ephemeral is now a nixosModule

modules/ephemeral.nix

@@ -0,0 +1,74 @@
+{ lib, config, ... }:
+
+let
+  cfg = config.ephemeral;
+in
+{
+  options.ephemeral = {
+    enable = lib.mkEnableOption "ephemeral root with automatic rollback";
+
+    rootDevice = lib.mkOption {
+      type = lib.types.str;
+      example = "/dev/mapper/cryptroot";
+      description = "Device path for the root btrfs filesystem";
+    };
+
+    rootSubvolume = lib.mkOption {
+      type = lib.types.str;
+      example = "@root";
+      description = "Name of the root btrfs subvolume";
+    };
+
+    oldRootRetentionDays = lib.mkOption {
+      type = lib.types.int;
+      default = 30;
+      description = "Number of days to keep old root snapshots before deletion";
+    };
+  };
+
+  config = lib.mkIf cfg.enable {
+    boot.initrd.systemd.services.recreate-root = {
+      description = "Rolling over and creating new filesystem root";
+      requires = [ "initrd-root-device.target" ];
+      after = [
+        "local-fs-pre.target"
+        "initrd-root-device.target"
+      ];
+      requiredBy = [ "initrd-root-fs.target" ];
+      before = [ "sysroot.mount" ];
+      unitConfig = {
+        AssertPathExists = "/etc/initrd-release";
+        DefaultDependencies = false;
+      };
+      serviceConfig = {
+        Type = "oneshot";
+        RemainAfterExit = true;
+      };
+      script = ''
+        mkdir /btrfs_tmp
+        mount ${cfg.rootDevice} /btrfs_tmp
+
+        if [[ -e /btrfs_tmp/${cfg.rootSubvolume} ]]; then
+          mkdir -p /btrfs_tmp/old_roots
+          timestamp=$(date --date="@$(stat -c %Y /btrfs_tmp/${cfg.rootSubvolume})" "+%Y-%m-%-d_%H:%M:%S")
+          mv /btrfs_tmp/${cfg.rootSubvolume} "/btrfs_tmp/old_roots/$timestamp"
+        fi
+
+        delete_subvolume_recursively() {
+          IFS=$'\n'
+          for i in $(btrfs subvolume list -o "$1" | cut -f 9- -d ' '); do
+             delete_subvolume_recursively "/btrfs_tmp/$i"
+          done
+          btrfs subvolume delete "$1"
+        }
+
+        for i in $(find /btrfs_tmp/old_roots/ -maxdepth 1 -mtime +${toString cfg.oldRootRetentionDays}); do
+          delete_subvolume_recursively "$i"
+        done
+
+        btrfs subvolume create /btrfs_tmp/${cfg.rootSubvolume}
+        umount /btrfs_tmp
+      '';
+    };
+  };
+}