Group shared system settings

system/shared/impermanence/autowipe.nix

@@ -0,0 +1,72 @@
+{ config, lib, ... }: let
+  inherit (lib) mkIf concatStringsSep flatten mapAttrsToList;
+  cfg = config.myOptions.system.impermanence.autoWipeBtrfs;
+in
+{
+  config = mkIf cfg.enable {
+    boot.initrd.systemd = {
+      enable = true; # This enables systemd support in stage 1 - required for below setup
+
+      services.rollback = {
+        description = "Rollback BTRFS subvolumes to a pristine state";
+        enable = true;
+        wantedBy = [ "initrd.target" ];
+        # Make sure it's done after decryption (i.e. LUKS/TPM process)
+        after = [ "systemd-cryptsetup@cryptfs.service" ];
+        # mount the root fs before clearing
+        before = [ "sysroot.mount" ];
+        unitConfig.DefaultDependencies = "no";
+        serviceConfig.Type = "oneshot";
+        script = let
+          # TODO: Consider adding support for moving the subvolume to something like old-roots
+          #      instead of deleting it. This would allow for easier recovery in case of a mistake.
+          # The subvolume can live here for a certain amount of time, before it's deleted, or there
+          # can be a certain amount of total roots kept. See the example configuration here:
+          # <https://github.com/nix-community/impermanence> which demonstrates keepping 30 days
+          # worth of old roots.
+          wipeScript = devicePath: subvolumes: ''
+            # Mount the BTRFS device root to a temporary mount point
+            echo "Mounting BTRFS root from ${devicePath} to /mnt"
+            mount --mkdir "${devicePath}" /mnt
+
+            # Recreate each specified subvolume
+            ${concatStringsSep "\n" (map (subvolume: ''
+              delete_subvolume_recursively "/mnt/${subvolume}"
+              btrfs subvolume create "/mnt/${subvolume}"
+            '') subvolumes)}
+
+            # Cleanup: unmount the device
+            echo "Unmounting BTRFS root from ${devicePath}"
+            umount /mnt
+          '';
+        in ''
+          # Simply deleting a subvolume with btrfs subvolume delete will not work,
+          # if that subvolume contains other btrfs subvolumes. Because of that, we
+          # instead use this function to delete subvolumes, whihc will first perform
+          # a recursive deletion of any nested subvolumes.
+          #
+          # This is necessary, because the root subvolume will actually usually contain
+          # other subvolumes, even if the user haven't created those explicitly. It seems
+          # that NixOS creates these automatically. Namely, I observed these in root subvol:
+          # - root/srv
+          # - root/var/lib/portables
+          # - root/var/lib/machines
+          # - root/var/tmp
+          delete_subvolume_recursively() {
+            IFS=$'\n'
+            for i in $(btrfs subvolume list -o "$1" | cut -f 9- -d ' '); do
+              delete_subvolume_recursively "/mnt/$i"
+            done
+
+            echo "Deleting subvolume $1"
+            btrfs subvolume delete "$1"
+          }
+
+          ${concatStringsSep "\n" (mapAttrsToList (devicePath: deviceOpts:
+            wipeScript devicePath deviceOpts.subvolumes
+          ) cfg.devices)}
+        '';
+      };
+    };
+  };
+}