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

[Eric Blake]

On 03/02/2016 12:04 PM, Markus Armbruster wrote:
> 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.

And yet it was the review on the earlier posting that caused me to add
asserts; maybe re-reading that thread will help refresh memory, and spur
an idea for how to better express it in the commit message:
https://lists.gnu.org/archive/html/qemu-devel/2016-01/msg04726.html

> 
>> 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

[Ever wonder if I intentionally stick in a typo, just to see who will
notice? Or maybe it really was a slip of the finger...]

>> +++ 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.

The old condition:
arg_type and arg_type.members

New condition:
arg_type and (arg_type.members or arg_type.variants)

But we know there are no variants, since unions cannot (yet) be passed
as event 'data', so the condition is the same effect now, and
future-proofing for a future patch when I do allow unions in events.

>> +++ 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.

And here, bases cannot (yet) have variants, but that's also on my plate
of things I'd like to support in the future.

> 
> Haven't reviewed for completeness.
> 

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

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