Re: [Qemu-devel] [PATCH v8 10/18] qapi: Move union tag quirks into subclass

[Markus Armbruster <armbru@redhat.com>]

Eric Blake <eblake@redhat.com> writes:

> Right now, simple unions have a quirk of using 'kind' in the C
> struct to match the QMP wire name 'type'.  This has resulted in
> messy clients each doing special cases.  While we plan to
> eventually rename things to match, it is better in the meantime
> to consolidate the quirks into a special subclass, by adding a
> new member.c_name() function.  This will also make it easier
> for reworking how alternate types are laid out in a future
> patch.  Use the new c_name() function where possible.

Terminology: "the new c_name() method".

I don't like having both function c_name() and method c_name(), because
it's very easy to use the function when you should use the method, and
the mistakes will make things break only rarely, so they can go
undetected easily.  I'm okay with this patch only because you assure me
the whole thing is temporary: "# TODO Drop this class once we no longer
have the 'type'/'kind' mismatch".

> No change to generated code.
>
> Signed-off-by: Eric Blake <eblake@redhat.com>
>
> ---

v8: use + instead of interpolation in a few places, rebase to simpler
.is_implicit(), use .c_name() more.

> v7: new patch, but borrows idea of subclass from v6 10/12, as
> well as c_name() from 7/12
> ---
>  scripts/qapi-commands.py |  8 ++++----
>  scripts/qapi-types.py    | 12 +++++-------
>  scripts/qapi-visit.py    | 17 +++++------------
>  scripts/qapi.py          | 26 +++++++++++++++++++-------
>  4 files changed, 33 insertions(+), 30 deletions(-)
>
> diff --git a/scripts/qapi-commands.py b/scripts/qapi-commands.py
> index 43a893b..53a79ab 100644
> --- a/scripts/qapi-commands.py
> +++ b/scripts/qapi-commands.py
> @@ -32,8 +32,8 @@ def gen_call(name, arg_type, ret_type):
>      if arg_type:
>          for memb in arg_type.members:
>              if memb.optional:
> -                argstr += 'has_%s, ' % c_name(memb.name)
> -            argstr += '%s, ' % c_name(memb.name)
> +                argstr += 'has_' + memb.c_name() + ', '
> +            argstr += memb.c_name() + ', '

I like to use + instead of % in sufficiently simple cases myself.  Not
sure this is one.  See also change to gen_params() below.

>
>      lhs = ''
>      if ret_type:
> @@ -77,11 +77,11 @@ def gen_marshal_vars(arg_type, ret_type):
>                  ret += mcgen('''
>      bool has_%(c_name)s = false;
>  ''',
> -                             c_name=c_name(memb.name))
> +                             c_name=memb.c_name())
>              ret += mcgen('''
>      %(c_type)s %(c_name)s = %(c_null)s;
>  ''',
> -                         c_name=c_name(memb.name),
> +                         c_name=memb.c_name(),
>                           c_type=memb.type.c_type(),
>                           c_null=memb.type.c_null())
>          ret += '\n'
> diff --git a/scripts/qapi-types.py b/scripts/qapi-types.py
> index 4fe618e..e37d529 100644
> --- a/scripts/qapi-types.py
> +++ b/scripts/qapi-types.py
> @@ -136,9 +136,10 @@ struct %(c_name)s {
>  ''')
>      else:
>          ret += mcgen('''
> -    %(c_type)s kind;
> +    %(c_type)s %(c_name)s;
>  ''',
> -                     c_type=c_name(variants.tag_member.type.name))
> +                     c_type=variants.tag_member.type.c_name(),
> +                     c_name=variants.tag_member.c_name())
>
>      # FIXME: What purpose does data serve, besides preventing a union that
>      # has a branch named 'data'? We use it in qapi-visit.py to decide
> @@ -152,10 +153,7 @@ struct %(c_name)s {
>      union { /* union tag is @%(c_name)s */
>          void *data;
>  ''',
> -                 # TODO ugly special case for simple union
> -                 # Use same tag name in C as on the wire to get rid of
> -                 # it, then: c_name=c_name(variants.tag_member.name)
> -                 c_name=c_name(variants.tag_name or 'kind'))
> +                 c_name=variants.tag_member.c_name())
>
>      for var in variants.variants:
>          # Ugly special case for simple union TODO get rid of it
> @@ -164,7 +162,7 @@ struct %(c_name)s {
>          %(c_type)s %(c_name)s;
>  ''',
>                       c_type=typ.c_type(),
> -                     c_name=c_name(var.name))
> +                     c_name=var.c_name())
>
>      ret += mcgen('''
>      };
> diff --git a/scripts/qapi-visit.py b/scripts/qapi-visit.py
> index 2a9fab8..cb251b2 100644
> --- a/scripts/qapi-visit.py
> +++ b/scripts/qapi-visit.py
> @@ -196,7 +196,7 @@ void visit_type_%(c_name)s(Visitor *v, %(c_name)s **obj, const char *name, Error
>                       case=c_enum_const(variants.tag_member.type.name,
>                                         var.name),
>                       c_type=var.type.c_name(),
> -                     c_name=c_name(var.name))
> +                     c_name=var.c_name())
>
>      ret += mcgen('''
>      default:
> @@ -249,10 +249,6 @@ void visit_type_%(c_name)s(Visitor *v, %(c_name)s **obj, const char *name, Error
>                       c_name=c_name(name))
>          ret += gen_err_check(label='out_obj')
>
> -    tag_key = variants.tag_member.name
> -    if not variants.tag_name:
> -        # we pointlessly use a different key for simple unions
> -        tag_key = 'type'
>      ret += mcgen('''
>      visit_type_%(c_type)s(v, &(*obj)->%(c_name)s, "%(name)s", &err);
>      if (err) {
> @@ -264,11 +260,8 @@ void visit_type_%(c_name)s(Visitor *v, %(c_name)s **obj, const char *name, Error
>      switch ((*obj)->%(c_name)s) {
>  ''',
>                   c_type=variants.tag_member.type.c_name(),
> -                 # TODO ugly special case for simple union
> -                 # Use same tag name in C as on the wire to get rid of
> -                 # it, then: c_name=c_name(variants.tag_member.name)
> -                 c_name=c_name(variants.tag_name or 'kind'),
> -                 name=tag_key)
> +                 c_name=variants.tag_member.c_name(),
> +                 name=variants.tag_member.name)
>
>      for var in variants.variants:
>          # TODO ugly special case for simple union
> @@ -283,13 +276,13 @@ void visit_type_%(c_name)s(Visitor *v, %(c_name)s **obj, const char *name, Error
>          visit_type_%(c_type)s(v, &(*obj)->%(c_name)s, "data", &err);
>  ''',
>                           c_type=simple_union_type.c_name(),
> -                         c_name=c_name(var.name))
> +                         c_name=var.c_name())
>          else:
>              ret += mcgen('''
>          visit_type_implicit_%(c_type)s(v, &(*obj)->%(c_name)s, &err);
>  ''',
>                           c_type=var.type.c_name(),
> -                         c_name=c_name(var.name))
> +                         c_name=var.c_name())
>          ret += mcgen('''
>          break;
>  ''')
> diff --git a/scripts/qapi.py b/scripts/qapi.py
> index 5b66264..80c026b 100644
> --- a/scripts/qapi.py
> +++ b/scripts/qapi.py
> @@ -975,7 +975,7 @@ class QAPISchemaObjectType(QAPISchemaType):
>              members = []
>          seen = {}
>          for m in members:
> -            assert c_name(m.name) not in seen
> +            assert m.c_name() not in seen
>              seen[m.name] = m
>          for m in self.local_members:
>              m.check(schema, members, seen)
> @@ -1022,6 +1022,18 @@ class QAPISchemaObjectTypeMember(object):
>          all_members.append(self)
>          seen[self.name] = self
>
> +    def c_name(self):
> +        return c_name(self.name)
> +
> +
> +# TODO Drop this class once we no longer have the 'type'/'kind' mismatch
> +class QAPISchemaObjectTypeUnionTag(QAPISchemaObjectTypeMember):
> +    def c_name(self):
> +        assert self.name == 'type'
> +        assert isinstance(self.type, QAPISchemaEnumType)
> +        assert self.type.is_implicit()
> +        return 'kind'
> +
>
>  class QAPISchemaObjectTypeVariants(object):
>      def __init__(self, tag_name, tag_member, variants):
> @@ -1248,7 +1260,7 @@ class QAPISchema(object):
>      def _make_implicit_tag(self, type_name, variants):
>          typ = self._make_implicit_enum_type(type_name,
>                                              [v.name for v in variants])
> -        return QAPISchemaObjectTypeMember('type', typ, False)
> +        return QAPISchemaObjectTypeUnionTag('type', typ, False)
>
>      def _def_union_type(self, expr, info):
>          name = expr['union']
> @@ -1555,8 +1567,8 @@ def gen_params(arg_type, extra):
>          ret += sep
>          sep = ', '
>          if memb.optional:
> -            ret += 'bool has_%s, ' % c_name(memb.name)
> -        ret += '%s %s' % (memb.type.c_type(is_param=True), c_name(memb.name))
> +            ret += 'bool has_' + memb.c_name() + sep
> +        ret += '%s %s' % (memb.type.c_type(is_param=True), memb.c_name())
>      if extra:
>          ret += sep + extra
>      return ret

New hunk in v8, to remain consistent with gen_call().

I doubt replacing literal ', ' by sep is making things clearer.

> @@ -1585,13 +1597,13 @@ def gen_visit_fields(members, prefix='', need_cast=False, skiperr=False):
>              ret += mcgen('''
>      visit_optional(v, &%(prefix)shas_%(c_name)s, "%(name)s", %(errp)s);
>  ''',
> -                         prefix=prefix, c_name=c_name(memb.name),
> +                         prefix=prefix, c_name=memb.c_name(),
>                           name=memb.name, errp=errparg)
>              ret += gen_err_check(skiperr=skiperr)
>              ret += mcgen('''
>      if (%(prefix)shas_%(c_name)s) {
>  ''',
> -                         prefix=prefix, c_name=c_name(memb.name))
> +                         prefix=prefix, c_name=memb.c_name())
>              push_indent()
>
>          # Ugly: sometimes we need to cast away const
> @@ -1604,7 +1616,7 @@ def gen_visit_fields(members, prefix='', need_cast=False, skiperr=False):
>      visit_type_%(c_type)s(v, %(cast)s&%(prefix)s%(c_name)s, "%(name)s", %(errp)s);
>  ''',
>                       c_type=memb.type.c_name(), prefix=prefix, cast=cast,
> -                     c_name=c_name(memb.name), name=memb.name,
> +                     c_name=memb.c_name(), name=memb.name,
>                       errp=errparg)
>          ret += gen_err_check(skiperr=skiperr)

New hunks in v8.  Do you change from function c_name() to method
.c_name() Just for consistency, or is there a more serious reason?