rewrite containerd docker tarballs

rewrite-docker-tar

#!/bin/bash
#
# rewrite-docker-tar ./image.tar ./output.tar
#
# Exporting 'docker' tarballs with buildctl is creating manifests with no file
# extensions (no .json or .tar.gz on the config or layers).  This works with
# dockerd because it never looks at the extensions just tries various things with 
# safe fallbacks.  Other tooling relies on extensions and checks for
# .json, .tar, and .tar.gz.
#
# This script will rebuild one of these containerd tarballs with file
# extensions so most other tooling can process it.
#
# This is probably brittle as hell.

set -euo pipefail

image=${1:-''}
result=${2:-''}

if [[ -z "${image}" || -z "${result}" ]]; then
  echo "Usage: rewrite-docker-tar in.tar out.tar" && exit 1
fi

workdir=$(mktemp -d -t rewrite-docker-tar.XXXX)
image_dir=${workdir}/image
output_dir=${workdir}/output

mkdir -p ${image_dir} ${output_dir}

cleanup(){
  rc=${1:-0}
  find ${workdir}
  test -d ${workdir} && rm -rf ${workdir}
  exit ${rc}
}

trap cleanup EXIT

contents=$(tar -tf ${image})
tar -C ${image_dir} -xf ${image}

manifest=${image_dir}/manifest.json

# Extract the current manifest, I have only seen these fields in produced images.
config_json=$(jq -r '.[].Config' ${manifest})

layers=()
for layer in $(jq -r '.[].Layers[]' ${manifest}); do
  layers+=(${layer})
done

# Rename files
mv ${image_dir}/${config_json}{,.json}

for layer in ${layers[@]}; do
  mv ${image_dir}/${layer}{,.tar.gz}
done

# Rewrite json
#jq --arg manifest "${manifest}.json" '.[].Config = $manifest' ${image_dir}/manifest.json
sed -i -e 's|'"${config_json}"'|'"${config_json}"'.json|' ${image_dir}/manifest.json
for layer in ${layers[@]}; do
  sed -i -e 's|'"${layer}"'|'"${layer}"'.tar.gz|' ${image_dir}/manifest.json
done

# Build output tar, this could totally be incomplete if the format changes in containerd.
# 

pushd ${image_dir} > /dev/null
tar -cf ${output_dir}/out.tar -T /dev/null > /dev/null
for file in *; do
  tar -rf ${output_dir}/out.tar ${file}
done
popd > /dev/null
mv ${output_dir}/out.tar ${result}