#!/bin/sh
# shellcheck disable=SC1091,SC1090
# vim: set noexpandtab tabstop=4 shiftwidth=4 softtabstop=4 :

[ -n "$1" ] || exit

. /lib/functions.sh

# retrieve args
OMR_TRACKER_INTERFACE="$1"
shift

# export vars
export OMR_TRACKER_INTERFACE
export OMR_TRACKER_HOST
export OMR_TRACKER_TIMEOUT
export OMR_TRACKER_STATUS
export OMR_TRACKER_DEVICE
export OMR_TRACKER_DEVICE_IP
export OMR_TRACKER_DEVICE_GATEWAY

dscp=56   # set DSCP CS7 (56) in outgoing packets
initial_hosts="$OMR_TRACKER_HOSTS"
initial_timeout="$OMR_TRACKER_TIMEOUT"

# set constants for rto updating
# we've changed the default values of the RFC to K=3 and beta=0.25 instead of K=4 and beta=0.125
# this allows us to lower the gap between rtt and rto when the latency increases quickly as our
# measurements are less frequent than the standard timeout computation of the TCP stack
_init_rto() {
	srtt=
	rttvar=
	rto_init=$(( initial_timeout * 1000 ))
	rto=$rto_init
}

# update rto as explained in rfc6298
# rto    : retransmission timeout.
# rtt    : round trip time
# rttvar : round trip time variance
# srtt   : smoothed round trip time
# alpha  : smoothing coeff for srtt
# beta   : smoothing coeff for rttvar
# K      : coeff to increase rttvar impact on rto computation
_update_rto() {
	if [ -z "$srtt" ]; then
		srtt=$1
		rttvar=$(($1 / 2))
	else
		diff=$((srtt - $1))
		rttvar=$(((75 * rttvar + 25 * (diff >= 0 ? diff : -diff)) / 100))
		srtt=$(((75 * srtt + 25 * $1) / 100))
	fi
	rto=$((tmp = srtt + 3 * rttvar, tmp > rto_init ? tmp : rto_init))
}

_restart() {
	OMR_TRACKER_HOST="${OMR_TRACKER_HOSTS%% *}"
	[ "$OMR_TRACKER_HOST" = "$OMR_TRACKER_HOSTS" ] || {
		OMR_TRACKER_HOSTS="${OMR_TRACKER_HOSTS#* } $OMR_TRACKER_HOST"
		_init_rto
	}
}

_post_tracking() {
	for tracker_bin in /usr/share/omr/post-tracking.d/*; do
		[ -x "$tracker_bin" ] && (
			_log() {
				logger -t "post-tracking-${tracker_bin##*/}" "$*"
			}
			. "$tracker_bin"
		)
	done
}

_restart

# main loop
while true; do
	# setup tracker variables
	OMR_TRACKER_DEVICE_IP=
	OMR_TRACKER_STATUS="ERROR"
	OMR_TRACKER_LATENCY=
	OMR_TRACKER_TIMEOUT=$((rto / 1000 + (rto % 1000 ? 1 : 0)))

	if [ -d "/sys/class/net/$OMR_TRACKER_DEVICE" ]; then
		# retrieve iface ip and gateway
		OMR_TRACKER_DEVICE_IP=$(ip -4 -br addr ls dev "$OMR_TRACKER_DEVICE" | awk -F'[ /]+' '{print $3}')

		# execute specific tracker
		if [ -n "$OMR_TRACKER_DEVICE_IP" ] && [ -n "$OMR_TRACKER_DEVICE_GATEWAY" ]; then
			retgw=$(ping "$OMR_TRACKER_DEVICE_GATEWAY" \
				-I "$OMR_TRACKER_DEVICE_IP" \
				-w 1 \
				-c 1 \
				-q
			) && echo "$retgw" | grep -sq "0% packet loss" && {
				# setup loop variable
				tries="$OMR_TRACKER_TRIES"
				ip route replace "$OMR_TRACKER_HOST" via "$OMR_TRACKER_DEVICE_GATEWAY" dev "$OMR_TRACKER_DEVICE" src "$OMR_TRACKER_DEVICE_IP"
				# loop until tries attempts have been reached
				while [ "$tries" -gt 0 ]; do
					ret=$(ping "$OMR_TRACKER_HOST" \
						-I "$OMR_TRACKER_DEVICE_IP" \
						-w "$OMR_TRACKER_TIMEOUT" \
						-c 1 \
						-q
					) && echo "$ret" | grep -sq "0% packet loss" && {
						OMR_TRACKER_LATENCY=$(echo "$ret" | cut -d "/" -s -f4 | cut -d "." -f1)
						_update_rto "$OMR_TRACKER_LATENCY"
						OMR_TRACKER_STATUS="OK"
						break
					}
					tries=$((tries - 1))
					OMR_TRACKER_TIMEOUT=$((OMR_TRACKER_TIMEOUT * 2))
				done
				ip route del "$OMR_TRACKER_HOST" via "$OMR_TRACKER_DEVICE_GATEWAY" dev "$OMR_TRACKER_DEVICE" src "$OMR_TRACKER_DEVICE_IP"
			}
		fi
	fi

	[ "$OMR_TRACKER_HOSTS" = "$initial_hosts" ] || [ "$OMR_TRACKER_STATUS" = "OK" ] && _post_tracking
	[ "$OMR_TRACKER_STATUS" = "ERROR" ] && _restart

	sleep "$OMR_TRACKER_INTERVAL"
done