nixdots/system/shared/impermanence/autowipe.nix

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{ 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.
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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 ${devicePath}"
umount /mnt
'';
in ''
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# Simply deleting a subvolume with btrfs subvolume delete will not work,
# if that subvolume contains other btrfs subvolumes. Because of that, we
# instead use this function to delete subvolumes, whihc will first perform
# a recursive deletion of any nested subvolumes.
#
# This is necessary, because the root subvolume will actually usually contain
# other subvolumes, even if the user haven't created those explicitly. It seems
# that NixOS creates these automatically. Namely, I observed these in root subvol:
# - root/srv
# - root/var/lib/portables
# - root/var/lib/machines
# - root/var/tmp
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delete_subvolume_recursively() {
IFS=$'\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)}
'';
};
};
};
}