add user-level impermanence

modules/environment/impermanence.nix

@@ -7,7 +7,10 @@
   ...
 }:
 with lib; {
-  imports = ["${impermanence}/nixos.nix"];
+  imports = [
+    "${impermanence}/nixos.nix"
+    ./user-impermanence.nix
+  ];
   options = {
     environment.impermanence.enable = mkEnableOption "Enables impermanence";
   };

modules/environment/user-impermanence.nix

@@ -0,0 +1,71 @@
+{
+  config,
+  lib,
+  ...
+}:
+with lib; {
+  options = {
+    environment.impermanence.users = mkOption {
+      description = "Which users to clean up the home directory for";
+      default = [];
+      type = types.listOf types.str;
+    };
+  };
+
+  config = mkIf config.environment.impermanence.enable {
+    systemd.services = listToAttrs (flatten (map (name: let
+        cfg = config.users.users.${name};
+      in [
+        {
+          name = "cleanup-home-${name}";
+          description = "Clean home directory for ${name}";
+          value = {
+            wantedBy = [
+              "user-${toString cfg.uid}.slice"
+            ];
+            before = [
+              "user-${toString cfg.uid}.slice"
+              "home-manager-${name}.slice"
+            ];
+            serviceConfig.Type = "oneshot";
+            script = ''
+              if [[ -e ${cfg.home} ]]; then
+                timestamp=$(date --date="@$(stat -c %X ${cfg.home})" "+%Y-%m-%d_%H:%M:%S")
+                mkdir - p /persistent/old-homedirs/${name}
+                mv ${cfg.home} /home/old-homedirs/${name}/$timestamp
+              fi
+
+              delete_subvolume_recursively() {
+                IFS=$'\n'
+                for i in $(btrfs subvolume list -o "$1" | cut -f 9- -d ' '); do
+                  delete_subvolume_recursively "/persistent/old-homedirs/${name}/$i"
+                done
+                btrfs subvolume delete "$1"
+              }
+
+              for i in $(find /persistent/old-homedirs/${name} -maxdepth 1 -mtime +30); do
+                delete_subvolume_recursively "$i"
+              done
+
+              btrfs subvolume create ${cfg.home}
+            '';
+          };
+        }
+        {
+          name = "home-manager-${name}";
+          value = {
+            wantedBy = mkForce [
+              "user-${toString cfg.uid}.slice"
+            ];
+            after = [
+              "cleanup-home-${name}.service"
+            ];
+            before = [
+              "user-${toString cfg.uid}.slice"
+            ];
+          };
+        }
+      ])
+      config.environment.impermanence.users));
+  };
+}

users/root/default.nix

@@ -15,4 +15,5 @@
     files = ["/root/.bash_history"];
   };
   home-manager.users.root = {};
+  environment.impermanence.users = ["root"];
 }