Re: [Qemu-devel] [PATCH v7 11/14] qapi: Move duplicate member checks to schema check()

[Eric Blake <eblake@redhat.com>]

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On 10/13/2015 01:08 AM, Markus Armbruster wrote:
> Eric Blake <eblake@redhat.com> writes:
> 
>> On 10/12/2015 09:53 AM, Markus Armbruster wrote:
> [...]
>>> It would be simpler if we could make C clash only when QMP clashes.
>>
>> If a QMP clash always caused a C clash, life would be a bit simpler. We
>> aren't going to get there (because in a flat union, hiding the members
>> of the branch type behind a single pointer means those members don't
>> clash with any C names of the overall union), but I can certainly try to
>> make the comments explain what is going on.
> 
> (1) If QMP clashed exactly when C clashed, we'd have to think only about
> one of them, and the C compiler would check for clashes statically.

Although deferring the error about the clash until compile time is a bit
long, compared to reporting the clash at generator time.

> 
> (2) If a QMP clash implied a C clash, we'd only have to think about C,
> and the C compiler would check statically.
> 
> (3) If a C clash implied a QMP clash, we'd only have to think about QMP.
> 
> (4) If they can clash independently, we need to think about both
> independently.
> 
> We currently have (4), and having to juggle both QMP and C in my mind
> has been taxing.
> 
> My remark was about (3): C clashes only when QMP clashes <=> C clash
> implies QMP clash.
> 
> Your remark was about (2).  More useful than (3), but as you say not
> feasible.
> 
> Do you think we can get to (3)?

C clash that does not imply a QMP clash:

{ 'union':'U', 'data':{ 'a':'int', 'A':'str' } }

C clash (the implicit enum has two U_KIND_A values) but not a QMP clash
('a' and 'A' don't even appear in QMP; and even if they did, they are
distinct in C as branch names).  Might be possible to avoid the C clash
by munging case labels of the generated enum differently, but I'm not
sure it is worth the effort.

QMP clash that does not (currently) imply a C clash (using abbreviated
notation):

{ 'union':'U', 'base':{ 'type':'Enum', 'member':'int' },
  'discriminator':'type',
  'data':{ 'branch':{ 'member':'str' } } }

QMP clash (the field 'member' is part of the base type, and also part of
the 'branch' variant type), but not a C clash (because the C layout
accesses the branch through a box named 'branch').

But we cannot just remove the boxing, either, because then we'd have a
clash in C that is NOT a clash in QMP:

{ 'union':'U', 'base':{ 'type':'Enum' }, 'discriminator':'type',
'data':{ 'branch1':{ 'member':'str' }, 'branch2': { 'member':'int' } } }

If the two 'member' fields were at the same C level as 'type', then we
could use C collisions to detect a QMP clash between a variant's members
and the non-variant fields - but it also has the drawback of declaring a
C collision between the two branches.

So no, I don't think we can get to (3).

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


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