#!/usr/bin/env bash
set -eo pipefail

# hello-world                      latest              ef872312fe1b        3 months ago        910 B
# hello-world                      latest              ef872312fe1bbc5e05aae626791a47ee9b032efa8f3bda39cc0be7b56bfe59b9   3 months ago        910 B

# debian                           latest              f6fab3b798be        10 weeks ago        85.1 MB
# debian                           latest              f6fab3b798be3174f45aa1eb731f8182705555f89c9026d8c1ef230cbf8301dd   10 weeks ago        85.1 MB

if ! command -v curl &> /dev/null; then
	echo >&2 'error: "curl" not found!'
	exit 1
fi
if ! command -v jq &> /dev/null; then
	echo >&2 'error: "jq" not found!'
	exit 1
fi

usage() {
	echo "usage: $0 dir image[:tag][@digest] ..."
	echo "       $0 /tmp/old-hello-world hello-world:latest@sha256:8be990ef2aeb16dbcb9271ddfe2610fa6658d13f6dfb8bc72074cc1ca36966a7"
	[ -z "$1" ] || exit "$1"
}

dir="$1" # dir for building tar in
shift || usage 1 >&2

[ $# -gt 0 -a "$dir" ] || usage 2 >&2
mkdir -p "$dir"

# hacky workarounds for Bash 3 support (no associative arrays)
images=()
rm -f "$dir"/tags-*.tmp
manifestJsonEntries=()
doNotGenerateManifestJson=
# repositories[busybox]='"latest": "...", "ubuntu-14.04": "..."'

# bash v4 on Windows CI requires CRLF separator
newlineIFS=$'\n'
if [ "$(go env GOHOSTOS)" = 'windows' ]; then
	major=$(echo ${BASH_VERSION%%[^0.9]} | cut -d. -f1)
	if [ "$major" -ge 4 ]; then
		newlineIFS=$'\r\n'
	fi
fi

registryBase='https://registry-1.docker.io'
authBase='https://auth.docker.io'
authService='registry.docker.io'

# https://github.com/moby/moby/issues/33700
fetch_blob() {
	local token="$1"; shift
	local image="$1"; shift
	local digest="$1"; shift
	local targetFile="$1"; shift
	local curlArgs=( "$@" )

	local curlHeaders="$(
		curl -S "${curlArgs[@]}" \
			-H "Authorization: Bearer $token" \
			"$registryBase/v2/$image/blobs/$digest" \
			-o "$targetFile" \
			-D-
	)"
	curlHeaders="$(echo "$curlHeaders" | tr -d '\r')"
	if [ "$(echo "$curlHeaders" | awk 'NR == 1 { print $2; exit }')" != '200' ]; then
		rm -f "$targetFile"

		local blobRedirect="$(echo "$curlHeaders" | awk -F ': ' 'tolower($1) == "location" { print $2; exit }')"
		if [ -z "$blobRedirect" ]; then
			echo >&2 "error: failed fetching '$image' blob '$digest'"
			echo "$curlHeaders" | head -1 >&2
			return 1
		fi

		curl -fSL "${curlArgs[@]}" \
			"$blobRedirect" \
			-o "$targetFile"
	fi
}

while [ $# -gt 0 ]; do
	imageTag="$1"
	shift
	image="${imageTag%%[:@]*}"
	imageTag="${imageTag#*:}"
	digest="${imageTag##*@}"
	tag="${imageTag%%@*}"

	# add prefix library if passed official image
	if [[ "$image" != *"/"* ]]; then
		image="library/$image"
	fi

	imageFile="${image//\//_}" # "/" can't be in filenames :)

	token="$(curl -fsSL "$authBase/token?service=$authService&scope=repository:$image:pull" | jq --raw-output '.token')"

	manifestJson="$(
		curl -fsSL \
			-H "Authorization: Bearer $token" \
			-H 'Accept: application/vnd.docker.distribution.manifest.v2+json' \
			-H 'Accept: application/vnd.docker.distribution.manifest.v1+json' \
			"$registryBase/v2/$image/manifests/$digest"
	)"
	if [ "${manifestJson:0:1}" != '{' ]; then
		echo >&2 "error: /v2/$image/manifests/$digest returned something unexpected:"
		echo >&2 "  $manifestJson"
		exit 1
	fi

	imageIdentifier="$image:$tag@$digest"

	schemaVersion="$(echo "$manifestJson" | jq --raw-output '.schemaVersion')"
	case "$schemaVersion" in
		2)
			mediaType="$(echo "$manifestJson" | jq --raw-output '.mediaType')"

			case "$mediaType" in
				application/vnd.docker.distribution.manifest.v2+json)
					configDigest="$(echo "$manifestJson" | jq --raw-output '.config.digest')"
					imageId="${configDigest#*:}" # strip off "sha256:"

					configFile="$imageId.json"
					fetch_blob "$token" "$image" "$configDigest" "$dir/$configFile" -s

					layersFs="$(echo "$manifestJson" | jq --raw-output --compact-output '.layers[]')"
					IFS="$newlineIFS"
					layers=( $layersFs )
					unset IFS

					echo "Downloading '$imageIdentifier' (${#layers[@]} layers)..."
					layerId=
					layerFiles=()
					for i in "${!layers[@]}"; do
						layerMeta="${layers[$i]}"

						layerMediaType="$(echo "$layerMeta" | jq --raw-output '.mediaType')"
						layerDigest="$(echo "$layerMeta" | jq --raw-output '.digest')"

						# save the previous layer's ID
						parentId="$layerId"
						# create a new fake layer ID based on this layer's digest and the previous layer's fake ID
						layerId="$(echo "$parentId"$'\n'"$layerDigest" | sha256sum | cut -d' ' -f1)"
						# this accounts for the possibility that an image contains the same layer twice (and thus has a duplicate digest value)

						mkdir -p "$dir/$layerId"
						echo '1.0' > "$dir/$layerId/VERSION"

						if [ ! -s "$dir/$layerId/json" ]; then
							parentJson="$(printf ', parent: "%s"' "$parentId")"
							addJson="$(printf '{ id: "%s"%s }' "$layerId" "${parentId:+$parentJson}")"
							# this starter JSON is taken directly from Docker's own "docker save" output for unimportant layers
							jq "$addJson + ." > "$dir/$layerId/json" <<-'EOJSON'
								{
									"created": "0001-01-01T00:00:00Z",
									"container_config": {
										"Hostname": "",
										"Domainname": "",
										"User": "",
										"AttachStdin": false,
										"AttachStdout": false,
										"AttachStderr": false,
										"Tty": false,
										"OpenStdin": false,
										"StdinOnce": false,
										"Env": null,
										"Cmd": null,
										"Image": "",
										"Volumes": null,
										"WorkingDir": "",
										"Entrypoint": null,
										"OnBuild": null,
										"Labels": null
									}
								}
							EOJSON
						fi

						case "$layerMediaType" in
							application/vnd.docker.image.rootfs.diff.tar.gzip)
								layerTar="$layerId/layer.tar"
								layerFiles=( "${layerFiles[@]}" "$layerTar" )
								# TODO figure out why "-C -" doesn't work here
								# "curl: (33) HTTP server doesn't seem to support byte ranges. Cannot resume."
								# "HTTP/1.1 416 Requested Range Not Satisfiable"
								if [ -f "$dir/$layerTar" ]; then
									# TODO hackpatch for no -C support :'(
									echo "skipping existing ${layerId:0:12}"
									continue
								fi
								token="$(curl -fsSL "$authBase/token?service=$authService&scope=repository:$image:pull" | jq --raw-output '.token')"
								fetch_blob "$token" "$image" "$layerDigest" "$dir/$layerTar" --progress
								;;

							*)
								echo >&2 "error: unknown layer mediaType ($imageIdentifier, $layerDigest): '$layerMediaType'"
								exit 1
								;;
						esac
					done

					# change "$imageId" to be the ID of the last layer we added (needed for old-style "repositories" file which is created later -- specifically for older Docker daemons)
					imageId="$layerId"

					# munge the top layer image manifest to have the appropriate image configuration for older daemons
					imageOldConfig="$(jq --raw-output --compact-output '{ id: .id } + if .parent then { parent: .parent } else {} end' "$dir/$imageId/json")"
					jq --raw-output "$imageOldConfig + del(.history, .rootfs)" "$dir/$configFile" > "$dir/$imageId/json"

					manifestJsonEntry="$(
						echo '{}' | jq --raw-output '. + {
							Config: "'"$configFile"'",
							RepoTags: ["'"${image#library\/}:$tag"'"],
							Layers: '"$(echo '[]' | jq --raw-output ".$(for layerFile in "${layerFiles[@]}"; do echo " + [ \"$layerFile\" ]"; done)")"'
						}'
					)"
					manifestJsonEntries=( "${manifestJsonEntries[@]}" "$manifestJsonEntry" )
					;;

				*)
					echo >&2 "error: unknown manifest mediaType ($imageIdentifier): '$mediaType'"
					exit 1
					;;
			esac
			;;

		1)
			if [ -z "$doNotGenerateManifestJson" ]; then
				echo >&2 "warning: '$imageIdentifier' uses schemaVersion '$schemaVersion'"
				echo >&2 "  this script cannot (currently) recreate the 'image config' to put in a 'manifest.json' (thus any schemaVersion 2+ images will be imported in the old way, and their 'docker history' will suffer)"
				echo >&2
				doNotGenerateManifestJson=1
			fi

			layersFs="$(echo "$manifestJson" | jq --raw-output '.fsLayers | .[] | .blobSum')"
			IFS="$newlineIFS"
			layers=( $layersFs )
			unset IFS

			history="$(echo "$manifestJson" | jq '.history | [.[] | .v1Compatibility]')"
			imageId="$(echo "$history" | jq --raw-output '.[0]' | jq --raw-output '.id')"

			echo "Downloading '$imageIdentifier' (${#layers[@]} layers)..."
			for i in "${!layers[@]}"; do
				imageJson="$(echo "$history" | jq --raw-output ".[${i}]")"
				layerId="$(echo "$imageJson" | jq --raw-output '.id')"
				imageLayer="${layers[$i]}"

				mkdir -p "$dir/$layerId"
				echo '1.0' > "$dir/$layerId/VERSION"

				echo "$imageJson" > "$dir/$layerId/json"

				# TODO figure out why "-C -" doesn't work here
				# "curl: (33) HTTP server doesn't seem to support byte ranges. Cannot resume."
				# "HTTP/1.1 416 Requested Range Not Satisfiable"
				if [ -f "$dir/$layerId/layer.tar" ]; then
					# TODO hackpatch for no -C support :'(
					echo "skipping existing ${layerId:0:12}"
					continue
				fi
				token="$(curl -fsSL "$authBase/token?service=$authService&scope=repository:$image:pull" | jq --raw-output '.token')"
				fetch_blob "$token" "$image" "$imageLayer" "$dir/$layerId/layer.tar" --progress
			done
			;;

		*)
			echo >&2 "error: unknown manifest schemaVersion ($imageIdentifier): '$schemaVersion'"
			exit 1
			;;
	esac

	echo

	if [ -s "$dir/tags-$imageFile.tmp" ]; then
		echo -n ', ' >> "$dir/tags-$imageFile.tmp"
	else
		images=( "${images[@]}" "$image" )
	fi
	echo -n '"'"$tag"'": "'"$imageId"'"' >> "$dir/tags-$imageFile.tmp"
done

echo -n '{' > "$dir/repositories"
firstImage=1
for image in "${images[@]}"; do
	imageFile="${image//\//_}" # "/" can't be in filenames :)
	image="${image#library\/}"

	[ "$firstImage" ] || echo -n ',' >> "$dir/repositories"
	firstImage=
	echo -n $'\n\t' >> "$dir/repositories"
	echo -n '"'"$image"'": { '"$(cat "$dir/tags-$imageFile.tmp")"' }' >> "$dir/repositories"
done
echo -n $'\n}\n' >> "$dir/repositories"

rm -f "$dir"/tags-*.tmp

if [ -z "$doNotGenerateManifestJson" ] && [ "${#manifestJsonEntries[@]}" -gt 0 ]; then
	echo '[]' | jq --raw-output ".$(for entry in "${manifestJsonEntries[@]}"; do echo " + [ $entry ]"; done)" > "$dir/manifest.json"
else
	rm -f "$dir/manifest.json"
fi

echo "Download of images into '$dir' complete."
echo "Use something like the following to load the result into a Docker daemon:"
echo "  tar -cC '$dir' . | docker load"