[PATCH wayland] client: add wl_proxy_get_version function

[Rémi Denis-Courmont]

Le 2014-10-06 11:26, Pekka Paalanen a écrit :
>> > 4 or higher as a wl_surface version 3? If not, I cannot rely on
>> > 'wl_proxy_get_version(surface) >= 2'...
>>
>> Changing the semantics[5] of wl_surface.damage is a good example 
>> where
>> this would fail indeed. If a library only knows up to version 3, but
>> the app gives it a wl_surface of version 4, the damage coordinates
>> would be wrong.
>>
>> Originally we had the idea, that all version bumps are backwards
>> compatible. Simply testing for version >= X would always work. The
>> wl_surface.damage change is not backwards compatible in that sense.
>>
>> Looks to me like if we don't fix damage, then this versioning 
>> problem
>> would be solvable... :-(
>
> Oh I'm being silly, we only need to fix damage in a backward 
> compatible
> way: add a new request, don't modify the behaviour of the old one.
>
> Keeping backward compatibility wrt. version >= X checks seems like a
> good idea and should reduce the problem set somewhat.

You could stick to adding new requests rather than redefine existing 
ones.

But what about events. AFAICT, they would still require either:
- never adding events in new versions,
  or
   - adding events that only fire after a new requests was sent,
    and
   - never copying the listener structure ever in the client library
     (to prevent out-of-bounds read).

Otherwise, it would still fail the code path setting the object 
listener targets an older interface version than the code path creating 
the object.

-- 
Rémi Denis-Courmont