zsnapshots: allow filesystem selection when no argument is specified

zfsbootmenu/bin/zsnapshots

@@ -1,4 +1,5 @@
 #!/bin/bash
+# vim: softtabstop=2 shiftwidth=2 expandtab
 
 sources=(
   /lib/profiling-lib.sh
@@ -19,18 +20,61 @@ done
 
 unset src sources
 
+list_root_filesystems() {
+  local fs mountpoint active ret
+
+  ret=1
+  while IFS=$'\t' read -r fs mountpoint active; do
+    if [ "${mountpoint}" = "/" ]; then
+      [ "${active}" = "off" ] && continue;
+    elif [ "${mountpoint}" = "legacy" ]; then
+      [ "${active}" = "on" ] || continue;
+    else
+      continue;
+    fi
+
+    echo "${fs}"
+    ret=0
+  done <<< "$( zfs list -H -t filesystem -o name,mountpoint,org.zfsbootmenu:active )"
+  return "${ret}"
+}
+
 # Replace the global_header function with a stub
 global_header() {
   echo -n -e "\\033[1;33m[ Recover from snapshot ]"
 }
 
-if [ $# -ne 1 ] ; then
- echo "Usage: $0 filesystem"
- exit
-fi
+fs_header() {
+  echo -n -e "\\033[1;33m[ Select a filesystem ]"
+}
 
 fs="${1}"
 
+if [ -z "${fs}" ]; then
+  if ! candidates="$(list_root_filesystems 2>/dev/null)"; then
+    zerror "no root candidates found; specify a filesystem manually"
+    exit 1
+  fi
+
+  header="$( column_wrap "^[RETURN] select:[ESCAPE] cancel" )"
+
+  if ! fs="$( 
+      fzf --header="${header}" --prompt "Filesystem > " \
+        ${HAS_BORDER:+--border-label="$( fs_header )"} \
+        ${HAS_BORDER:+--preview-label-pos=2:bottom} \
+        --preview-window="up:${PREVIEW_HEIGHT}${HAS_BORDER:+,border-sharp}" \
+        --preview="/libexec/zfsbootmenu-preview {} '${BOOTFS}'" <<< "${candidates}" \
+  )"; then
+    tput clear
+    exit 0;
+  fi
+fi
+
+if [ -z "${fs}" ]; then
+  zerror "a filesystem must be selected to browse snapshots"
+  exit 1
+fi
+
 if ! is_zfs_filesystem "${fs}" ; then
   zerror "'${fs}' is not a ZFS filesystem"
   exit 1