Sharing a single wl_display (client) by multiple toolkits

[Simon Ser]

On Saturday, March 23, 2019 1:50 PM, Scott Anderson <scott.anderson at collabora.com> wrote:
> On 22/03/19 8:37 am, Jaroslaw Kubik wrote:
>
> > Hi,
> > Does wayland-client support sharing a single connection object between
> > unrelated pieces of code? Such feature could be useful for integrating
> > multiple toolkits (ie. showing GTK widget in a Qt application). I am trying
> > to get something similar (but less spectacular) done at the moment, and
> > I am facing the following issue: When two unrelated pieces of code
> > (ie. GTK and Qt) use the same wl_display and both create a proxy to a
> > global object (ie. wl_output), then from the server perspective both these
> > proxies are indistinguishable. When sending events like wl_surface.enter,
> > the server has to pick a single wl_resource as an argument. That wl_resource
> > is bound to a specific wl_output proxy on the client side. If the server
> > chooses wrong, then the window created and managed by Qt will receive
> > wl_surface.enter event with wl_output proxy instance that belongs to GTK.
> > It will attempt to access it's user-data and crash because it's not the data
> > it is expecting.
> > In the above scenario the server has no way of knowing which of the proxies
> > are expected as parameters in any given case. And the client has no way of
> > "discovering" equivalent proxies in case it would like to straighten it out.
> > This leads me to believe that the design of the wayland protocol makes it
> > impossible to share a connection object between unrelated code and that
> > use-case is simply not supported.
> > Is that correct? Are there any plans to address this?
> > Best Regards,
> > Jaroslaw Kubik
>
> Hi,
>
> Wayland can handle this perfectly fine. Separate sections of code can
> create their own wl_registries and bind their own wl_globals
> independently of each other, including using different versions of each
> global. The only requirement is that they use the same wl_display
> object, otherwise they will actually be two separate clients to the
> compositor.

The question is about "sharing a single connection object". This isn't
possible, an object (ie. proxy) can only be owned by a single consumer.

However, it's possible to make pieces of code use different objects,
and Scott gives here EGL's example. There are over examples too, for
instance some libraries can draw to a subsurface and the main program
can attach the subsurface to one of its surfaces.

tl;dr "sharing an object" is not possible, though the use-case you're
asking about can probably work.

> For example, the EGL and Vulkan Wayland platform is implemented this
> way, where you give it a reference to your wl_display and wl_surface,
> and it handles setting up and committing the buffer separately from
> anything else.
>
> If some global is bound multiple times, you will receive an event on
> each of them and there won't be any mix up regarding user-data, so you
> don't need to worry about whether the right global gets used.
>
> Scott
>
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