +                             info: Optional[QAPISourceInfo],
> +                             ifcond: QAPISchemaIfCond,
> +                             features: List[QAPISchemaFeature],
> +                             alternatives: QAPISchemaAlternatives) -> None:
> +        self._record(features)
> +
> +    def visit_command(self,
> +                      name: str,
> +                      info: Optional[QAPISourceInfo],
> +                      ifcond: QAPISchemaIfCond,
> +                      features: List[QAPISchemaFeature],
> +                      arg_type: Optional[QAPISchemaObjectType],
> +                      ret_type: Optional[QAPISchemaType],
> +                      gen: bool,
> +                      success_response: bool,
> +                      boxed: bool,
> +                      allow_oob: bool,
> +                      allow_preconfig: bool,
> +                      coroutine: bool) -> None:
> +        self._record(features)
> +
> +    def visit_event(self,
> +                    name: str,
> +                    info: Optional[QAPISourceInfo],
> +                    ifcond: QAPISchemaIfCond,
> +                    features: List[QAPISchemaFeature],
> +                    arg_type: Optional[QAPISchemaObjectType],
> +                    boxed: bool) -> None:
> +        self._record(features)
> +

pycodestyle-3 gripes

    scripts/qapi/features.py:129:1: E302 expected 2 blank lines, found 1

> +def gen_features(schema: QAPISchema,
> +                 output_dir: str,
> +                 prefix: str) -> None:
> +    vis = QAPISchemaGenFeatureVisitor(prefix)
> +    schema.visit(vis)
> +    vis.write(output_dir)

We have another gen_features() in gen.py.  Not a show stopper, but could
we find equally good names that don't clash?

> diff --git a/scripts/qapi/main.py b/scripts/qapi/main.py
> index 316736b6a2..2b9a2c0c02 100644
> --- a/scripts/qapi/main.py
> +++ b/scripts/qapi/main.py
> @@ -18,6 +18,7 @@
>  from .introspect import gen_introspect
>  from .schema import QAPISchema
>  from .types import gen_types
> +from .features import gen_features
>  from .visit import gen_visit
>  
>  
> @@ -49,6 +50,7 @@ def generate(schema_file: str,
>  
>      schema = QAPISchema(schema_file)
>      gen_types(schema, output_dir, prefix, builtins)
> +    gen_features(schema, output_dir, prefix)
>      gen_visit(schema, output_dir, prefix, builtins)
>      gen_commands(schema, output_dir, prefix, gen_tracing)
>      gen_events(schema, output_dir, prefix)
> diff --git a/scripts/qapi/schema.py b/scripts/qapi/schema.py
> index d65c35f6ee..160ce0a7c0 100644
> --- a/scripts/qapi/schema.py
> +++ b/scripts/qapi/schema.py
> @@ -933,8 +933,11 @@ def connect_doc(self, doc: Optional[QAPIDoc]) -> None:
>  class QAPISchemaFeature(QAPISchemaMember):
>      role = 'feature'
>  
> +    # Features which are standardized across all schemas
> +    SPECIAL_NAMES = ['deprecated', 'unstable']
> +
>      def is_special(self) -> bool:
> -        return self.name in ('deprecated', 'unstable')
> +        return self.name in QAPISchemaFeature.SPECIAL_NAMES
>  
>  
>  class QAPISchemaObjectTypeMember(QAPISchemaMember):
> diff --git a/scripts/qapi/types.py b/scripts/qapi/types.py
> index b2d26c2ea8..3435f1b0b0 100644
> --- a/scripts/qapi/types.py
> +++ b/scripts/qapi/types.py
> @@ -313,7 +313,9 @@ def _begin_user_module(self, name: str) -> None:
>                                        types=types, visit=visit))
>          self._genh.preamble_add(mcgen('''
>  #include "qapi/qapi-builtin-types.h"
> -'''))
> +#include "%(prefix)sqapi-features.h"

This works, because it pulls in qapi-features.h basically everywhere.

It's actually needed only where we use enum QapiFeature.  So far, we
only ever generate uses with gen.gen_features().  Callers:

* gen_register_command() for qapi-init-commands.c.

* gen_enum_lookup() for qapi-types*.c and qapi-emit-events.c.

* gen_visit_object_members() for qapi-visit*.c.

Please include it just in these generated .c files.

> +''',
> +                                      prefix=self._prefix))
>  
>      def visit_begin(self, schema: QAPISchema) -> None:
>          # gen_object() is recursive, ensure it doesn't visit the empty type