Allow installer to use different backends

[nikodunk]

Problem

The current installer uses libdistinst to install elementary OS packages to disk I believe. Thanks to @jumpyvi we now have a mkosi/systemd sysupdate PR that can generate an elementary OS 9 image, and then update it forever from there (even over major OS release versions 8=>9=>10!). We need a way to initially copy this image to the users' disk though.

Proposal

@danirabbit suggested I open a ticket here to start a discussion on the installer supporting different backends.

The installer could work the same as it does now, but on the last step instead of copying packages with libdistinst (or debootstrap?), do something like:

sudo dd if=Elementary_build_x86-64.raw of=/dev/sdX bs=4M status=progress conv=fsync

under the hood. Then reboot into the new image, as it does now.

We could even reuse the current live-build live iso generation, but simply copy the elementary OS image to that iso and then have the installer run the above command to the desired disk.

Prior Art (Optional)

• Gnome OS proto installer - (Rust wrapper around systemd repart AFAICT)
• KDE Linux (mkosi + Calamares) - (hybrid iso image AFAICT)
• https://gitlab.gnome.org/p3732/os-installer
• https://github.com/ublue-os/titanoboa
• https://github.com/FyraLabs/readymade

[jumpyvi]

I think we can use systemd-repart (cli) to setup the disk + flash the disk using user-provided options for encryption

[nikodunk]

I think we can use systemd-repart (cli) to setup the disk + flash the disk using user-provided options for encryption

@jumpyvi interesting prior art (now that I'm filling in the template above) at https://invent.kde.org/kde-linux/kde-linux/-/blob/master/build.sh#L203

The resulting file is both a valid GPT disk image and a bootable El Torito ISO.

[nikodunk]

Current distinst backend: https://github.com/elementary/installer/blob/main/daemon/DistinstBackend.vala#L181

[nikodunk]

Hey, we might not need a new backend. Elementary OS uses libdistinst, which takes a squashfs and puts it onto the disk.

@jumpyvi if we can create that squashfs with the new method, the installer will be able to copy it to disk IMO. Did I understand that correctly @ryonakano ?

[ryonakano]

I'm sorry, I'm not familiar around here……

[nikodunk]

For posterity and to not leave this ticket in a confusing state, @danirabbit @jumpyvi and I discussed in discord, and we said:

danirabbit

If we can replace distinst with some calls to systemd I think that would be considered a win personally
Is there a dbus API for systemd repart?

nikodunk

i dont think there is. Can vala do subprocess?

danirabbit

Yes and we already have a priveleged process in installer for other disk operations so I can write a backend for this
Is there a good way to check if we’re on a mkosi image vs a livebuild image? Right now we check for Casper to see if we’re in a live environment

jumpyvi

From what I've seen from gnome/kde, it's a different UKI for live/real

nikodunk

i think we check for uki for mkosi image (vs casper for livebuild image)