semaphore implemented as shell script (version without |fork()|/|exec()| per test cycle)
Posted by Anonymous on Mon 18th Nov 2019 17:45
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modification of post by Anonymous (view diff)

#!/usr/bin/ksh93

# should work for ksh88/ksh93/bash4

########################################################################
#                                                                      #
#               This software is part of the ast package               #
#                    Copyright (c) 2019 Roland Mainz                   #
#                      and is licensed under the                       #
#                 Eclipse Public License, Version 1.0                  #
#                    by AT&T Intellectual Property                     #
#                                                                      #
#                A copy of the License is available at                 #
#          http://www.eclipse.org/org/documents/epl-v10.html           #
#         (with md5 checksum b35adb5213ca9657e911e9befb180842)         #
#                                                                      #
#                                                                      #
#                 Roland Mainz <roland.mainz@nrubsig.org>              #
#                                                                      #
########################################################################

#
# Copyright (c) 2019, Roland Mainz. All rights reserved.
#

#
# shsemaphore.sh - wait until <n> customers are waiting
# at the semaphore
#
# Usage:
# 1. init semaphore ("mysemaname" is the 'semaphore address')
# $ shsemaphore.sh init mysemaname
#
# 2. let two semaphore customers (names are 'Bear' and 'Fox')
# wait for each other:
# $ shsemaphore.sh wait mysemaname Bear 2 &
# $ shsemaphore.sh wait mysemaname Fox 2
#
# 3. Cleanup all the mess if you are really done:
# $ shsemaphore.sh cleanup mysemaname
#

# POSIX path for mkdir, ls, rm, rmdir ...
export PATH='/bin:/usr/bin'

# constants
typeset -r SEMA_BASE_DIR='/tmp/'

# arguments
typeset sema_cmd="$1"
typeset sema_address="${SEMA_BASE_DIR}/shsemaphore_$2"
typeset sema_name="${sema_address}/$3"
typeset -i sema_count="$4"

# Debug:
#printf "cmd=%s, address=%s, name=%s, count=%d\n" \
#       "${sema_cmd}" "${sema_address}" "${sema_name}" "${sema_count}"

function sema_cleanup
{
        rm -Rf "${sema_address}"
}

function sema_init
{
        if [[ -z "$SEMA_BASE_DIR" || -z "$sema_address" ]] ; then
                printf "%s: Illegal arguments.\n" "$0" 1>&2
                return 1
        fi

        mkdir -p "${SEMA_BASE_DIR}"

        sema_cleanup
        mkdir "${sema_address}"
}

function sema_wait
{
        typeset IFS

        if [[ -z "$SEMA_BASE_DIR" || -z "$sema_address" || -z "$sema_name" || -z "$sema_count" ]] ; then
                printf "%s: Illegal arguments.\n" "$0" 1>&2
                return 1
        fi

        if ! mkdir "${sema_name}" ; then
                printf "%s: Count not set name.\n" "$0" 1>&2
                return 1
        fi

        # is semaphore still valid ?
        cd "${sema_address}" || return 4

        IFS=$'\n'
        while (( 1 )) ; do
                # 1. we rely on shell globing to avoid |fork()|/|exec()|
                # for ls(1)
                # 2. we count '.' and '..', too. Both special dirs
                # will disappear if the directory "${sema_address}"
                # gets removed
                sfiles=( $(printf "%s\n" */ .*/ 2>'/dev/null') )

                # Debug:
                #printf 'sfiles=|%s| ' "${sfiles[@]}" ; printf '\n'

                # semaphore condition fullfilled ?
                (( ${#sfiles[@]} >= (sema_count+2) )) && return 0

                # semaphore gone ?
                # In that case we get only two entries from globbing
                # above, one for '*/' and one for '.*/'
                (( ${#sfiles[@]} <= 2 )) && return 4

                sleep 2
        done
        # notreached
}

# main
case "${sema_cmd}" in
        'init')
                sema_init
                exit $?
                ;;
        'wait')
                sema_wait
                exit $?
                ;;
        'cleanup')
                sema_cleanup
                exit $?
                ;;
        *)
                printf "%s: Unknown command.\n" "$0" 1>&1
                exit 1
                ;;
esac