app: script: Don't try to restart PPP if UART disappear

If the USB serial device is removed while PPP is running
the script might try to restart the connection or try
to run shutdown script and end up writing to a device
files that don't exist.

Add check, if devices are still present.

Signed-off-by: Seppo Takalo <seppo.takalo@nordicsemi.no>

Author: SeppoTakalo

app/scripts/sm_start_ppp.sh

@@ -254,8 +254,20 @@ echo "Connect and wait for PPP link..."
 
 chat $CHATOPT -t$TIMEOUT "${CHAT_SCRIPT[@]}" >$AT_CMUX <$AT_CMUX
 
+check_devices_or_exit() {
+	# Verify that UART devices are still present
+	if [ ! -c $PPP_CMUX ] || [ ! -c $AT_CMUX ] || [ ! -c $MODEM ]; then
+		echo "Error: UART devices not found, exiting..."
+		start-stop-daemon --stop --pidfile $TRACE_PID_FILE --remove-pidfile \
+				  --oknodo --retry 1
+		pkill ldattach
+		exit 1
+	fi
+}
+
 shutdown_modem() {
 	set +eu
+	check_devices_or_exit
 	start-stop-daemon --stop --pidfile $TRACE_PID_FILE --remove-pidfile --oknodo --retry 1
 	chat $CHATOPT -t5 '' $SHUTDOWN_SCRIPT >$AT_CMUX <$AT_CMUX
 	CHAT_ERR=$?
@@ -275,6 +287,7 @@ shutdown_modem() {
 ppp_start() {
 	set +eu
 	set -x
+	check_devices_or_exit
 	pppd $PPP_CMUX $PPP_OPTIONS
 	if [ "$?" -eq 5 ]; then
 		echo "pppd terminated with signal, shutting down modem..."
@@ -300,6 +313,7 @@ export TRACE_PID_FILE
 export -f ppp_start
 export -f shutdown_modem
 export -f cmux_close
+export -f check_devices_or_exit
 
 # Start PPPD in a subshell
 # Logs go to syslog so redirect output to /dev/null