The second part of my project in Endless Orange Week was to look at state persistence for apps and sessions. At its core, this means providing
- some way for apps to save and restore their state (such as which part of the UI you’re looking at, what’s selected, and unsaved content);
- code in the session manager to save and restore the state of all applications, and a list of which applications are running, when shutting down/restarting/logging out/logging in/starting up.
Those two bullet points hide a lot of complexity, and it’s not surprising that I didn’t get particularly far in this project! It requires coordinated changes in a lot of components: GLib, GTK, gnome-session and applications themselves.
A lot of these changes have been prototyped or worked on before, by various people, but nothing has yet come together. In fact, gnome-session used to fully support restoring apps to a certain degree — before it was ported away from XSMP, it used to support saving the set of apps when closing a session, and re-starting those apps when starting the session again. It did not support restoring the state of each app, though, just the fact that it was running.
Breaking down the problem
Updating and building on the proposals and branches from others so far, we end up with:
- Changes to GLib to add a ‘restart data’ property to GApplication, which allows the application to expose its current state to the session manager (to be saved), and which is initialised with the restored state from the session manager on startup. These build heavily on changes proposed by Bastien Nocera, but my tweaks to them are still pretty exploratory and not ready for review.
- Code in GTK to support serialising the widget tree. This implements saving the state of the UI, and was prototyped by Matthias Clasen. My additions (also not yet ready for review) tie it in to the gnome-session API. Further work (but not very much of it) would have to be done to tie Matthias’ proposals into the final shape of the GLib API.
- Preparatory cleanups of old code in gnome-session (this one is ready to review and hopefully merge!).
- Work to re-support session restore in gnome-session. This is mostly ready, but needs tidying up and testing (which is likely to be a significant amount of work). It ties in with systemd transient scope work which Benjamin Berg and Iain Lane have been working on.
The final two pieces of the puzzle above took most of my week, and included a lot of time spent learning the architecture of gnome-session, and working out a bit of a protocol for apps to register for session restore and, in particular, for gnome-session to be able to re-launch apps the right way when restoring a session. That’s something which deserves a blog post of its own at some point in the future, once I’m sure I’ve got everything straight in my head.
In summary, there’s not been much progress in terms of creating merge requests. But the week was, I think, well spent: I’ve learned a lot about the shape of the problem, and feel like I have a better idea of how to split it up into chunks which it might be possible to land over time to get the feature working. Many thanks to Endless for giving me the opportunity to do so.
Certainly, I think this project is too big to easily do in a single GNOME release. There’s too much coordination required between different projects with different cadences and development resources.
The next step will be to land the preparatory gnome-session cleanups, and to discuss and land the GLib API so that people can start building with that.