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Shell quiz: implement a mutex

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# 8  
Old 05-21-2009

Code:
progname=${0##*/}
lockdir=/tmp/$progname.lockdir
if mkdir -m 400 "$lockdir" 2>/dev/null
then
  trap 'rmdir "$lockdir"' EXIT
else
  printf "%s is already running\n" "$progname" >&2
  exit 1
fi
##  script follows
sleep 20

# 9  
Old 05-21-2009
Quote:
Originally Posted by cfajohnson
Code:
if mkdir -m 400 "$lockdir" 2>/dev/null

Yup, this is the standard solution.
Check lock and set lock is an atomic operation.
Simple and working... except of course when a SIGKILL hits.

I am puzzled about -m 400, what is the pourpose of it?
If the directory already exists, the mkdir command would fail regardless of the mode bits.
# 10  
Old 05-21-2009
Quote:
Originally Posted by colemar
Ok, now I got it: the serialization protocol dictates that the pid appearing in line 1 of the interlock file is the one whose process is allowed to go on, while the other processes are inside a wait-and-check loop (well, perhaps it is smarter to just wait $pid1 where pid1 is read from line 1); when the process ends, it removes its pid; then another pid is in line 1... and so on, until the pid queue is empty.
Nah, there are still problems.
To remove its pid from the top, a shell script has to rewrite the whole interlock file. What if in the meantime another instance is appending its pid?
This issue is basically a consequence of the fact that unix provides no automatic exclusive write access to files.

I believe it would go away if we make the following substitutions:
  • interlock file <= directory
  • lines of interlock file <= filenames inside the directory
# 11  
Old 05-21-2009
Quote:
Originally Posted by colemar
Yup, this is the standard solution.
Check lock and set lock is an atomic operation.
Simple and working... except of course when a SIGKILL hits.

Anyone throwing SIGKILLs around should expect to have to do some manual cleanup!
Quote:
I am puzzled about -m 400, what is the pourpose of it?
If the directory already exists, the mkdir command would fail regardless of the mode bits.

It's intended to prevent anyone putting a file in there and preventing rmdir from working. rm -rf "$lockdir" would fix that, too.
# 12  
Old 05-21-2009
Quote:
Originally Posted by cfajohnson
mkdir
Let see if I can add something useful:

Code:
progname=${0##*/}
lockdir=/tmp/$progname.lockdir
if mkdir -m 755 "$lockdir" 2>/dev/null
then
  : > "$lockdir/$$"
  trap 'rm -rf "$lockdir"' EXIT
else
  pid=$(echo "$lockdir"/*);  pid=${pid##*/}
  printf "%s is already running with pid %s\n" "$progname" $pid >&2
  exit 1
fi
##  script follows
sleep 20

And a version that doesn't drop the ball:

Code:
progname=${0##*/}
lockdir=/tmp/$progname.lockdir
trap '[[ -f "$lockdir/$$" ]] && rm -rf "$lockdir"' EXIT
until mkdir -m 755 "$lockdir" 2>/dev/null
do
  pid=$(echo "$lockdir"/*);  pid=${pid##*/}
  printf "%s is already running with pid %s\n" "$progname" $pid >&2
  sleep 1
done
: > "$lockdir/$$"

##  script follows
sleep 20

Last edited by colemar; 05-22-2009 at 04:44 AM.. Reason: A small fix to pid=
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