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.
literalExpression triggers the following error when building the
manual:
Cacheable portion of option doc build failed.
Usually this means that an option attribute that ends up in documentation (eg `default` or `description`) depends on the restricted module arguments `config` or `pkgs`.
Allow giving a custom package containing the `systemd-repart` binary.
Defaults to `pkgs.systemd`. This option opens up the possibility to use
a different package for the image builder and the system configuration.
For example, someone could use this option to build an image with a
patched systemd while still using the upstream nixpkgs systemd package
(i.e., `pkgs.systemd`) for the system configuration installed to the
created image.
Output the amended repart definitions to a well-known directory in
$TMPDIR instead of using a temporary directory with a random directory
name.
The output file `repart-output.json` also contains the full path to the
repart definition file used to create the partition. As
`amend-repart-definitions.py` uses `tempfile.mkdtemp`, this introduces
an impurity:
```json
{
"type" : "root-x86-64",
"label" : "rootfs",
"uuid" : "f2fa2e49-e443-45d2-a2e2-c3754cab6363",
"file" : "/build/tmppjo7kv5o/rootfs.conf",
"node" : "image.raw2",
"offset" : 135266304,
"old_size" : 0,
"raw_size" : 1651101696,
"old_padding" : 0,
"raw_padding" : 0,
"activity" : "create",
}
```
This commit changes the parent directory of the amended repart
definitions to `/build/amended-repart.d/`.
Write the output of `systemd-repart` as a JSON file to
`$out/repart-output.json`.
Depending on the repart configuration, the output of `systemd-repart`
contains important information, for example, when creating verity
partitions:
> The verity root hash itself will be included in the output of
> systemd-repart.
See `Verity=` in repart.d(5).
