feat: Impermanence (system)

I had to disable initrd's systemd stuff.  I just /couldn't/ get a rollback
service working; believe me, not for a lack of effort!  I've been working on
this on-and-off for a month or two now.

modules/nixos/impermanence.nix

@@ -1,4 +1,3 @@
-# Requires boot.initrd.enable = true and boot.initrd.systemd.enable = true!
 { config, lib, pkgs, ... }:
 
 with lib;
@@ -6,41 +5,38 @@ with lib;
 let
   cfg = config.sydnix.impermanence;
 in {
+  imports = [
+    ./impermanence/rollback.nix
+  ];
+
   options = {
     sydnix.impermanence = {
-      enable = mkOption {
-        description = "Enable Impermanence";
-        type = types.bool;
-        default = false;
-      };
+      enable = mkEnableOption "Impermanence";
 
       directories = mkOption {
-        description = "";
         type = with types; listOf anything;
         default = [];
       };
 
+      files = mkOption {
+        type = with types; listOf anything;
+        default = [];
+      };
+
+      persistDirectory = mkOption {
+        default = "/persist";
+        type = types.str;
+        description = ''
+          The directory in which persistent files live.
+        '';
+      };
+
       persistGroupName = mkOption {
         default = "persist";
         type = types.str;
-      };
-
-      files = mkOption {
-        description = "";
-
-        type = with types; listOf anything;
-        default = [];
-      };
-
-      device = mkOption {
-        description = "Device to be wiped";
-        type = types.str;
-      };
-
-      archiveLimit = mkOption {
-        type = types.ints.positive;
-        description = "The number of previous roots to preserve.";
-        default = 3;
+        description = ''
+          Name of the group whose members have access to the persist directory.
+        '';
       };
     };
   };
@@ -51,115 +47,19 @@ in {
       name = cfg.persistGroupName;
     };
 
-    systemd.tmpfiles.settings = {
-      "10-persist" = {
-        # Permit members of `cfg.persistGroupName` to read, write, and execute
-        # /persist.
-        "/persist" = {
-          z = {
-            group = cfg.persistGroupName;
-            mode = "2775";
-          };
-        };
+    # Permit members of `cfg.persistGroupName` to read, write, and execute
+    # /persist.
+    systemd.tmpfiles.settings."10-persist".${cfg.persistDirectory} = {
+      z = {
+        group = cfg.persistGroupName;
+        mode = "2775";
       };
     };
 
     # TODO: Move this somewhere else.
     programs.fuse.userAllowOther = true;
 
-    # sudo mount -o ro -t virtiofs mount-dots /persist/dots
-    boot.initrd.systemd = {
-      # `pkgs.babashka` calls `pkgs.babashka-unwrapped`, and will explode if it
-      # cannot find it.
-      storePaths = [ pkgs.babashka-unwrapped ];
-
-      initrdBin = with pkgs; [
-        babashka
-        util-linux
-      ];
-    };
-
-    boot.initrd.systemd.services.erase-darlings =
-      let service = {
-            description = "Rollback filesystem to a blank state on boot";
-            wantedBy = [
-              "initrd.target"
-            ];
-            before = [
-              "sysroot.mount"
-            ];
-            path = with pkgs; [ util-linux babashka ];
-            unitConfig.DefaultDependencies = "no";
-            serviceConfig = {
-              Type = "oneshot";
-              RemainAfterExit = true;
-            };
-            script =
-              let bb-script = "/sysroot" + ./impermanence/erase-darlings.clj;
-              in ''${pkgs.babashka}/bin/bb "${bb-script}" -n "${toString cfg.archiveLimit}" --device "${cfg.device}"'';
-          };
-      in if config.boot.initrd.systemd.enable
-         then service
-         else throw "sydnix.impermanence currently requires config.boot.initrd.systemd.enable'!";
-
-    # TODO: Remove this.
-    # boot.initrd.systemd.services.erase-darlings-old =
-    #   let service = {
-    #         description = "Rollback filesystem to a blank state on boot";
-    #         wantedBy = [
-    #           "initrd.target"
-    #         ];
-    #         after = [
-    #           # "zfs-import.service"
-    #           "zfs-import-rpool.service"
-    #         ];
-    #         before = [
-    #           "sysroot.mount"
-    #         ];
-    #         path = [ pkgs.zfs ];
-    #         unitConfig.DefaultDependencies = "no";
-    #         serviceConfig = {
-    #           Type = "oneshot";
-    #           RemainAfterExit = true;
-    #         };
-    #         script = /* bash */ ''
-    #           zfs rollback -r rpool/local/root@blank \
-    #             && echo ">> >> rollback complete << <<"
-    #         '';
-    #       };
-    #   in if config.boot.initrd.systemd.enable
-    #      then service
-    #      else throw "sydnix.impermanence currently requires config.boot.initrd.systemd.enable'!";
-
-    # systemd.services =
-    #   let erase-home-darlings = {
-    #         description = "Rollback home to a blank state on boot";
-    #         wantedBy = [
-    #           "local-fs-pre.target"
-    #           "zfs-mount.service"
-    #         ];
-    #         before = [
-    #           "local-fs.target"
-    #           "local-fs-pre.target"
-    #           "zfs-mount.service"
-    #         ];
-    #         path = [ pkgs.zfs pkgs.babashka pkgs.util-linux ];
-    #         unitConfig.DefaultDependencies = "no";
-    #         serviceConfig = {
-    #           Type = "oneshot";
-    #           RemainAfterExit = true;
-    #           ExecStart =
-    #             let script = ./erase-home-darlings.clj;
-    #             in ''${pkgs.babashka}/bin/bb "${script}" -n "${toString cfg.archiveLimit}" --dataset "${cfg.dataset}" --rollback-to "${cfg.rollbackTo}"'';
-    #         };
-    #         stopIfChanged = false;
-    #         restartIfChanged = false;
-    #       };
-    #   in {
-    #     # inherit erase-home-darlings;
-    #   };
-
-    environment.persistence."/persist/root" = {
+    environment.persistence."${cfg.persistDirectory}/root" = {
       directories = cfg.directories;
       files = cfg.files;
     };