#! /bin/sh
###
### Make fake snapshots by remounting a filesystem readonly
###
### (c) 2011 Mark Wooding
###

###----- Licensing notice ---------------------------------------------------
###
### This program is free software; you can redistribute it and/or modify
### it under the terms of the GNU General Public License as published by
### the Free Software Foundation; either version 2 of the License, or
### (at your option) any later version.
###
### This program is distributed in the hope that it will be useful,
### but WITHOUT ANY WARRANTY; without even the implied warranty of
### MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
### GNU General Public License for more details.
###
### You should have received a copy of the GNU General Public License
### along with this program; if not, write to the Free Software Foundation,
### Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.

set -e
quis=${0##*/}

###--------------------------------------------------------------------------
### Parse the command line.

## Provide help or version information.
usage="usage: $quis DEVICE [KEY=VALUE ...]"
version="$quis, version 1.0.0"
case "$#,$1" in
  0,*) echo >&2 "$usage"; exit 1 ;;
  *,-v | *,--version) echo "$version"; exit ;;
  *,-h | *,--help)
    cat <<EOF
$version
$usage

Option keys:
  dir=MOUNTPT		Mount point of filesystem.
  op=OPERATION		\`snap' to create snapshot, or \`unsnap' to remove.
  tag=TAG		Disambiguation tag to store in filesystem.
EOF
    exit
    ;;
esac

## Scan the option keys.
dev=$1; shift
Oop=snap Otag=snap
win=t
for i in "$@"; do
  case "$i" in
    ?*=*) ;;
    *) echo >&2 "$quis: malformed option \`$i'"; exit 1 ;;
  esac
  k=${i%%=*} v=${i#*=}
  case "$k" in *.ro) k=${k%.ro} ;; ?*.?*) continue ;; esac
  case "$k" in
    op | tag | dir) eval "O$k=\$v" ;;
    *) echo >&2 "$quis: unknown option \`$k'"; win=nil ;;
  esac
done
case $win in nil) exit 1 ;; esac

## Find a mount point if none was given.
sdn='s/^b[^ ]*  *[^ ]*  *[^ ]*  *[^ ]*  *\([0-9]*\),  *\([0-9]*\) .*$/\1:\2/'
case "${Odir+t}" in
  t) ;;
  *)
    exec 3</etc/mtab
    devno=$(ls -lL "/dev/$dev" | sed "$sdn")
    case "$devno" in
      *:*) ;;
      *) echo >&2 "$quis: $dev is not a block device"; exit 1 ;;
    esac
    while read <&3 d m fs hunoz; do
      if [ ! -b "$d" ]; then continue; fi
      dn=$(ls -lL "$d" | sed "$sdn")
      case "$dn" in
	"$devno")
	  case "${Odir+t}" in
	    t) echo >&2 "$quis: /dev/$dev mounted multiple times"; exit 1 ;;
	  esac
	  Odir=$m
	  ;;
      esac
    done
    exec 3>&-
    ;;
esac
case "${Odir+t}" in
  t) ;; *) echo >&2 "$quis: /dev/$dev apparently not mounted"; exit 1 ;;
esac

###--------------------------------------------------------------------------
### Take or remove the snapshot.

case "$Oop" in

  snap)
    echo "$Otag" >"$Odir/.snap"
    mount -oremount,ro "/dev/$dev" "$Odir"
    echo "$dev"
    ;;

  unsnap)
    if [ ! -f "$Odir/.snap" ]; then
      echo >&2 "$quis: no snapshot tag"
      exit 1
    fi
    read tag <"$Odir/.snap"
    case "$tag" in
      "$Otag") ;;
      *)
	echo >&2 "$quis: tag mismatch (found \`$tag' but expected \`$Otag')"
	exit 1
	;;
    esac
    mount -oremount,rw "/dev/$dev" "$Odir"
    rm "$Odir/.snap"
    ;;

  *)
    echo >&2 "$quis: unknown operation \`$Oop'"
    exit 1
    ;;

esac

###----- That's all, folks --------------------------------------------------