common/scripts/filemutexdemo1.sh

#!/usr/bin/ksh93

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#
# Copyright (c) 2008, 2010, Oracle and/or its affiliates. All rights reserved.
#

#
# filemutexdemo1 - a simple locking demo which supports read/write
# locks and critical sections (like JAVA's "syncronized" keyword)
#

# Solaris needs /usr/xpg6/bin:/usr/xpg4/bin because the tools in /usr/bin are not POSIX-conformant
export PATH=/usr/xpg6/bin:/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't work
        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't work
        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
    {
        (( _.subshell == .sh.subshell )) && return 0
        print -u2 -f "filemutex_t.%s(%s): Wrong subshell level\n" "$1" "${_.name}"
        return 1
    }

    function try_lock_shared
    {
        _.check_subshell "try_lock_shared" || return 1

        mkdir "${_.lock_dirname}/shared_${PPID}_$$" 2>/dev/null || return 1
        _.locked_shared="true"
        return 0
    }

    function lock_shared
    {
        float interval=0.2

        _.check_subshell "lock_shared" || return 1

        while ! _.try_lock_shared ; do sleep ${interval} ; (( interval+=interval/10. )) ; done
        return 0
    }

    function try_lock_exclusive
    {
        _.check_subshell "try_lock_exclusive" || return 1

        rmdir "${_.lock_dirname}" 2>/dev/null || return 1
        _.locked_exclusive="true"
        return 0
    }

    function lock_exclusive
    {
        float interval=0.2

        _.check_subshell "lock_exclusive" || return 1

        while ! _.try_lock_exclusive ; do sleep ${interval} ; (( interval+=interval/10. )) ; done
        return 0
    }

    # critical section support (like java's "synchronized" keyword)
    function synchronized
    {
        integer retcode

        _.check_subshell "synchronized" || return 1

        _.lock_exclusive

        "$@"
        (( retcode=$? ))

        _.unlock

        return ${retcode}
    }

    # critical section support with shared lock
    function synchronized_shared
    {
        integer retcode

        _.check_subshell "synchronized_shared" || return 1

        _.lock_shared

        "$@"
        (( retcode=$? ))

        _.unlock

        return ${retcode}
    }

    function unlock
    {
        # return an error if rmdir/mkdir/check_subshell fail...
        set -o errexit

        _.check_subshell "unlock"

        if ${_.locked_shared} ; then
            rmdir "${_.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 "destroy" || return 1

        (${_.locked_exclusive} || ${_.locked_shared}) && _.unlock
        rmdir "${_.lock_dirname}"
        return 0
    }
)

# main
builtin mkdir
builtin rmdir

print "## Start."

typeset -r mymutexname="hello_world"

filemutex_t fs

fs.create "${mymutexname}" || print -u2 "Mutex init failed."

print "# Starting child which keeps an exclusive lock for 10 seconds..."
(
    filemutex_t child_fs

    child_fs.create_child "${mymutexname}"

    child_fs.lock_exclusive
    sleep 10
    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 -lad /tmp/filemutex*/*

printf "%T: # Executing child which runs printf '|%%s|\\\n' 'hello' 'world' inside a synchronized section\n"
(
    filemutex_t child_fs

    child_fs.create_child "${mymutexname}"

    child_fs.synchronized printf '|%s|\n' 'hello' 'world'
) &

printf "%T: # Sleeping 5 secs while holding the shared lock...\n"
sleep 5.

printf "%T: # Releasing shared lock...\n"
fs.unlock

sleep 5.
print "# Destroying lock..."
fs.destroy

print "## Done."

exit 0