Locking implementation

rc-locking.sh (text/plain), 11.13 KB, created by Oldrich Jedlicka on 2006-01-09 08:20:41 UTC

# Copyright 2006 Oldrich Jedlicka
# Distributed under the terms of the GNU General Public License v2

# RC functions for using named locks and "thread" variables - variables
# belonging to a subshell. The basic checks for consistency are made as well
# as the cleanup on EXIT from the subshell - this is important if somebody
# terminated the script with SIGINT. Any number of locking within the same
# subshell is allowed, all locks are cleared automatically when the subshell
# terminates.
#
# The internal functionality depends on calls to svc_replace_signals - this is
# called every time there is a lock/unlock or a thread variable usage.
#
# Errors in locking and unlocking are reported and exit status 1 is returned.
# Exit status 0 is for all successful calls.
#
# NOTE ######################################################################## 
#
#   Keep in mind that changes in a subshell are not visible to a parent shell!
#
# END NOTE #################################################################### 
#
# Good example 1: Simple lock
#
#    lock "name" || die "Cannot lock"
#    ... do something exclusively
#    unlock "name" || die "Cannot unlock"
#    ... here is no lock active
#
# Good example 2: Double lock in the same shell
#
#    lock "name"
#    lock "name"
#    ... do something exclusively
#    unlock "name"
#    ... still do something exclusively
#    unlock "name"
#    ... here is no lock active
#
# Good example 3: Double locking from the same process, but different subshell
#    lock "name"
#    ... do something exclusively
#    (
#        lock "name"
#        ... do something and forget to unlock
#    )
#    ... here is the lock from subshell already released
#    ... main lock is still active
#    unlock "name"
#    ... here is no lock active
#
# Good example 4:
#    use_thread_variable VARIABLE
#    VARIABLE=10
#    echo $VARIABLE             # 10
#    (
#        use_thread_variable VARIABLE
#        echo $VARIABLE         # Empty string
#        VARIABLE=5
#        echo $VARIABLE         # 5
#    )
#    echo $VARIABLE             # 10
#
# Wrong example 5: Normal subshell variable behaviour
#    use_thread_variable VARIABLE
#    VARIABLE=10
#    echo $VARIABLE             # 10
#    (
#        echo $VARIABLE         # 10
#        VARIABLE=5
#        echo $VARIABLE         # 5
#    )
#    echo $VARIABLE             # 10
#
# Wrong but somehow working example 6: Locking and thread variables
#    use_thread_variable VARIABLE
#    VARIABLE=10
#    echo $VARIABLE             # 10
#    (
#        echo $VARIABLE         # 10
#        lock "name"            # Does the job for cleaning the variables
#        echo $VARIABLE         # Empty string
#        VARIABLE=5
#        echo $VARIABLE         # 5
#    )
#    echo $VARIABLE             # 10
#

RC_GOT_LOCKING="yes"

[[ ${RC_GOT_FUNCTIONS} != "yes" ]] && source /sbin/functions.sh

svclock=${svclock:-/var/lock/rcscripts}

# bool is_in_list(char *item, char *list)
#
#    Returns 0 if the item is in the list, otherwise 1.
#
#    To work properly with arrays it has to be called as
#
#        is_in_list "value" "$list1[@]" "$list2[@]" ...
#
#    For normal operation it can be called as
#
#        is_in_list "value" "item1" "item2" "item3" ...
#
is_in_list() {
	local x retval=1 item="$1"
	shift
	for x in "$@"
	do
		if [[ "$x" == "$item" ]]
		then
			retval=0
			break
		fi
	done
	return ${retval}
}

# void use_thread_variable(char *name)
#
#    Adds a variable to the list of thread variables. Those variables are
#    unset in a subshell. This should be called before anything is assigned
#    to the variable.
#
use_thread_variable() {
	svc_replace_signals
	if ! is_in_list "$1" $SVC_THREADVAR
	then
		SVC_THREADVAR="$1 $SVC_THREADVAR"
	fi
}

# void add_cleanup_function(char *function)
#
#   Adds the function to the list of cleanup actions executed, when the
#   current subshell exits.
#
add_cleanup_function() {
	svc_replace_signals
	if [[ $(type -t "$1") == "function" ]]
	then
		if ! is_in_list "$1" "${SVC_CLEANUP[@]}"
		then
			SVC_CLEANUP[${#SVC_CLEANUP}]="$1"
		fi
	else
		echo "$1 is not a function!" > /dev/stderr
	fi
}

# bool trylock(char *name)
#
#   Tries to acquire a named lock. Returns 0, if the locking was successful,
#   otherwise 1.
#
trylock() {
	local path="${svclock}/$1"
	local pid retval newcount
	
	if [[ ! -w ${svclock} ]]
	then
		# TODO: No locking possible. Error or silently ignore?
		return 0
	fi

	add_cleanup_function svc_restore_locks

	pid=$(cat "$path" 2>/dev/null)
	pid=( $pid )

	# Save lock count

	if [[ "$$" == "${pid[0]}" ]]
	then
		# We are already the owner
		svc_save_lock_count "$1" "${pid[1]}"
		newcount=$((${pid[1]}+1))
		echo "$$ ${newcount}" > "$path"
		retval=$?
	else
		# We want to get an ownership
		svc_save_lock_count "$1" 0
		(set -C; echo "$$ 1" > "$path") 2> /dev/null
		retval=$?
	fi
	return ${retval}
}

# void lock(char *name)
#
#   Acquire the named lock.
#
lock() {
	if [[ ! -w ${svclock} ]]
	then
		return 0
	fi

	while [ 1 ]; do
		if trylock "$1"; then
			return 0
		else
			sleep 0.1
		fi
	done
}

# void unlock(char *name, bool force)
#
#    Release the named lock. Returns 1 if the process was not the owner of the
#    lock or there was an error during unlocking.
#
unlock() {
	local path="${svclock}/$1"
	local pid newcount mincount locked error
	
	if [[ ! -w ${svclock} ]]
	then
		# TODO: No locking possible. Error or silently ignore?
		return 0
	fi

	# If we cannot get the contents of the file, it doesn't exist - we are
	# not the owner of the file
	pid=$(cat "$path" 2>/dev/null)
	pid=( $pid )

	svc_was_locked "$1" && locked=1 || locked=0
	
	if [[ "$$" == "${pid[0]}" && $locked -eq 1 ]]
	then
		newcount=$((${pid[1]}-1))
		mincount=$( svc_get_lock_count "$1" )
		if [[ $newcount -lt $mincount ]]
		then
			error="Unlocking more times than locking!"
		else
			if [[ $newcount == 0 ]];
			then
				rm -f "$path"
			else
				echo "$$ ${newcount}" > "$path"
			fi
			retval=$?
		fi
	elif [[ $locked -eq 1 ]]
	then
		if [[ -z ${pid[0]} ]]
		then
			error="Unlocking already unlocked lock!"
		else
			error="Unlocking not owned lock!"
		fi
	else
		# The lock was not locked by us
		if [[ -z "${pid[0]}" ]]
		then
			error="Unlocking not locked lock!"
		else
			error="Unlocking not owned lock!"
		fi
	fi

	if [[ -n $error ]]
	then
		echo " ${BAD}***${NORMAL} $error"
		echo " ${BAD}**${GOOD}*${NORMAL} ${HILITE}HINT:${NORMAL} Maybe you have forgotten to make a lock."
		echo " ${BAD}*${GOOD}**${NORMAL} ${HILITE}HINT:${NORMAL} Maybe you did not lock in the same subshell."
		echo " ${GOOD}***${NORMAL} ${HILITE}HINT:${NORMAL} If you lock in a subshell, locks recover when you leave it."
		retval=1
	fi
	
	return ${retval}
}

##############################################################################
#                                                                            #
# This should be the last public code in here, please add all public above!! #
#                                                                            #
# *** START PRIVATE CODE ***                                                 #
#                                                                            #
##############################################################################

# void svc_do_cleanup(void)
#
#    This is normal EXIT signal handler. It does all actions defined with 
#    add_cleanup_function.
#
svc_do_cleanup() {
	for name in $SVC_CLEANUP
	do
		eval $name
	done
}

# void svc_replace_signals(void)
#
#    Replace the EXIT signal handler with our implementation. Delete all
#    thread variables if the new signal handler is installed.
#
svc_replace_signals() {
	local traps tmpfile var
	tmpfile=`mktemp /tmp/traps.XXXXXXXX`
	if [[ -z $tmpfile ]]
	then
		# Fatal error! Report and exit
		( RC_QUIET_STDOUT="no"
		eerror "Cannot create temporary file in /tmp!"
		)
		exit 1
	fi

	# The only way how to get traps from the current shell is to redirect
	# output to a temportary file and read it back.
	trap -p EXIT > $tmpfile
	traps=$( < $tmpfile )
	rm -f $tmpfile
	
	case $traps in
	*'svc_do_cleanup'* )
		# Alredy installed, do nothing
		:
		;;
	*)
		# New installation, clean all known thread variables
		SVC_CLEANUP=""
		for var in $SVC_THREADVAR
		do
			unset $var
		done
		trap svc_do_cleanup EXIT
		;;
	esac
}

# void svc_addlock(char *name)
#
#   Adds named lock to lock list.
#
svc_addlock() {
	if ! is_in_list "$1" "${SVC_LOCK[@]}"
	then
		SVC_LOCK[${#SVC_LOCK}]="$1"
	fi
}

# void svc_save_lock_count(char *name, int count)
#
#    Save the lock count in the corresponding SVC_LOCKCOUNT_<name> thread 
#    variable.
#
svc_save_lock_count() {
	local lockcountvar lockcount
	
	lockcountvar="SVC_LOCKCOUNT_"$( bash_variable "$1" )
	use_thread_variable $lockcountvar
	lockcount=${!lockcountvar}

	if [[ -z $lockcount ]]
	then
		if [[ -z $2 ]]
		then
			lockcount=0
		else
			lockcount=$2
		fi
		eval "$lockcountvar=$lockcount"
		svc_addlock "$1"
	fi
}

# int svc_get_lock_count(char *name)
#
#    Get the saved lock count from the thread variable SVC_LOCKCOUNT_<name>.
#    Returns the number 0, if the lock was not used before in the current
#    subshell.
#
svc_get_lock_count() {
	local lockcountvar
	lockcountvar="SVC_LOCKCOUNT_"$( bash_variable "$1" )
	use_thread_variable $lockcountvar
	echo ${!lockcountvar:-0}
}

# bool svc_was_locked(char *name)
#
#    Returns 0 if the lock was used in the current subshell before, otherwise 1.
#
svc_was_locked() {
	local lockcountvar
	lockcountvar="SVC_LOCKCOUNT_"$( bash_variable "$1" )
	use_thread_variable $lockcountvar
	[[ -n ${!lockcountvar} ]]
}

# void svc_restorelocks()
#
#    Restore all locks from the saved values. It has to be called only from the
#    installed signal handler svc_do_cleanup, otherwise it will not work!
#
svc_restore_locks() {
	local pid name path lockcountvar lockcount
	for name in $SVC_LOCK
	do
		path="${svclock}/$name"
		pid=$(cat "$path" 2>/dev/null)
		pid=( $pid )
		if [[ "$$" == "${pid[0]}" ]]
		then
			# Get the lock cound. 
			lockcount=$( svc_get_lock_count "$name" )
			if [[ $lockcount -eq 0 ]]
			then
				rm -f "$path" 2> /dev/null
			else
				echo "$$ ${lockcount}" > "$path"
			fi
		fi
	done
	exit 1
}


##############################################################################
#                                                                            #
# This should be the last code in here, please add all functions above!!     #
#                                                                            #
# *** START LAST CODE ***                                                    #
#                                                                            #
##############################################################################

if [[ ! -d "${svclock}" ]] ; then
	mkdir -p -m 0755 "${svclock}"
fi

##############################################################################
#                                                                            #
# *** END LAST CODE ***                                                      #
#                                                                            #
# This should be the last code in here, please add all functions above!!     #
#                                                                            #
##############################################################################


# vim:ts=4