On Fri, Apr 11, 2014 at 2:03 AM, Pekka Paalanen <[email protected]> wrote:
> On Thu, 10 Apr 2014 09:42:55 -0500 > Jason Ekstrand <[email protected]> wrote: > > > On Thu, Apr 10, 2014 at 6:37 AM, Pekka Paalanen <[email protected]> > 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 <[email protected]> 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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