#!/bin/bash

if [[ -z "$1" ]]; then
  echo "Not enough input!"
  echo "Usage: $0 %k"
  exit 1
fi

# Prevent conflicts between parallel processes by creating a lock dir 
# and using it as a queue ticket. Only the process that can create it 
# (meaning it doesn't already exist from another process) is allowed to 
# proceed, others have to wait. Maximum queue time is 5 seconds, queue 
# check happens every 20 milliseconds.
lock()
{
  [[ -d /dev/.udev ]] ||
    if ! mkdir -p /dev/.udev; then
      echo "Cannot create /dev/.udev!"
      exit 1
    fi
  LOCK_DIR=/dev/.udev/.lock-cdsymlink
  local retry=250
  while ! mkdir $LOCK_DIR 2>/dev/null; do
    if ((retry==0)); then
       echo "Cannot lock $LOCK_DIR!" >&2
       exit 2
    fi
    sleep 0.02
    ((retry-=1))
  done
}

# Once the process is finished, delete lock dir so that the next process 
# may step forward in queue.
unlock()
{
  [[ "$LOCK_DIR" ]] || return 0
  rm -r $LOCK_DIR 2>/dev/null || return 0
}

# Find the next available symlink name for device. $1 is the device name 
# from kernel (udev's %k value), $2 is the desired symlink basename 
# (e.g. basename "cdrom" will be used to create symlinks /dev/cdrom, 
# /dev/cdrom1, etc.)
echo_link()
{
  local num
  while [[ -e /dev/$2$num ]] &&
        [[ "$(readlink /dev/$2$num)" != "$1" ]]; do
    ((num+=1))
  done
  echo -n "$2$num "
}

lock

# Output symlink name for cdrom, and then for dvd if needed.
echo_link $1 cdrom
[[ "$ID_CDROM_DVD" ]] && echo_link $1 dvd

unlock

.