WIP: nixos: system: add persist

This is the module that takes care of configuring impermanence at the
system level.

WIP:
    * address FIXMEs
    * activate home-manager persistence?
        * set `programs.fuse.userAllowOther = true;` ?
    * point `age` to persisted paths [1] ?
    * make sure all services and modules are persisted correctly...

[1]: b1d18d25b8

modules/nixos/system/default.nix

@@ -9,6 +9,7 @@
     ./language
     ./nix
     ./packages
+    ./persist
     ./podman
     ./printing
     ./users

modules/nixos/system/persist/default.nix

@@ -0,0 +1,67 @@
+# Ephemeral root configuration
+{ config, inputs, lib, ... }:
+let
+  cfg = config.my.system.persist;
+in
+{
+  imports = [
+    inputs.impermanence.nixosModules.impermanence
+  ];
+
+  options.my.system.persist = with lib; {
+    enable = mkEnableOption "stateless system configuration";
+
+    mountPoint = lib.mkOption {
+      type = types.str;
+      default = "/persistent";
+      example = "/etc/nix/persist";
+      description = ''
+        Which mount point should be used to persist this system's files and
+        directories.
+      '';
+    };
+
+    files = lib.mkOption {
+      type = with types; listOf str;
+      default = [ ];
+      example = [
+        "/etc/nix/id_rsa"
+      ];
+      description = ''
+        Additional files in the root to link to persistent storage.
+      '';
+    };
+
+    directories = lib.mkOption {
+      type = with types; listOf str;
+      default = [ ];
+      example = [
+        "/var/lib/libvirt"
+      ];
+      description = ''
+        Additional directories in the root to link to persistent storage.
+      '';
+    };
+  };
+
+  config = lib.mkIf cfg.enable {
+    environment.persistence."${cfg.mountPoint}" = {
+      files = [
+        "/etc/machine-id"
+      ]
+      ++ cfg.files
+      ;
+
+      directories = [
+        "/etc/nixos"
+        "/var/log"
+        "/var/lib/systemd/coredump"
+      ]
+      ++ (lib.optionals config.virtualisation.docker.enable [
+        "/var/lib/docker"
+      ])
+      ++ cfg.directories
+      ;
+    };
+  };
+}