nixos/nixos-option: Show values inside aggregate options uniformly

[chkno]

1. This makes aggregates of submodules (including the very important
   nixos-option users.users.<username> case) behave the same way as any
   other you-need-to-keep-typing-to-get-to-an-option-leaf (eg:
   "nixos-option environment").

Before e0780c5:

  $ nixos-option users.users.root
  error: At 'root' in path 'users.users.root': Attribute not found
  An error occurred while looking for attribute names. Are you sure that 'users.users.root' exists?

After e0780c5 but before this change, this query just printed out a raw
thing, which is behavior that belongs in nix eval, nix-instantiate --eval, or nix repl <<<:

  $ nixos-option users.users.root
  {
    _module = {
      args = { name = "root"; };
      check = true;
    };
    createHome = false;
    cryptHomeLuks = null;
    description = "System administrator";
    ...

After this change:

  $ nixos-option users.users.root
  This attribute set contains:
  createHome
  cryptHomeLuks
  description
  extraGroups
  group
  hashedPassword
  ...

2. For aggregates of other types (not submodules), print out the option
   that contains them rather than printing an error message.

Before:

  $ nixos-option environment.shellAliases.l
  error: At 'l' in path 'environment.shellAliases.l': Attribute not found
  An error occurred while looking for attribute names. Are you sure that 'environment.shellAliases.l' exists?

After:

  $ nixos-option environment.shellAliases.l
  Note: showing environment.shellAliases instead of environment.shellAliases.l
  Value:
  {
    l = "ls -alh";
    ll = "ls -l";
    ls = "ls --color=tty";
  }
  ...

Motivation for this change

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nix-review --run "nix-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.

Notify maintainers

cc @chkno

[Ma27]

After e0780c5 but before this change, this query just printed out a raw
thing, which is behavior that belongs in nix eval, nix-instantiate --eval, or nix repl <<<:

I don't think that this is a bad thing: nixos-option is supposed to be used to introspect the current configuration's values. Especially for non-power-users it is probably unintuitive to hack a fairly long nix eval call together. If you simply want to see options, man configuration.nix should be sufficient.

For aggregates of other types (not submodules), print out the option
that contains them rather than printing an error message.

Nice! Thanks a lot for taking care of this!

[chkno]

I don't think that this is a bad thing: nixos-option is supposed to be used to introspect the current configuration's values. Especially for non-power-users it is probably unintuitive to hack a fairly long nix eval call together. If you simply want to see options, man configuration.nix should be sufficient.

(man configuration.nix - Oh, that's a neat manpage. Thanks! I added it to man nixos-option's see-also in NixOS#75787 )

It doesn't make sense to me for options to be treated differently based on whether they're submodules or not.

I agree that dumping the values of options is much more helpful to humans than just their names. This seems true to me whether or not the option is in a submodule or not -- compare users.users.root (a submodule) and powerManagement (not a submodule):

Current	Seems Better
$ nixos-option powerManagement This attribute set contains: cpuFreqGovernor cpufreq enable powerDownCommands powerUpCommands powertop resumeCommands scsiLinkPolicy	$ nixos-option --all | grep ^powerManagement powerManagement.cpuFreqGovernor = null; powerManagement.cpufreq.max = null; powerManagement.cpufreq.min = null; powerManagement.enable = true; powerManagement.powerDownCommands = ""; powerManagement.powerUpCommands = ""; powerManagement.powertop.enable = false; powerManagement.resumeCommands = ""; powerManagement.scsiLinkPolicy = null;
$ nixos-option users.users.root This attribute set contains: createHome cryptHomeLuks description extraGroups group hashedPassword home initialHashedPassword initialPassword isNormalUser isSystemUser name openssh packages password passwordFile shell subGidRanges subUidRanges uid useDefaultShell	$ nixos-option --all | grep ^users.users.root users.users.root.createHome = false; users.users.root.cryptHomeLuks = null; users.users.root.description = "System administrator"; users.users.root.extraGroups = [ ]; users.users.root.group = "root"; users.users.root.hashedPassword = null; users.users.root.home = "/root"; users.users.root.initialHashedPassword = "!"; users.users.root.initialPassword = null; users.users.root.isNormalUser = false; users.users.root.isSystemUser = false; users.users.root.name = "root"; users.users.root.openssh.authorizedKeys.keyFiles = [ ]; users.users.root.openssh.authorizedKeys.keys = [ ]; users.users.root.packages = [ ]; users.users.root.password = null; users.users.root.passwordFile = null; users.users.root.shell = «derivation /nix/store/...-bash-interactive-4.4-p23.drv»; users.users.root.subGidRanges = [ ]; users.users.root.subUidRanges = [ ]; users.users.root.uid = 0; users.users.root.useDefaultShell = false;

The nixos-option --all | grep ^... trick doesn't work for any options with multi-line values, and writing a little sed program that would do the right thing to --all output is at least as far out of reach of novice users as invoking nix repl <<< ... correctly.

What if the output format of --all could be used with only part of the option tree? Suppose it goes behind a -r for "recursive" flag such that nixos-option -r powerManagement and nixos-option -r users.users.root would produce the second-column output above? (nixos-option --all would then have the same meaning as nixos-option -r "")

I don't know if it makes sense to make this the default format, though. The current content-free format is much faster to print, more robust against broken configs with errors or circular references or whatever, doesn't ever surprise you by usually printing ~10 lines and occasionally printing ~10,000, and is very easy to parse with scripts, which makes me fear that changing it would break someone's scripts.

Thoughts?

[Ma27]

First of all thanks a lot for your hard and continuous work on nixos-option! I'm actually pretty happy about the rewrite, the new implementation is way easier to work with and seems way more stable!

It doesn't make sense to me for options to be treated differently based on whether they're submodules or not.

Fair enough. The thing is, I'd really love to see this for submodule roots while I don't consider it that important for attr sets with options (like users) since the latter is part a well-defined structure.

I agree that dumping the values of options is much more helpful to humans than just their names. This seems true to me whether or not the option is in a submodule or not -- compare users.users.root (a submodule) and powerManagement (not a submodule):

👍 If we fix this for both cases, I'm fine with this as well :)

I don't know if it makes sense to make this the default format, though. The current content-free format is much faster to print, more robust against broken configs with errors or circular references or whatever, doesn't ever surprise you by usually printing ~10 lines and occasionally printing ~10,000, and is very easy to parse with scripts, which makes me fear that changing it would break someone's scripts.

👍

[chkno]

I added the -r flag to this PR.

$ nixos-option -r users.users.root
users.users.root.createHome = false;
users.users.root.cryptHomeLuks = null;
users.users.root.description = "System administrator";
users.users.root.extraGroups = [ ];
users.users.root.group = "root";
users.users.root.hashedPassword = null;
users.users.root.home = "/root";
...

$ nixos-option -r powerManagement
powerManagement.cpuFreqGovernor = null;
powerManagement.cpufreq.max = null;
powerManagement.cpufreq.min = null;
powerManagement.enable = true;
powerManagement.powerDownCommands = "";
powerManagement.powerUpCommands = "";
powerManagement.powertop.enable = false;
powerManagement.resumeCommands = "";
powerManagement.scsiLinkPolicy = null;

[Ma27]

@chkno thanks!