meta-digi-dey: suspend: add critical section

Add critical section in suspend script to avoid that more than
one instance can be executed concurrently. This happens, for
instance, when the power button key is pressed while the system
is resumming, causing issues on the wifi interface.

https://jira.digi.com/browse/DEL-3431

Signed-off-by: Pedro Perez de Heredia <pedro.perez@digi.com>

meta-digi-dey/recipes-core/busybox/busybox-1.23.2/ccimx6ul/suspend

@@ -3,7 +3,7 @@
 #
 #  suspend
 #
-#  Copyright (C) 2016 by Digi International Inc.
+#  Copyright (C) 2016, 2017 by Digi International Inc.
 #  All rights reserved.
 #
 #  This program is free software; you can redistribute it and/or modify it
@@ -17,6 +17,8 @@
 
 scriptname="$(basename ${0})"
 syspower="/sys/power/state"
+lockfile="/var/lock/${scriptname}.lock"
+lockfd="9"
 
 usage() {
 	printf "\nSuspend system to RAM memory\n"
@@ -40,6 +42,21 @@ resume_interfaces() {
 	fi
 }
 
+enter_critical_section() {
+	# Create lock file
+	eval "exec ${lockfd}>${lockfile}"
+
+	# Acquire the lock in non blocking mode. Otherwise, additional calls
+	# to the script will be queued and the system will endlessly go in
+	# and out of suspend to ram
+	flock -n "${lockfd}" || exit 0
+}
+
+exit_critical_section() {
+	# Release the lock
+	flock -u "${lockfd}"
+}
+
 while getopts "h" c; do
 	case "${c}" in
 		h) usage; exit;;
@@ -47,6 +64,9 @@ while getopts "h" c; do
 done
 
 if [ -f "${syspower}" ]; then
+	# Avoid running multiple instances of this script in parallel
+	enter_critical_section
+
 	# Pre-suspend actions
 	suspend_interfaces
 
@@ -56,6 +76,8 @@ if [ -f "${syspower}" ]; then
 
 	# Post-resume actions
 	resume_interfaces
+
+	exit_critical_section
 else
 	printf "\n[ERROR] File ${syspower} not found\n\n"
 fi