nixos/switch-to-configuration: Proper unit file parser and clean/fix lower part of the script

[dasJ]

Motivation for this change

This replaces the naive K=V unit parser with a proper INI parser from a
library and adds proper support for override files. Also adds a bunch of
comments about parsing, I hope this makes it easier to understand and
maintain in the future.

There are multiple reasons to do so, the first one is just general
correctness with is nice imo. But to get to more serious reasons (I
didn't put in all that effort for nothing) is that this is the first
step torwards more clever restart/reload handling. By using a library
like Data::Compare a future PR could replace the current way of
fingerprinting units (which is to compare store paths) by comparing the
hashes. This is more precise because units won't get restarted because
the order of the options change, comments are added, some dependency of
writeText changes, .... Also this allows us to add a feature like
X-Reload-Triggers so the unit can either be reloaded when these change
or restarted when everything else changes, giving module authors the
ability to have their services reloaded without having to fear that
updates are not applied because the service doesn't get restarted.
Another reason why this feature is nice is that now that the unit files
are parsed correctly (and values are just extracted from one section),
potential future rewrites can just rely on some INI library without
having to implement their own weird parser that is compatible with this
script.

This also comes with a new subroutine to handle systemd booleans because
I thought the current way of handling it was just ugly. This also allows
overriding values this script reads in an override file.

Apart from making this script more compatible with the world around it,
this also fixes two issues I saw bugging exactly 0 (zero) people. First
is that this script now supports multiple override files, also ones that
are not called override.conf and the second one is that 1 and on are
treated as bools by systemd but were previously not parsed as such by
switch-to-configuration.

Related to #49528.

Things done

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• For non-Linux: Is sandbox = true set in nix.conf? (See Nix manual)
• Tested, as applicable:
  • NixOS test(s) (look inside nixos/tests)
  • and/or package tests
  • or, for functions and "core" functionality, tests in lib/tests or pkgs/test
  • made sure NixOS tests are linked to the relevant packages
• Tested compilation of all packages that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review rev HEAD". Note: all changes have to be committed, also see nixpkgs-review usage
• Tested basic functionality of all binary files (usually in ./result/bin/)
• 22.05 Release Notes (or backporting 21.11 Release notes)
  • (Package updates) Added a release notes entry if the change is major or breaking
  • (Module updates) Added a release notes entry if the change is significant
  • (Module addition) Added a release notes entry if adding a new NixOS module
  • (Release notes changes) Ran nixos/doc/manual/md-to-db.sh to update generated release notes
• Fits CONTRIBUTING.md.

[dasJ]

About me removing the comment from parseKeyValues: This is actually correct when parsing the output of systemctl show and not parsing a unit file.
This does not matter however because I also plan on dropping that subroutine entirely and replacing it by systemctl show --value --property so we don't have to do the splitting ourselves and can just let systemctl do it for us

I added a second commit that cleans up the lower part of the script and gets rid of parseKeyValues

[dasJ]

A classic while writing the test: I found that the current behaviour that detects whether a unit is in auto-restart was broken. This is now fixed and slightly refactored (second commit) and a proper test case is added.

[dasJ]

Any updates on this? I'm really hesitant to self-merge, especially with no reviews, but I really want to see things moving forward and to push out the next changes that I have prepared.

[stigtsp]

LGTM

• Perl changes seem OK
• Config::IniFiles is a stable and maintained package
• Tests in switch-test.nix pass, and coverage seems good

(It would be nice if someone who knows this part of nixos better than me could have a closer look)