snap_recompress.sh

#! /usr/bin/env sh

snaps_dir="/var/lib/snapd/snaps"
snaps_mnt="/snap"
nsfs_mnt="/run/snapd/ns"

def_compression="xz"
new_compression="zstd"

main() {
  snap list --all | awk '{print $1, $3}' | tail -n +2 |
  while read name rev; do
    echo "Processing $name"
    snap_file="${snaps_dir}/${name}_${rev}.snap"
    snap_compression=$(unsquashfs -s "$snap_file" | grep Compression | awk '{print $2}')

    if [ "$snap_compression" = "$def_compression" ]; then
      echo "$snap_file is compressed with $def_compression, recompressing"
      recompress "$name" "$rev"
      unset name rev
    fi
  done
}

recompress() {
  name="$1"
  rev="$2"

  name_esc="$(printf $name | sed 's|-|\\x2d|g')"
  snap_file="${snaps_dir}/${name}_${rev}.snap"
  snap_file_recompressed="${snap_file}.recompressed"
  snap_mnt_unit="snap-${name_esc}-${rev}.mount"

  # Default compression level is 15, which is pretty slow (decompression is fast as usual)
  mksquashfs "${snaps_mnt}/${name}/${rev}" "$snap_file_recompressed" -noappend -comp "$new_compression"
  chmod 600 "$snap_file_recompressed"

  systemctl stop "$snap_mnt_unit"
  umount "${snaps_mnt}/${name}/${rev}"
  umount "${nsfs_mnt}/${name}.mnt"

  rm -f "$snap_file"
  mv "$snap_file_recompressed" "$snap_file"

  systemctl start "$snap_mnt_unit"
}

set_niceness() {
  if [ -x "$(command -v nice)" ]; then
    renice -n 19 $$
  fi

  if [ -x "$(command -v ionice)" ]; then
    ionice -c 3 -p $$
  fi
}

set_niceness
main