Re: [Qemu-devel] [PATCH v4 08/10] qapi: Allow anonymous base for flat union

[Markus Armbruster]

Eric Blake <address@hidden> writes:

> On 03/08/2016 09:23 AM, Markus Armbruster wrote:
>> Eric Blake <address@hidden> writes:
>> 
>>> Rather than requiring all flat unions to explicitly create
>>> a separate base struct, we can allow the qapi schema to specify
>>> the common members via an inline dictionary. This is similar to
>>> how commands can specify an inline anonymous type for its 'data'.
>>> We already have several struct types that only exist to serve as
>>> a single flat union's base; the next commit will clean them up.
>>>
>
>> 
>> I think you also
>> 
>> * fixed a missing allow_optional=True in check_union()
>
> More on that below.
>
>> 
>> * fixed a missing "non-optional" in qapi-code-gen.txt (mention in commit
>>   message?  separate patch?)
>> 
>> * renamed readonly to read-only in an example in qapi-code-gen.txt to be
>>   closer to the code (separate patch?)
>
> Could separate those two cleanups if it helps.

I'd like the fix recorded in git-log.  For that, a mention in the commit
message would suffice, but I think I'd rather separate them.

>>> @@ -560,9 +562,10 @@ def check_union(expr, expr_info):
>>>
>>>      # Else, it's a flat union.
>>>      else:
>>> -        # The object must have a string member 'base'.
>>> +        # The object must have a string or dictionary 'base'.
>>>          check_type(expr_info, "'base' for union '%s'" % name,
>>> -                   base, allow_metas=['struct'])
>>> +                   base, allow_dict=True, allow_optional=True,
>>> +                   allow_metas=['struct'])
>> 
>> This is where you added allow_optional=True.
>
> allow_optional only matters if allow_dict is True.  We have places where
> we allow a dict but do not allow optionals (namely, simple unions); but
> where we are creating an anonymous type, we want to allow optionals at
> the same time we allow a dict.  I think this is just a case where the
> commit message needs to be beefed up.

Perhaps not even that.  I'm spoiled by your meticulous patch versioning,
and when once in a blue moon I spot something you didn't announce there,
I take note.

>>> +A flat union definition avoids nesting on the wire, and specifies a
>>> +set of common members that occur in all variants of the union.  The
>>> +'base' key must specifiy either a type name (the type must be a
>>> +struct, not a union), or a dictionary representing an anonymous type.
>>> +All branches of the union must be complex types, and the top-level
>>> +members of the union dictionary on the wire will be combination of
>>> +members from both the base type and the appropriate branch type (when
>>> +merging two dictionaries, there must be no keys in common).  The
>>> +'discriminator' member must be the name of a non-optional enum-typed
>> 
>> This is where you supplied the missing "non-optional".
>> 
>>> +member of the base struct.
>>>
>> 
>> And below, you rename readonly to read-only.
>
> They touch the same paragraph, but I can separate them if it would make
> this patch feel cleaner.