[RFC libwayland] Track protocol object versions inside wl_proxy.

[Jason Ekstrand]

Ping

On Fri, Apr 11, 2014 at 1:36 AM, Jason Ekstrand <jason at jlekstrand.net>
wrote:

> On Fri, Apr 11, 2014 at 2:03 AM, Pekka Paalanen <ppaalanen at gmail.com>
> wrote:
>
>> On Thu, 10 Apr 2014 09:42:55 -0500
>> Jason Ekstrand <jason at jlekstrand.net> wrote:
>>
>> > On Thu, Apr 10, 2014 at 6:37 AM, Pekka Paalanen <ppaalanen at gmail.com>
>> wrote:
>> >
>> > > Hi Jason,
>> > >
>> > > thanks for working on this, it does seem very useful, practically a
>> > > mandatory feature to support.
>> > >
>> >
>> > Hi Pekka,
>> > Yeah, I've been itching to knock this out for a while.  Just finally got
>> > around to it.  Comments below.
>>
>> ...
>>
>> > > On Wed, 2 Apr 2014 09:48:29 -0500
>> > > Jason Ekstrand <jason at jlekstrand.net> wrote:
>> > >
>> > > > It's worth noting that there is one small backwards-compatability
>> issue
>> > > > here.  Namely, if the client is built against protocol stubs from an
>> > > > earlier version of libwayland but links against a library built
>> against a
>> > > > newer version, then all objects created by the client will report a
>> > > version
>> > > > of 1.  This is because the old api uses
>> wl_proxy_marshal_constructor in
>> > > > wl_registry_bind so all objects will inherit the protocol version of
>> > > > wl_display which is 1.  The library the client linked against is
>> aware of
>> > > > the wl_proxy_version function but has no way of knowing that the
>> library
>> > > > does not.
>> > >
>> > > I was about to say that wl_registry_bind() does pass the version to
>> > > wl_proxy_marshal_constructor, but that indeed is in the request
>> > > arguments and not a mandatory argument.
>> > >
>> > > But wl_proxy_marshal_constructor() would still have all the
>> information
>> > > it needs, if we special-case wl_registry.bind inside it. Ugly, but I
>> > > guess it'd work for wl_registry. Would that make the
>> > > backward-compatibility issue go away? In all other cases you would
>> take
>> > > the version from the parent wl_proxy, which you always have available
>> > > in wl_proxy_marshal_constructor(), and the versioned variant would not
>> > > be needed?
>> > >
>> >
>> > Yes, we could do that, and I considered it.  However, it would only bump
>> > the compatibility issue back to wl_proxy_marshal_constructor.  Older
>> code
>> > that uses wl_proxy_create inside of wl_registry_bind is still in
>> trouble.
>> >  I don't think it's a huge savings to just bump the compatibility issues
>> > back to 1.3 rather than 1.4/1.5.  In the long run, I don't think it's
>> worth
>> > the mess that we would create inside wl_proxy_marshal_constructor.
>>
>> Ok, I didn't even know to think that far back. Makes sense.
>>
>> > > But I guess it would still be broken on any other request that used
>> the
>> > > interfaceless format of new_id?
>> > >
>> >
>> > Nope, that's not a problem.  The wayland-scanner program doesn't
>> actually
>> > special case wl_registry_bind but interfaceless new_id's in general.
>> >  Anything else that specifies a new_id with no interface will hit the
>> same
>> > code-path and get wl_proxy_marshal_constructor_versioned.
>>
>> I meant if we special-case wl_registry.bind, then all other requests
>> using interfaceless new_ids would still be in trouble. But yes, a moot
>> point now.
>>
>> > > > One possible solution for this is to set the version of wl_display
>> to
>> > > zero
>> > > > and use zero to mean "unversioned".  Then, if a library wants to use
>> > > > something that's not strictly backwards-compatable, it can check
>> for zero
>> > > > and use whatever it's non-versioned fallback is.
>> > >
>> >
>> > Thoughts on this? ^^
>>
>> Well... if you don't know the version, is there a difference between
>> assuming it is 1, and knowning it is unknown and then assuming whatever?
>>
>> As I see it, the only benefit of knowing when you don't know, is that
>> you could then explicitly assume a higher version than 1, and then die
>> on a protocol error if you are wrong. I'm not sure if that is better
>> than assuming 1 which will always work if the application only accepts
>> that.
>>
>
> There is a case where I would do something different on unknown vs.
> version == 1.  In fact, it's the exact came case that inspired me to
> actually sit down and write this.  The wl_surface.damage request, from the
> perspective of EGL implementations, is completely broken on wl_surface
> versions 2 and 3 (trying to fix it for 4).  An EGL implementation could
> check for unknown, 2, or 3 and just do wl_surface.damage(0, 0, INT32_MAX,
> INT32_MAX) in both eglSwapBuffers and eglSwapBuffersWithDamageEXT to work
> around this.  Then, if the version is 2 or >= 4, they could just damage the
> surface correctly.  It's kind of a specific example and the only reason why
> we care is because we broke stuff at wl_surface version 2 but it's an
> example.
>
> --Jason
>
>
>> It looks like a trade-off between an "unknown method" protocol error /
>> events that never come, and the application complaining the server is
>> too old, when things go wrong.
>>
>> I can't say.
>>
>>
>> Thanks,
>> pq
>>
>
>
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