Re: [Qemu-devel] [PATCH v2 11/19] qapi: Add type.is_empty() helper

[Markus Armbruster]

Eric Blake <address@hidden> writes:

> And use it in qapi-types and qapi-event.  Down the road, we may
> want to lift our artificial restriction of no variants at the
> top level of an event, at which point, inlining our check for
> whether members is empty will no longer be sufficient, but
> adding a check for variants adds verbosity; in the meantime,
> add some asserts in places where we don't handle variants.

Perhaps I'm just running out of steam for today, but I've read this
twice, and still don't get why adding these assertions goes in the same
patch as adding the helper, or what it has to do with events.

> More immediately, the new .is_empty() helper will help fix a bug
> in qapi-visit in the next patch, where the generator did not
> handle an explicit empty type in the same was as a missing type.

same way

>
> No change to generated code.
>
> Signed-off-by: Eric Blake <address@hidden>
>
> ---
> v2: no change
> v1: add some asserts
> Previously posted as part of qapi cleanup subset E:
> v9: improve commit message
> v8: no change
> v7: rebase to context change
> v6: new patch
> ---
>  scripts/qapi.py       | 5 +++++
>  scripts/qapi-event.py | 7 ++++---
>  scripts/qapi-types.py | 2 +-
>  3 files changed, 10 insertions(+), 4 deletions(-)
>
> diff --git a/scripts/qapi.py b/scripts/qapi.py
> index 6c52fe5..83080b3 100644
> --- a/scripts/qapi.py
> +++ b/scripts/qapi.py
> @@ -962,6 +962,7 @@ class QAPISchemaObjectType(QAPISchemaType):
>              assert isinstance(self.base, QAPISchemaObjectType)
>              self.base.check(schema)
>              self.base.check_clash(schema, self.info, seen)
> +            assert not self.base.variants
>          for m in self.local_members:
>              m.check(schema)
>              m.check_clash(self.info, seen)

This is the "some asserts" ;)

> @@ -983,6 +984,10 @@ class QAPISchemaObjectType(QAPISchemaType):
>          # See QAPISchema._make_implicit_object_type()
>          return self.name[0] == ':'
>
> +    def is_empty(self):
> +        assert self.members is not None
> +        return not self.members and not self.variants
> +
>      def c_name(self):
>          assert not self.is_implicit()
>          return QAPISchemaType.c_name(self)
> diff --git a/scripts/qapi-event.py b/scripts/qapi-event.py
> index fb579dd..808ed80 100644
> --- a/scripts/qapi-event.py
> +++ b/scripts/qapi-event.py
> @@ -39,7 +39,7 @@ def gen_event_send(name, arg_type):
>  ''',
>                  proto=gen_event_send_proto(name, arg_type))
>
> -    if arg_type and arg_type.members:
> +    if arg_type and not arg_type.is_empty():
>          ret += mcgen('''
>      QmpOutputVisitor *qov;
>      Visitor *v;

Oh, you don't just add a helper, you actually *change* the condition!
Perhaps the commit message would be easier to understand if it explained
that first.

> @@ -58,7 +58,8 @@ def gen_event_send(name, arg_type):
>  ''',
>                   name=name)
>
> -    if arg_type and arg_type.members:
> +    if arg_type and not arg_type.is_empty():
> +        assert not arg_type.variants
>          ret += mcgen('''
>      qov = qmp_output_visitor_new();
>      v = qmp_output_get_visitor(qov);
> @@ -88,7 +89,7 @@ out_obj:
>  ''',
>                   c_enum=c_enum_const(event_enum_name, name))
>
> -    if arg_type and arg_type.members:
> +    if arg_type and not arg_type.is_empty():
>          ret += mcgen('''
>  out:
>      qmp_output_visitor_cleanup(qov);
> diff --git a/scripts/qapi-types.py b/scripts/qapi-types.py
> index 0306a88..6c1923d 100644
> --- a/scripts/qapi-types.py
> +++ b/scripts/qapi-types.py
> @@ -90,7 +90,7 @@ struct %(c_name)s {
>      # potential issues with attempting to malloc space for zero-length
>      # structs in C, and also incompatibility with C++ (where an empty
>      # struct is size 1).
> -    if not (base and base.members) and not members and not variants:
> +    if (not base or base.is_empty()) and not members and not variants:
>          ret += mcgen('''
>      char qapi_dummy_for_empty_struct;
>  ''')

I figure the case for the helper based on this patch alone is making the
code a bit more future-proof.  Suggest you try to explain that in your
commit message, including against what future change exactly you're
proofing the code.

Haven't reviewed for completeness.