feat: provide nvd and nix-diff view of facter changes

Provides a `debug` namespace within `config.facter` which can be used to display a diff between the system closure with facter enabled, and without.

The diffs are in the form of `nvd` or `nix-diff` output e.g. higher level summary versus detailed breakdown.

Usage:

- `nix run .#nixosConfigurations.basic.config.facter.debug.nvd`
- `nix run .#nixosConfigurations.basic.config.facter.debug.nix-diff`

Examples:

```
❯ nix run .#nixosConfigurations.basic.config.facter.debug.nvd
<<< /nix/store/fqbia5p8hfnyzxipfjmzxn8v3b69mjvs-nixos-system-nixos-24.11.20240831.12228ff
>>> /nix/store/ijkh9sq03y72pfrjvncrsrhwh7g6k8q0-nixos-system-nixos-24.11.20240831.12228ff
Added packages:
[A.]  #1  X-Reload-Triggers-systemd-networkd         <none>
[A.]  #2  X-Restart-Triggers-systemd-networkd        <none>
[A.]  #3  X-Restart-Triggers-systemd-resolved        <none>
[A.]  #4  etc-systemd-networkd.conf                  <none>
[A.]  #5  etc-systemd-resolved.conf                  <none>
[A.]  #6  graphics-driver.conf                       <none>
[A.]  #7  graphics-drivers                           <none>
[A.]  #8  hwdata                                     0.385
[A.]  #9  libXfixes                                  6.0.1
[A.]  #10  libXxf86vm                                 1.1.5
[A.]  #11  libdrm                                     2.4.122
[A.]  #12  libpciaccess                               0.18.1
[A.]  #13  libxshmfence                               1.3.2
[A.]  #14  llvm                                       18.1.8-lib
[A.]  #15  lm-sensors                                 3.6.0
[A.]  #16  mesa                                       24.2.1, 24.2.1-drivers
[A.]  #17  unit                                       99-ethernet-default-dhcp.network, 99-wireless-client-dhcp.network
[A.]  #18  unit-systemd-network-wait-online-.service  <none>
[A.]  #19  unit-systemd-networkd-wait-online.service  <none>
[A.]  #20  unit-systemd-networkd.service              <none>
[A.]  #21  unit-systemd-networkd.socket               <none>
[A.]  #22  unit-systemd-resolved.service              <none>
[A.]  #23  vulkan-loader                              1.3.283.0
[A.]  #24  wayland                                    1.23.0
[A.]  #25  xcb-util-keysyms                           0.4.1
Removed packages:
[R.]  #1  X-Restart-Triggers-resolvconf    <none>
[R-]  #2  dhcpcd                           10.0.6
[R.]  #3  dhcpcd.conf                      <none>
[R-]  #4  openresolv                       3.13.2
[R.]  #5  unit-dhcpcd.service              <none>
[R.]  #6  unit-network-setup.service       <none>
[R.]  #7  unit-resolvconf.service          <none>
[R.]  #8  unit-script-network-setup-start  <none>
Closure size: 562 -> 581 (55 paths added, 36 paths removed, delta +19, disk usage +700.6MiB).
```

```
❯ nix run .#nixosConfigurations.basic.config.facter.debug.nix-diff
- /nix/store/fqbia5p8hfnyzxipfjmzxn8v3b69mjvs-nixos-system-nixos-24.11.20240831.12228ff:{out}
+ /nix/store/ijkh9sq03y72pfrjvncrsrhwh7g6k8q0-nixos-system-nixos-24.11.20240831.12228ff:{out}
• The input derivation named `boot.json` differs
  - /nix/store/zcmdy29f5di6rrfkkld3x773q1d7c1bv-boot.json.drv:{out}
  + /nix/store/j0wxsmw6fxgzd59pdcljkpq430jbk8jn-boot.json.drv:{out}
  • The input derivation named `initrd-linux-6.6.48` differs
    - /nix/store/3z3jgifv3rj5wrh4cx23gzc9hlrwrxwj-initrd-linux-6.6.48.drv:{out}
    + /nix/store/ds7pcp97k33p93z460b5vzrii02jjkcf-initrd-linux-6.6.48.drv:{out}
    • The input derivation named `initrd-nixos.conf` differs
      - /nix/store/y458hw08d5hv2bcf295fv9wvbsfvprrq-initrd-nixos.conf.drv:{out}
      + /nix/store/989v278ws16mnvlx6fwsa925vgx9a0ia-initrd-nixos.conf.drv:{out}
      • The environments do not match:
          text=''
          virtio_balloon
          virtio_console
          virtio_rng
          virtio_gpu
          bochs
          dm_mod
      ''
    • The input derivation named `linux-6.6.48-modules-shrunk` differs
      - /nix/store/dk2xshi6hr85wjyia60nizahl2rq31sz-linux-6.6.48-modules-shrunk.drv:{out}
      + /nix/store/xxk67p9wy7hxq33gd1mfz5dqcsm2mhsr-linux-6.6.48-modules-shrunk.drv:{out}
      • The environments do not match:
          rootModules=''
          virtio_net virtio_pci virtio_mmio virtio_blk 9p 9pnet_virtio uhci_hcd ata_piix floppy virtio_blk virtio_pci ext2 ext4 autofs tpm-tis tpm-crb efivarfs ahci sata_nv sata_via sata_sis sata_uli ata_piix pata_marvell nvme sd_mod sr_mod mmc_block uhci_hcd ehci_hcd ehci_pci ohci_hcd ohci_pci xhci_hcd xhci_pci usbhid hid_generic hid_lenovo hid_apple hid_roccat hid_logitech_hidpp hid_logitech_dj hid_microsoft hid_cherry hid_corsair pcips2 atkbd i8042 rtc_cmos virtio_balloon virtio_console virtio_rng virtio_gpu bochs dm_mod
      ''
    • Skipping environment comparison
  • Skipping environment comparison
...
...
...
```

Co-authored-by: magicrb <richard@brezak.sk>

Author: brianmcgee

modules/nixos/debug.nix

@@ -0,0 +1,73 @@
+{
+  lib,
+  pkgs,
+  config,
+  extendModules,
+  ...
+}:
+{
+
+  options = {
+
+    system.build = {
+      noFacter = lib.mkOption {
+        type = lib.types.unspecified;
+        description = "A version of the system closure with facter disabled";
+      };
+    };
+
+    facter.debug = {
+      nvd = lib.mkOption {
+        type = lib.types.package;
+        description = ''
+          A shell application which will produce an nvd diff of the system closure with and without facter enabled.
+        '';
+      };
+      nix-diff = lib.mkOption {
+        type = lib.types.package;
+        description = ''
+          A shell application which will produce a nix-diff of the system closure with and without facter enabled.
+        '';
+      };
+    };
+
+  };
+
+  config.system.build = {
+    noFacter = extendModules {
+      modules = [
+        {
+          # we 'disable' facter by overriding the report and setting it to empty with one caveat: hostPlatform
+          config.facter.report = lib.mkForce {
+            system = config.nixpkgs.hostPlatform;
+          };
+        }
+      ];
+    };
+  };
+
+  config.facter.debug = {
+
+    nvd = pkgs.writeShellApplication {
+      name = "facter-diff";
+      runtimeInputs = [ pkgs.nvd ];
+      text = ''
+        nvd diff \
+                ${config.system.build.noFacter.config.system.build.toplevel} \
+                ${config.system.build.toplevel}
+      '';
+    };
+
+    nix-diff = pkgs.writeShellApplication {
+      name = "facter-diff";
+      runtimeInputs = [ pkgs.nix-diff ];
+      text = ''
+        nix-diff \
+                ${config.system.build.noFacter.config.system.build.toplevel} \
+                ${config.system.build.toplevel}
+      '';
+    };
+
+  };
+
+}

modules/nixos/facter.nix

@@ -7,6 +7,7 @@
   imports = [
     ./bluetooth.nix
     ./disk.nix
+    ./debug.nix
     ./fingerprint
     ./firmware.nix
     ./graphics