a "baggageclaim-driver"-a-like script leveraging `hdiutil` and `clonefile(2)`

apfs.sh

#!/bin/bash

# apfs - a "baggageclaim-driver"-a-like script leveraging `hdiutil` and
#        `clonefile(2)`.
#
# Usage: apfs.sh (create|cow|destroy)
#
# Commands:
#   create   vol-id                  desc: creates a volume
#   cow      vol-id parent-vol-id    desc: creates a cow layer based on `parent-vol-id`
#   destroy  vol-id                  desc: destroys a volume
#

set -e -x

readonly IMAGES_PATH="/tmp/images"
readonly VOLUMES_PATH="/tmp/volumes"

main() {
	case $1 in
	create)
		create_volume $2
		;;

	destroy)
		destroy_volume $2
		;;

	cow)
		create_cow $2 $3
		;;

	*)
		echo "Error: Unknown argument '$1'."
		echo "Usage: $0 (create|cow|destroy)"
		exit 1
		;;

	esac
}

# creates an underlying image (sparse file) to back the volume, then attaches
# such image to a `/dev` device which gets mounted to a mountpoint.
#
#   /images/uuid1.sparseimage                (1G sparse file)
#            |
#            *--> /dev/disk27s1              (block device)
#                       |
#                       *--> /volumes/uuid1  (mountpoint)
#
create_volume() {
	local handle=$1
	local volume_path disk_image

	if [[ -z $1 ]]; then
		echo "create_volume(handle): arg must not be empty"
		return 1
	fi

	disk_image="$IMAGES_PATH/${handle}.sparseimage"
	volume_path="$VOLUMES_PATH/${handle}"

	hdiutil create \
		-size 1g \
		-type SPARSE \
		-fs APFS \
		-volname $handle \
		$disk_image

	hdiutil attach \
		-mountpoint $volume_path \
		$disk_image
}

# clones the backing image of the parent volume and then mounts that
# to a different location, allowing us to modify the contents of the
# parent volume without interfering with the contents of the first
# (multi-file cow semantics!)
# 
#
#   /images/uuid1.sparseimage                (parent's 1G sparse file)
#           |
#           |
#           *->  clone this thing: clonefile(src,dst, flags)
#                    |
#                    *-> produces /volume/uuid12.sparseimage
#                                             |
#                                             *-> create disk & attach to it
#                                                          |
#                           mount to a volume location   <-*
#
#
create_cow() {
	local handle=$1
	local handle_parent=$2
	local volume_path disk_image disk_image_parent

	if [[ -z $handle || -z $handle_parent ]]; then
		echo "create_cow(handle, handle_parent): both args must not be empty"
		return 1
	fi

	disk_image="$IMAGES_PATH/${handle}.sparseimage"
	disk_image_parent="$IMAGES_PATH/${handle_parent}.sparseimage"
	volume_path="$VOLUMES_PATH/${handle}"

	cp -c $disk_image_parent $disk_image
	hdiutil attach \
		-mountpoint $volume_path \
		$disk_image
}

# destroys a volume by detaching it an removing the underlying disk image.
destroy_volume() {
	local handle=$1
	local volume_path disk_image

	if [[ -z $handle ]]; then
		echo "destroy_volume(handle): arg must not be empty"
		return 1
	fi

	disk_image="$IMAGES_PATH/${handle}.sparseimage"
	volume_path="$VOLUMES_PATH/${handle}"

	hdiutil detach $volume_path
	rm -f $disk_image
}

main "$@"