#!/bin/bash
#
# Script to update the resolver list for dnsmasq
#
# N.B. Resolvconf may run us even if dnsmasq is not running.
# If dnsmasq is installed then we go ahead and update
# the resolver list in case dnsmasq is started later.
#
# Assumption: On entry, PWD contains the resolv.conf-type files
#
# Requires bash because it uses a non-POSIX printf extension.
#
# Licensed under the GNU GPL.  See /usr/share/common-licenses/GPL.
#

set -e

RUN_DIR="/var/run/dnsmasq"
RSLVRLIST_FILE="${RUN_DIR}/resolv.conf"
TMP_FILE="${RSLVRLIST_FILE}_new.$$"

[ -x /usr/sbin/dnsmasq ] || exit 0
[ -x /lib/resolvconf/list-records ] || exit 1

PATH=/bin:/sbin

report_err() { echo "$0: Error: $*" >&2 ; }

# Stores arguments (minus duplicates) in RSLT, separated by spaces
# Doesn't work properly if an argument itself contain whitespace
uniquify()
{
    RSLT=""
    while [ "$1" ] ; do
        for E in $RSLT ; do
            [ "$1" = "$E" ] && { shift ; continue 2 ; }
        done
        RSLT="${RSLT:+$RSLT }$1"
        shift
    done
}

if [ ! -d "$RUN_DIR" ] && ! mkdir --parents --mode=0755 "$RUN_DIR" ; then
    report_err "Failed trying to create directory $RUN_DIR"
    exit 1
fi

RSLVCNFFILES="$(/lib/resolvconf/list-records | sed -e '/^lo.dnsmasq$/d')"

NMSRVRS=""
if [ "$RSLVCNFFILES" ] ; then
    uniquify $(sed -n -e 's/^[[:space:]]*nameserver[[:space:]]\+//p' $RSLVCNFFILES)
    NMSRVRS="$RSLT"
fi

# Dnsmasq uses the mtime of $RSLVRLIST_FILE, with a resolution of one second,
# to detect changes in the file. This means that if a resolvconf update occurs
# within one second of the previous one then dnsmasq may fail to notice the
# more recent change.  To work around this problem we sleep here to ensure
# that the new mtime is different.
if [ -f "$RSLVRLIST_FILE" ] && [ "$(ls -go --time-style='+%s' "$RSLVRLIST_FILE" | { read p h s t n ; echo "$t" ; })" = "$(date +%s)" ] ; then
    sleep 1
fi

clean_up() { rm -f "$TMP_FILE" ; }
trap clean_up EXIT
: >| "$TMP_FILE"
for N in $NMSRVRS ; do echo "nameserver $N" >> "$TMP_FILE" ; done
mv -f "$TMP_FILE" "$RSLVRLIST_FILE"