Create new root subvolume on boot instead of relying on a blank snapshot.

Also persist /var/lib/nixos by default.

common/impermanence.nix

@@ -34,41 +34,27 @@
   # reset / at each boot
   # Note `lib.mkBefore` is used instead of `lib.mkAfter` here.
   boot.initrd.postDeviceCommands = pkgs.lib.mkBefore ''
-    mkdir -p /mnt
+    mkdir /mnt
+    mount /dev/mapper/luksroot /mnt
+    if [[ -e /mnt/root ]]; then
+        mkdir -p /mnt/old_roots
+        timestamp=$(date --date="@$(stat -c %Y /mnt/root)" "+%Y-%m-%-d_%H:%M:%S")
+        mv /mnt/root "/mnt/old_roots/$timestamp"
+    fi
 
-    # We first mount the btrfs root to /mnt
-    # so we can manipulate btrfs subvolumes.
-    mount -o subvol=/ /dev/disk/by-label/btrfs /mnt
+    delete_subvolume_recursively() {
+        IFS=$'\n'
+        for i in $(btrfs subvolume list -o "$1" | cut -f 9- -d ' '); do
+            delete_subvolume_recursively "/mnt/$i"
+        done
+        btrfs subvolume delete "$1"
+    }
 
-    # While we're tempted to just delete /root and create
-    # a new snapshot from /root-blank, /root is already
-    # populated at this point with a number of subvolumes,
-    # which makes `btrfs subvolume delete` fail.
-    # So, we remove them first.
-    #
-    # /root contains subvolumes:
-    # - /root/var/lib/portables
-    # - /root/var/lib/machines
-    #
-    # I suspect these are related to systemd-nspawn, but
-    # since I don't use it I'm not 100% sure.
-    # Anyhow, deleting these subvolumes hasn't resulted
-    # in any issues so far, except for fairly
-    # benign-looking errors from systemd-tmpfiles.
-    btrfs subvolume list -o /mnt/root |
-    cut -f9 -d' ' |
-    while read subvolume; do
-      echo "deleting /$subvolume subvolume..."
-      btrfs subvolume delete "/mnt/$subvolume"
-    done &&
-    echo "deleting /root subvolume..." &&
-    btrfs subvolume delete /mnt/root
+    for i in $(find /mnt/old_roots/ -maxdepth 1 -mtime +30); do
+        delete_subvolume_recursively "$i"
+    done
 
-    echo "restoring blank /root subvolume..."
-    btrfs subvolume snapshot /mnt/root-blank /mnt/root
-
-    # Once we're done rolling back to a blank snapshot,
-    # we can unmount /mnt and continue on the boot process.
+    btrfs subvolume create /mnt/root
     umount /mnt
   '';
 }