[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>

2 files changed, 17 insertions, 2 deletions

diff --git a/archiso/initcpio/hooks/archiso b/archiso/initcpio/hooks/archiso
index 3ef4eb1..9d38f07 100644
--- a/archiso/initcpio/hooks/archiso
+++ b/archiso/initcpio/hooks/archiso
@@ -1,5 +1,5 @@
 # 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
diff --git a/archiso/initcpio/install/archiso b/archiso/initcpio/install/archiso
index e506271..300dfef 100644
--- a/archiso/initcpio/install/archiso
+++ b/archiso/initcpio/install/archiso
@@ -4,6 +4,7 @@ build() {
     add_module "cdrom"
     add_module "loop"
     add_module "dm-snapshot"
+    add_module "overlay"
 
     add_runscript