Re: [Qemu-devel] [PATCH v5 10/28] qapi: Segregate anonymous unions into alternates in generator

[Eric Blake]

On 03/26/2015 08:47 AM, Markus Armbruster wrote:
> Eric Blake <address@hidden> writes:
> 
>> Special-casing 'discriminator == {}' for handling anonymous unions
>> is getting awkward; since this particular type is not always a
>> dictionary on the wire, it is easier to treat it as a completely
>> different class of type, "alternate", so that if a type is listed
>> in the union_types array, we know it is not an anonymous union.
>>
>> This patch just further segregates union handling, to make sure that
>> anonymous unions are not stored in union_types, and splitting up
>> check_union() into separate functions.  A future patch will change
>> the qapi grammar, and having the segregation already in place will
>> make it easier to deal with the distinct meta-type.
>>
>> Signed-off-by: Eric Blake <address@hidden>
>> ---

>> @@ -535,7 +546,8 @@ def find_struct(name):
>>
>>  def add_union(definition):
>>      global union_types
>> -    union_types.append(definition)
>> +    if definition.get('discriminator') != {}:
>> +        union_types.append(definition)
>>
>>  def find_union(name):
>>      global union_types
> 
> This is the only unobvious hunk.
> 
> union_types is used only through find_union.  The hunk makes
> find_union(N) return None when N names an anonymous union.
> 
> find_union() is used in two places:
> 
> * find_alternate_member_qtype()
> 
>   Patched further up.  It really wants only non-anonymous unions, and
>   this change to find_union() renders its check for anonymous unions
>   superfluous.  Good.
> 
> * generate_visit_alternate()
> 
>   Asserts that each member's type is either a built-in type, a complex
>   type, a union type, or an enum type.
> 
>   The change relaxes the assertion not to trigger on anonymous union
>   types.  Why is that okay?

No, the change tightens the assertion so that it will now fail on an
anonymous union nested as a branch of another anonymous union (where
before it could silently pass the assertion), because the anonymous
union is no longer found by find_union().  And this is okay because the
earlier change to find_alternate_member_qtype means that we don't allow
nested anonymous unions, so making the assertion fail if an anonymous
union gets through anyway is correct.

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

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