nixpkgs/pkgs/build-support/docker/store-path-to-layer.sh

#!@shell@

set -eu

layerNumber=$1
shift

layerPath="./layers/$layerNumber"
echo "Creating layer #$layerNumber for $@"

mkdir -p "$layerPath"

# Make sure /nix and /nix/store appear first in the archive.
#
# We create the directories here and use them because
# when there are other things being added to the
# nix store, tar could fail, saying,
# "tar: /nix/store: file changed as we read it"
mkdir -p nix/store

# Then we change into the /nix/store in order to
# avoid a similar "file changed as we read it" error
# as above. Namely, if we use the absolute path of
# /nix/store/123-pkg and something new is added to the nix
# store while tar is running, it will detect a change to
# /nix/store and fail. Instead, if we cd into the nix store
# and copy the relative nix store path, tar will ignore
# changes to /nix/store. In order to create the correct
# structure in the tar file, we transform the relative nix
# store path to the absolute store path.
tarhash=$(
  basename -a "$@" |
    tar -cp nix \
      -C /nix/store --verbatim-files-from --files-from - \
      --hard-dereference --sort=name \
      --mtime="@$SOURCE_DATE_EPOCH" \
      --owner=0 --group=0 \
      --transform 's,^nix(/|$),/nix/,' \
      --transform 's,^[^/],/nix/store/\0,rS' |
    tee "$layerPath/layer.tar" |
    tarsum
)

# Add a 'checksum' field to the JSON, with the value set to the
# checksum of the tarball.
cat ./generic.json | jshon -s "$tarhash" -i checksum > $layerPath/json

# Indicate to docker that we're using schema version 1.0.
echo -n "1.0" > $layerPath/VERSION