Having access to the original Nix partition definitions in the builder
should make it a bit easier to manipulate them and still provide access
to the manipulated results.
As a follow-up to https://github.com/NixOS/nixpkgs/pull/294096 this
should further improve the flexibility around building OS images with
systemd-repart:
* Previously the attribute set `compression` needed to be fully
populated, including `algorithm` and `level` because
`compression.enable` was evaluated by bash, after being interpolated
as strings into the `buildCommand`. Now it's sufficient to pass
`compression.enable = false` to the builder, e.g. in `overrideAttrs`,
to disable the compression.
* Using mkDerivation allows for much more customization than the
previously used `runCommand`, making use of phases and pre/post hooks.
This is especially helpful for building multiple images from the same
system configuration, e.g. to build an image `Y` based on a partially
built raw image `X`, by injecting a UKI that depends on `X` into a
defered ESP.
* Before this change it was non-trivial to conduct further manipulations
on the amended repart definitions. Now, the definitions that
systemd-repart uses to build the image can be easily manipulated in
`postPatch` or `preBuild`.
Aside from this, the build is now executed in the build directory, rather
than `$out`. This allows references to relative paths in the build
environment to be used, especially for `--definitions`, which previously
required an absolute path.
Parameters passed to systemd-repart are now passed to the build script
via environment variable, which is defined as a list of strings in
combination with `__structuredAttrs = true`. This should make it easier
to customize the image build using `overrideAttrs`.
Both the script used to amend the repart definitions and the amended
definitions are now available via passthru.
The version option is needed if you want to implement partition &
systemd-boot based A/B booting where the version information is encoded
in the files on the ESP. See systemd-sysupate docs for more details on
this:
https://www.freedesktop.org/software/systemd/man/latest/sysupdate.d.html
Note, however, that this is not *only* useful for systemd-sysupdate but
also for other similar updating tools/mechanisms.
Since the repart image is built on the build platform, use
`buildPackages` to construct the image. This allows for systemd-repart
images for cross-compiled nixos configurations to work properly.
