[GSoC] Some questions about wayland clients with no outputs present

[Armin Krezović]

Hi all,

Now that I've analyzed the weston codebase a bit, I'm ready to
bombard you people with some questions :).

For those that aren't yet aware, I'll be working on making weston
able to run with no outputs and survive with all outputs plugged
out at runtime.

For starters, I've simply commented out the output backend load code
in src/main.c to see what would happen. To my surprise, the compositor
didn't crash, but the desktop-shell did.

I'm currently guessing that it crashes because the clients that use
clients/window.c code look for outputs in the registry. Of course,
there are none because the backend that updates the global registry
isn't loaded.

Now, my conclusion is: In order to make weston work without any outputs
present is to make clients work without any outputs present. This will
probably require people to update their toolkits and apps.

As I've originally proposed, I see two different ways in achieving this:

First and easiest way is use of a virtual output. I could modify the
DRM backend to add a virtual output if no physical outputs are found
and remove the output when a physical output is connected. Add similar
behavior on phyisical screen unplug. If there are none left, add a virtual
output. If I remember correctly, Pekka once noted that DRM backend already
supports output hotplugging, more or less, but it doesn't support the
primary output unplugging.

Now, the first question is: Even if this way is picked, would clients or
something else require much work to make them behave correctly when
the primary output is unplugged?

The second and more harder way is to simply cope with no outputs present and
watch for output plugging/unplugging. This would require clients to watch for
output changes and take apropriate action.

The second question then might be: What should clients do in such case?

1. Do they only need to schedule redraw and toggle back fullscreen if it was on?

2. Do they need to resize/scale relative to the size of new screen? (eg, a maximized
app on 1920x1080 screen would not be entirely visible on 1600x900 one).

3. Is there something as "move an app to different screen" that can be issued
by compositor? Or do apps need to do that too?

4. Is there anything special needed for XWayland or is it just another wayland client?

That would be all for now.

Armin.

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