hooks/archiso: implement searching for archisodevice

If `archisosearchuuid` is specified, assign `archisodevice`:
* to a device with `UUID=${archisosearchuuid}`, if it exists,
* to a device containing `archisosearchfilename` (`/boot/${archisosearchuuid}.uuid` by default).

This avoids having to specify `archisodevice`.

Related to archlinux/archiso#217

CHANGELOG.rst

@@ -8,6 +8,11 @@ Changelog
 Added
 -----
 
+- Implement searching for the archiso device using ``archisosearchuuid`` and ``archisosearchfilename``
+  (defaults to ``/boot/${archisosearchuuid}.uuid``) boot parameters. If a device with UUID matching
+  ``archisosearchuuid`` does not exist, then all available block devices are mounted and searched for the
+  ``archisosearchfilename`` file.
+
 Changed
 -------
 

docs/README.bootparams

@@ -23,6 +23,14 @@ INDEX
                     Default: "/dev/disk/by-label/${archisolabel}"
 * archisobasedir=   Set the base directory where all files reside.
                     Default: "arch"
+* archisosearchuuid=     UUID used in searching for a device where archiso
+                         files reside. If a device with such UUID does not
+                         exist, looks for ${archisosearchfilename} in all
+                         available devices.
+                         Overrides ${archisodevice}.
+                         Default: (unset)
+* archisosearchfilename= File path which confirms a device is an archiso medium.
+                         Default: "/boot/${archisosearchuuid}.uuid"
 * copytoram=        If set to "y", the root file system image is copied to
                     tmpfs.
                     If set to "auto", the root file system image is copied to

hooks/archiso

@@ -121,6 +121,37 @@ _mnt_dev() {
     fi
 }
 
+_search_for_archisodevice() {
+    local search_dev
+
+    # First try searching for a UUID matching $archisosearchuuid
+    if archisodevice="$(rootdelay="${rootdelay:-2}" resolve_device "UUID=${archisosearchuuid}")"; then
+        return 0
+    fi
+
+    install -dm755 /archisosearch
+    # Search for $archisosearchfilename in all available block devices
+    for search_dev in $(blkid -o device); do
+        msg ":: Searching for '${archisosearchfilename}' in '${search_dev}'"
+        umount /archisosearch 2>/dev/null
+        if mount -r "$search_dev" /archisosearch 2>/dev/null; then
+            if [ -e "/archisosearch${archisosearchfilename}" ]; then
+                archisodevice="$search_dev"
+                break
+            fi
+        fi
+    done
+    umount /archisosearch 2>/dev/null
+    rmdir /archisosearch
+
+    if [ -b "$archisodevice" ]; then
+        return 0
+    else
+        msg "ERROR: Device '${archisosearchuuid}' not found"
+        launch_interactive_shell
+    fi
+}
+
 _verify_checksum() {
     local _status
     cd "/run/archiso/bootmnt/${archisobasedir}/${arch}" || exit 1
@@ -184,6 +215,7 @@ run_hook() {
     # defined via initcpio's parse_cmdline()
     [ -z "${archisodevice}" ] && archisodevice="/dev/disk/by-label/${archisolabel}"
     [ -z "${cow_spacesize}" ] && cow_spacesize="256M"
+    [ -z "${archisosearchfilename}" ] && archisosearchfilename="/boot/${archisosearchuuid}.uuid"
     # shellcheck disable=SC2154
     # defined via initcpio's parse_cmdline()
     if [ -n "${cow_label}" ]; then
@@ -210,6 +242,10 @@ archiso_mount_handler() {
     local newroot="${1}"
     local sigfile cms_sigfile fs_img fs_img_size iso_blockdev
 
+    if [ -n "$archisosearchuuid" ]; then
+        _search_for_archisodevice
+    fi
+
     if ! mountpoint -q "/run/archiso/bootmnt"; then
         _mnt_dev "${archisodevice}" "/run/archiso/bootmnt" "-r" "defaults"
     fi

install/archiso

@@ -14,6 +14,7 @@ build() {
     add_binary dmsetup
     add_binary losetup
     add_binary openssl
+    add_binary blkid
 
     if command -v pv >/dev/null 2>&1; then
         add_binary pv