update flake.lock; refactor impermanence; other minor fixes

modules/core/persist.nix

@@ -10,15 +10,35 @@ in
 {
   options.my.persist = {
     enable = lib.mkEnableOption "persist";
-    location = lib.mkOption {
-      type = lib.types.str;
-      default = "/nix/persist";
-      example = lib.literalExpression ''
-        "/persistent"
-      '';
-      description = lib.mdDoc ''
-        Persistent location
-      '';
+    btrfs = lib.mkOption {
+      type = lib.types.submodule (
+        { ... }:
+        {
+          options = {
+            device = lib.mkOption {
+              type = lib.types.str;
+            };
+            zstdCompress = lib.mkOption {
+              type = lib.types.bool;
+              default = true;
+            };
+            persistSubvol = lib.mkOption {
+              type = lib.types.str;
+            };
+            rootSubvol = lib.mkOption {
+              type = lib.types.str;
+              default = "root";
+            };
+            mountPoint = lib.mkOption {
+              type = lib.types.str;
+              default = "/nix/persist";
+              example = lib.literalExpression ''
+                "/persistent"
+              '';
+            };
+          };
+        }
+      );
     };
     homeDirs = lib.mkOption {
       default = [ ];
@@ -69,8 +89,65 @@ in
   };
 
   config = lib.mkIf cfg.enable {
+    fileSystems.${cfg.btrfs.mountPoint} = {
+      device = cfg.btrfs.device;
+      fsType = "btrfs";
+      options = [
+        "subvol=${cfg.btrfs.persistSubvol}"
+      ]
+      ++ lib.optionals cfg.btrfs.zstdCompress [
+        "compress=zstd"
+      ];
+      neededForBoot = true;
+    };
+    fileSystems."/" = {
+      device = cfg.btrfs.device;
+      fsType = "btrfs";
+      options = [
+        "subvol=${cfg.btrfs.rootSubvol}"
+      ]
+      ++ lib.optionals cfg.btrfs.zstdCompress [
+        "compress=zstd"
+      ];
+    };
+
+    boot.initrd.systemd.services.wipe-root = {
+      description = "Rollback BTRFS rootfs";
+      wantedBy = [ "initrd.target" ];
+      before = [ "sysroot.mount" ];
+      after = [ "initrd-root-device.target" ];
+      unitConfig.DefaultDependencies = "no";
+      serviceConfig.Type = "oneshot";
+
+      script = ''
+        mkdir -p /btrfs_tmp
+        mount ${cfg.btrfs.device} /btrfs_tmp
+        mkdir -p /btrfs_tmp/old_roots
+
+        if [ -e /btrfs_tmp/root ]; then
+          timestamp=$(date -d "@$(stat -c %Y /btrfs_tmp/root)" "+%Y-%m-%d_%H:%M:%S" 2>/dev/null || date "+%Y-%m-%d_%H:%M:%S")
+          mv /btrfs_tmp/root "/btrfs_tmp/old_roots/$timestamp"
+        fi
+
+        delete_subvolume_recursively() {
+          IFS=$(printf '\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 $(find /btrfs_tmp/old_roots/ -maxdepth 1 -mtime +14); do
+          delete_subvolume_recursively "$i"
+        done
+
+        btrfs subvolume create /btrfs_tmp/${cfg.btrfs.rootSubvol}
+        umount /btrfs_tmp
+      '';
+    };
+
     programs.fuse.userAllowOther = true;
-    environment.persistence.${cfg.location} = {
+    environment.persistence.${cfg.btrfs.mountPoint} = {
       hideMounts = true;
       directories = cfg.nixosDirs;
       files = cfg.nixosFiles;