Re: CD Symlinks Solution Idea

[juuso.alasuutari@tamperelainen.org]

Continuing on the subject of cdrom symlinks creation.

I looked at the debian script and copied its use of a lock directory to enforce
a queue for parallel processes. Otherwise my script just normally looks in /dev
to see whether e.g. a symlink named cdrom already exists, and if so, checks if
cdrom1 exists, etc. until the first available name comes up.

I didn't find a way to make the link names persistent, so that e.g. cdrom would
always point at the same device. But on the other hand, I've never heard of
cdrom devices being loaded in random order during boot. Is it nevertheless
possible that this might happen, like it does with network devices? I can't
test it, because I only have one cdrom drive.

I'm posting this so that all those interested can evaluate and point out
possible bugs in the script. Thank you for your help and your time. :)

Juuso

== Begin script ==

#!/bin/bash

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

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