recovery shell: ESP-related tooling

Add a mount_esp helper with tab completion. This is a very thin wrapper
over the new mount_block function.

Add the relevant kernel modules to the optional modules list for both
dracut and mkinitcpio images. Explicitly add the same modules to the
release/recovery shared config.

docs/online/recovery-shell.rst

@@ -40,6 +40,10 @@ Common Commands
 
   Mount the filesystem at a unique location and print the mount point.
 
+**mount_esp** *device*
+
+  Mount an EFI System Partition at a unique location and print the mount point.
+
 **mount_efivarfs** *mode*
 
   Mount or remount *efivarfs* as read-write or read-only.

etc/zfsbootmenu/release.conf.d/common.conf

@@ -5,6 +5,9 @@ install_optional_items+=" /etc/zbm-commit-hash "
 
 omit_dracutmodules+=" crypt-ssh nfs lunmask "
 
+# kernel modules to allow mounting an ESP
+add_drivers+=" fat vfat nls_iso8859_1 nls_cp437 "
+
 # qemu drivers
 omit_dracutmodules+=" qemu "
 

zfsbootmenu/bin/mount_esp

@@ -0,0 +1,19 @@
+#!/bin/bash
+
+# shellcheck disable=SC1091
+sources=(
+  /lib/kmsg-log-lib.sh
+  /lib/zfsbootmenu-core.sh
+)
+
+for src in "${sources[@]}"; do
+  # shellcheck disable=SC1090
+  if ! source "${src}" >/dev/null 2>&1 ; then
+    echo "<3>ZFSBootMenu: unable to source '${src}' in $0" > /dev/kmsg
+    exit 1
+  fi
+done
+
+unset src sources
+
+mount_block "${1}"

zfsbootmenu/install-helpers.sh

@@ -69,6 +69,10 @@ zfsbootmenu_essential_modules=(
 zfsbootmenu_optional_modules=(
   "zlib_deflate"
   "zlib_inflate"
+  "fat"
+  "vfat"
+  "nls_iso8859_1"
+  "nls_cp437"
 )
 
 create_zbm_conf() {

zfsbootmenu/lib/zfsbootmenu-completions.sh

@@ -71,6 +71,23 @@ _mount_zfs() {
 }
 complete -F _mount_zfs mount_zfs
 
+_mount_esp() {
+  local ESP dev uuid
+  COMPREPLY=()
+
+  [ "${#COMP_WORDS[@]}" != "2" ] && return
+
+  while IFS=' ' read -r dev uuid; do
+    # magic uuid for an ESP
+    [ "${uuid,,}" == "c12a7328-f81f-11d2-ba4b-00a0c93ec93b" ] || continue
+    is_mounted "${dev}" >/dev/null 2>&1 || ESP+=( "${dev}" )
+  done <<<"$( lsblk -ln -o PATH,PARTTYPE )"
+
+  COMPREPLY=( $( compgen -W "${ESP[*]}" -- "${COMP_WORDS[1]}" ) )
+}
+
+complete -F _mount_esp mount_esp
+
 _zsnapshots() {
   local ZFS
   COMPREPLY=()

zfsbootmenu/lib/zfsbootmenu-core.sh

@@ -244,6 +244,49 @@ check_for_pools() {
   return 1
 }
 
+# arg1: device name
+# prints: mountpoint
+# returns: 0 on success
+
+mount_block() {
+  local device mnt output
+
+  device="${1}"
+
+  if [ -z "${device}" ]; then
+    zerror "device is undefined"
+    return 1
+  fi
+
+  if [ ! -b "${device}" ]; then
+    zerror "device does not exist or is not a block device"
+    return 1
+  fi
+
+  if mnt="$( is_mounted "${device}" )"; then
+    echo "${mnt}"
+    return 0
+  fi
+
+  mnt="/mnt/${device##*/}"
+
+  if [ -d "${mnt}" ] && is_mountpoint "${mnt}"; then
+    zerror "mountpoint '${mnt}' already in use"
+    return 1
+  fi
+
+  mkdir -p "${mnt}" || return 1
+
+  if output="$( mount "${device}" "${mnt}" 2>&1 )"; then
+    echo "${mnt}"
+    return 0
+  else
+    zerror "unable to mount '${device}' at '${mnt}'"
+    zerror "${output}"
+    return 1
+  fi
+}
+
 # arg1: ZFS filesystem name
 # prints: mountpoint
 # returns: 0 on success
@@ -2063,6 +2106,36 @@ is_mountpoint() {
   return 1
 }
 
+# arg1: device or dataset name
+# prints: mountpoint if mounted 
+# returns: 0 if the device is mounted, 1 if not
+
+is_mounted() {
+  local mount_path dev path opts
+
+  mount_dev="${1}"
+
+  if [ -z "${mount_dev}" ]; then
+    zerror "mount_dev undefined"
+    return 1
+  fi
+
+  if [ ! -r /proc/self/mounts ]; then
+    zerror "unable to read mount database"
+    return 1
+  fi
+
+  # shellcheck disable=SC2034
+  while read -r dev path opts; do
+    if [ "${dev}" = "${mount_dev}" ]; then
+      echo "${path}"
+      return 0
+    fi
+  done < /proc/self/mounts
+
+  return 1
+}
+
 # args: none
 # prints: nothing
 # returns: 0 if EFI is detected, 1 if not