Jennifer Pioch wrote:
>
> Any advice on a good way to implement a multiple readers/single writer
> lock in a ksh shell script? Multiple scripts should read from a file
> but only a single one should be allowed to update or replace the file.
[snip]
Attached (as "filemutexdemo1.ksh.txt" ; note that this demo needs
ksh93t- from the ksh93-integration update1 tree) is a demo which shows
an implementation of a filesystem-based mutex mechanism for shell
scripts (based on the idea thatt directory creation and destroytion are
"atomic" operations and that a directory cannot be removed by "rmdir"
when it contains any files/directories (which is used to implememet the
shared lock stuff)).
The code supports:
- "exclusive" locks
- "shared" locks (for multiple reader/single-writer setups)
- seperate methods to "try" to obtain a lock (without waiting).
- Critical sections via "syncronized" and "syncronized_shared"
Comments/suggestions/etc. welcome...
----
Bye,
Roland
--
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#!/usr/bin/ksh93
#
# CDDL HEADER START
#
# The contents of this file are subject to the terms of the
# Common Development and Distribution License (the "License").
# You may not use this file except in compliance with the License.
#
# You can obtain a copy of the license at usr/src/OPENSOLARIS.LICENSE
# or http://www.opensolaris.org/os/licensing.
# See the License for the specific language governing permissions
# and limitations under the License.
#
# When distributing Covered Code, include this CDDL HEADER in each
# file and include the License file at usr/src/OPENSOLARIS.LICENSE.
# If applicable, add the following below this CDDL HEADER, with the
# fields enclosed by brackets "[]" replaced with your own identifying
# information: Portions Copyright [yyyy] [name of copyright owner]
#
# CDDL HEADER END
#
#
# Copyright 2008 Sun Microsystems, Inc. All rights reserved.
# Use is subject to license terms.
#
# ident "%Z%%M% %I% %E% SMI"
#
#
# filemutexdemo1 - a simple locking demo which supports read/write
# locks and critical sections (like JAVA's "syncronized" keyword)
#
# Solaris needs /usr/xpg4/bin/ because the tools in /usr/bin are not
POSIX-conformant
export PATH=/usr/xpg4/bin:/bin:/usr/bin
# Make sure all math stuff runs in the "C" locale to avoid problems
# with alternative # radix point representations (e.g. ',' instead of
# '.' in de_DE.*-locales). This needs to be set _before_ any
# floating-point constants are defined in this script).
if [[ "${LC_ALL}" != "" ]] ; then
export \
LC_MONETARY="${LC_ALL}" \
LC_MESSAGES="${LC_ALL}" \
LC_COLLATE="${LC_ALL}" \
LC_CTYPE="${LC_ALL}"
unset LC_ALL
fi
export LC_NUMERIC=C
# Definition for a mutex which uses the filesystem for locking
typeset -T filemutex_t=(
typeset name
typeset lock_dirname
typeset locked_exclusive="false"
typeset locked_shared="false"
# keep track of subshell level. The problem is that we do not know a
# way to figure out whether someone calls "unlock" in a subshell and
then
# leaves the subshell and calls "unlock" again
integer subshell=-1
typeset lock_dirname
# create a filemutex instance (including lock directory)
function create
{
# make sure we return an error if the init didn' twork
set -o errexit
[[ "$1" == "" ]] && return 1
_.name="$1"
_.lock_dirname="/tmp/filemutex_t_${_.name}.lock"
mkdir "${_.lock_dirname}"
# last entry, used to mark the mutex as initalised+valid
(( _.subshell=.sh.subshell ))
return 0
}
# use a filemutex instance (same as "create" but without creating
# the lock directory)
function create_child
{
# make sure we return an error if the init didn' twork
set -o errexit
[[ "$1" == "" ]] && return 1
_.name="$1"
_.lock_dirname="/tmp/filemutex_t_${_.name}.lock"
# last entry, used to mark the mutex as initalised+valid
(( _.subshell=.sh.subshell ))
return 0
}
function check_subshell
{
if (( _.subshell != .sh.subshell )) ; then
print -u2 -f "filemutex_t.%s(%s): Wrong subshell
level\n" "$1" "${_.name}"
return 1
fi
return 0
}
function try_lock_shared
{
_.check_subshell "try_lock_shared" || return 1
touch "${_.lock_dirname}/shared_${PPID}_$$" 2>/dev/null ||
return 1
_.locked_shared="true"
return 0
}
function lock_shared
{
float interval=1.
_.check_subshell "lock_shared" || return 1
while ! _.try_lock_shared ; do sleep .5 ; ((
interval+=interval/5. )) ; done
return 0
}
function try_lock_exclusive
{
_.check_subshell "lock_exclusive" || return 1
rmdir "${_.lock_dirname}" 2>/dev/null || return 1
_.locked_exclusive="true"
return 0
}
function lock_exclusive
{
float interval=1.
_.check_subshell "lock_exclusive" || return 1
while ! _.try_lock_exclusive ; do sleep .5 ; ((
interval+=interval/5. )) ; done
return 0
}
# critical section support (like java's "synchronized" keyword)
function synchronized
{
integer retcode
_.lock_exclusive
"$@"
(( retcode=$? ))
_.unlock
return ${retcode}
}
# critical section support with shared lock
function synchronized_shared
{
integer retcode
_.lock_shared
"$@"
(( retcode=$? ))
_.unlock
return ${retcode}
}
function unlock
{
_.check_subshell "unlock" || return 1
if ${_.locked_shared} ; then
rm "${_.lock_dirname}/shared_${PPID}_$$"
_.locked_shared="false"
return 0
elif ${_.locked_exclusive} ; then
mkdir "${_.lock_dirname}"
_.locked_exclusive="false"
return 0
fi
print -u2 -f "filemutex_t.unlock(%s): mutex '%s' not locked."
"$1" "${_.name}"
return 1
}
# destroy mutex if noone is using it anymore (not the same as "unset"
!!))
function destroy
{
_.check_subshell "unlock" || return 1
(${_.locked_exclusive} || ${_.locked_shared}) && _.unlock
rmdir "${_.lock_dirname}"
return 0
}
)
# main
builtin mkdir
builtin rmdir
builtin rm
print "## Start."
filemutex_t fs
fs.create "hello_world" || print -u2 "Mutex init failed."
print "# Starting child which keeps an exclusive lock for 15 seconds"
(
filemutex_t child_fs
child_fs.create_child "hello_world"
child_fs.lock_exclusive
sleep 15
child_fs.unlock
) &
sleep 1
printf "%T: # Waiting to obtain a shared lock...\n"
fs.lock_shared
printf "%T: # Obtained shared lock\n"
printf "fs.locked_exclusive=%s, fs.locked_shared=%s\n" "${fs.locked_exclusive}"
"${fs.locked_shared}"
ls -l /tmp/filemutex*/*
print "# Unlocking..."
fs.unlock
print -r "# Executing printf '|%s|\n' 'hello' 'world' while holding an
exclusive lock"
fs.synchronized printf '|%s|\n' 'hello' 'world'
fs.destroy
print "## Done."
exit 0
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