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 -- __ . . __ (o.\ \/ /.o) roland.mainz at nrubsig.org \__\/\/__/ MPEG specialist, C&&JAVA&&Sun&&Unix programmer /O /==\ O\ TEL +49 641 7950090 (;O/ \/ \O;) -------------- next part -------------- #!/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
