fix and enable systemd watchdog

There were following problems:
  - configure.ac wrongly checked for defined HAVE_SYSTEMD; this
    wasn't working, so the watchdog code was not compiled in.
    Replace library search with explicit version check
  - sd_notify() watchdog call was unsetting NOTIFY_SOCKET from env;
    this means only first "watchdog ping" was delivered, each
    subsequent one did not have socket to be sent to and systemd
    was killing service
  - after those fixes, enable Watchdog in systemd unit with one
    minute intervals

configure.ac

@@ -136,6 +136,9 @@ fi
 AC_SUBST(TOR_SYSTEMD_CFLAGS)
 AC_SUBST(TOR_SYSTEMD_LIBS)
 
+PKG_CHECK_MODULES(SYSTEMD209, [systemd >= 209],
+    [AC_DEFINE(HAVE_SYSTEMD_209,1,[Have systemd v209 or more])], [])
+
 if test x$enable_systemd = xyes -a x$have_systemd != xyes ; then
     AC_MSG_ERROR([Explicitly requested systemd support, but systemd not found])
 fi
@@ -155,10 +158,6 @@ cpu workers lock up here, so I will disable threads.])
    esac
 fi
 
-ifdef([HAVE_SYSTEMD], [
-AC_SEARCH_LIBS([sd_watchdog_enabled], [systemd-daemon],
-    [AC_DEFINE(HAVE_SYSTEMD_209,1,[Have systemd v209 or more])], [])
-])
 
 case $host in
    *-*-solaris* )

contrib/dist/tor.service.in

@@ -11,6 +11,7 @@ ExecReload = /bin/kill -HUP ${MAINPID}
 KillSignal = SIGINT
 TimeoutSec = 30
 Restart = on-failure
+WatchdogSec = 1m
 LimitNOFILE = 32768
 
 # Hardening

src/or/main.c

@@ -1776,7 +1776,7 @@ static periodic_timer_t *systemd_watchdog_timer = NULL;
 static void
 systemd_watchdog_callback(periodic_timer_t *timer, void *arg)
 {
-  sd_notify(1, "WATCHDOG=1");
+  sd_notify(0, "WATCHDOG=1");
 }
 #endif