[archiso] Add optional OverlayFS support

This is the first attemp to test overlayfs in archiso.
The current dm-snapshot mode is keep and is enabled by default,
while the new mode is enabled via "-s sfs" to mkarchiso.
No new boot parameters are added, since archiso hooks detects
if the .sfs file is for dm-snapshot (airootfs.img inside)
or for overlayfs.
Persistence is supported in overlayfs mode using the same options
(cowlabel or cowdevice), but warning while in dm-snapshot mode,
only one file is used (airootfs.cow), in overlayfs mode internal
files for workdir/ and upperdir/ are allocated, so you can not use
VFAT or NTFS.

To test this, you need to enable [testing] in pacman.conf from
releng profile and edit build.sh then add "-s sfs" in make_prepare()

Look at:
    setarch ${arch} mkarchiso ${verbose} -w "${work_dir}" -D "${install_dir}" prepare
Replace with:
    setarch ${arch} mkarchiso ${verbose} -w "${work_dir}" -s sfs -D "${install_dir}" prepare

The build requires just half of space that the build for dm-snapshot,
since there is no ext4 img ;)

Just to remember: there is no space gain in .sfs (just about 2M)

There is at least one thing during boot with machine-id service:
Dec 24 03:31:39 archiso systemd-machine-id-commit[183]: Failed to unmount transient /etc/machine-id file in our private namespace: Invalid argument

Signed-off-by: Gerardo Exequiel Pozzi <vmlinuz386@gmail.com>

archiso/initcpio/hooks/archiso

 # args: source, newroot, mountpoint
-_mnt_fs() {
+_mnt_dmsnapshot() {
     local img="${1}"
     local newroot="${2}"
     local mnt="${3}"
@@ -37,6 +37,16 @@ _mnt_fs() {
     echo $(readlink -f /dev/mapper/${dm_snap_name}) >> /run/archiso/used_block_devices
 }
 
+# args: source, newroot, mountpoint
+_mnt_overlayfs() {
+    local src="${1}"
+    local newroot="${2}"
+    local mnt="${3}"
+    mkdir -p /run/archiso/cowspace/${cow_directory}/upperdir /run/archiso/cowspace/${cow_directory}/workdir
+    mount -t overlay -o lowerdir=${src},upperdir=/run/archiso/cowspace/${cow_directory}/upperdir,workdir=/run/archiso/cowspace/${cow_directory}/workdir airootfs "${newroot}${mnt}"
+}
+
+
 # args: /path/to/image_file, mountpoint
 _mnt_sfs() {
     local img="${1}"
@@ -165,7 +175,11 @@ archiso_mount_handler() {
     mkdir -p "/run/archiso/cowspace/${cow_directory}"
 
     _mnt_sfs "/run/archiso/bootmnt/${archisobasedir}/${arch}/airootfs.sfs" "/run/archiso/sfs/airootfs"
-    _mnt_fs "/run/archiso/sfs/airootfs/airootfs.img" "${newroot}" "/"
+    if [[ -f "/run/archiso/sfs/airootfs/airootfs.img" ]]; then
+        _mnt_dmsnapshot "/run/archiso/sfs/airootfs/airootfs.img" "${newroot}" "/"
+    else
+        _mnt_overlayfs "/run/archiso/sfs/airootfs" "${newroot}" "/"
+    fi
 
     if [[ "${copytoram}" == "y" ]]; then
         umount /run/archiso/bootmnt

archiso/initcpio/install/archiso

@@ -4,6 +4,7 @@ build() {
     add_module "cdrom"
     add_module "loop"
     add_module "dm-snapshot"
+    add_module "overlay"
 
     add_runscript
 

archiso/mkarchiso

@@ -16,6 +16,7 @@ iso_application="Arch Linux Live/Rescue CD"
 install_dir="arch"
 work_dir="work"
 out_dir="out"
+sfs_mode="img"
 sfs_comp="xz"
 
 # Show an INFO message
@@ -87,6 +88,10 @@ _usage ()
     echo "                     Default: '${work_dir}'"
     echo "    -o <out_dir>     Set the output directory"
     echo "                     Default: '${out_dir}'"
+    echo "    -s <sfs_mode>    Set SquashFS image mode (img or sfs)"
+    echo "                     img: prepare airootfs.sfs for dm-snapshot usage"
+    echo "                     sfs: prepare airootfs.sfs for overlayfs usage"
+    echo "                     Default: ${sfs_mode}"
     echo "    -c <comp_type>   Set SquashFS compression type (gzip, lzma, lzo, xz)"
     echo "                     Default: '${sfs_comp}'"
     echo "    -v               Enable verbose output"
@@ -193,8 +198,8 @@ _cleanup () {
     _msg_info "Done!"
 }
 
-# Makes a filesystem from a source directory.
-_mkairootfs () {
+# Makes a ext4 filesystem inside a SquashFS from a source directory.
+_mkairootfs_img () {
     if [[ ! -e "${work_dir}/airootfs" ]]; then
         _msg_error "The path '${work_dir}/airootfs' does not exist" 1
     fi
@@ -224,6 +229,22 @@ _mkairootfs () {
     rm ${work_dir}/airootfs.img
 }
 
+# Makes a SquashFS filesystem from a source directory.
+_mkairootfs_sfs () {
+    if [[ ! -e "${work_dir}/airootfs" ]]; then
+        _msg_error "The path '${work_dir}/airootfs' does not exist" 1
+    fi
+
+    mkdir -p "${work_dir}/iso/${install_dir}/${arch}"
+    _msg_info "Creating SquashFS image, this may take some time..."
+    if [[ "${quiet}" = "y" ]]; then
+        mksquashfs "${work_dir}/airootfs" "${work_dir}/iso/${install_dir}/${arch}/airootfs.sfs" -noappend -comp "${sfs_comp}" -no-progress &> /dev/null
+    else
+        mksquashfs "${work_dir}/airootfs" "${work_dir}/iso/${install_dir}/${arch}/airootfs.sfs" -noappend -comp "${sfs_comp}" -no-progress
+    fi
+    _msg_info "Done!"
+}
+
 _mkchecksum () {
     _msg_info "Creating checksum file for self-test..."
     cd "${work_dir}/iso/${install_dir}/${arch}"
@@ -292,7 +313,11 @@ command_prepare () {
     _show_config prepare
 
     _cleanup
-    _mkairootfs
+    if [[ ${sfs_mode} == "sfs" ]]; then
+        _mkairootfs_sfs
+    else
+        _mkairootfs_img
+    fi
     _mkchecksum
 }
 
@@ -330,7 +355,7 @@ if [[ ${EUID} -ne 0 ]]; then
     _msg_error "This script must be run as root." 1
 fi
 
-while getopts 'p:r:C:L:P:A:D:w:o:c:vh' arg; do
+while getopts 'p:r:C:L:P:A:D:w:o:s:c:vh' arg; do
     case "${arg}" in
         p) pkg_list="${pkg_list} ${OPTARG}" ;;
         r) run_cmd="${OPTARG}" ;;
@@ -341,6 +366,7 @@ while getopts 'p:r:C:L:P:A:D:w:o:c:vh' arg; do
         D) install_dir="${OPTARG}" ;;
         w) work_dir="${OPTARG}" ;;
         o) out_dir="${OPTARG}" ;;
+        s) sfs_mode="${OPTARG}" ;;
         c) sfs_comp="${OPTARG}" ;;
         v) quiet="n" ;;
         h|?) _usage 0 ;;