Fix bug when disabling heartbeats.

Callbacks can't return 0.

changes/bug19454

@@ -0,0 +1,3 @@
+  o Minor bugfixes (heartbeat):
+    - Fix regression that crashes Tor when disabling heartbeats. Fixes bug
+      19454; bugfix on tor-0.2.8.1-alpha. Reported by "kubaku".

src/or/main.c

@@ -2004,18 +2004,24 @@ check_fw_helper_app_callback(time_t now, const or_options_t *options)
   return PORT_FORWARDING_CHECK_INTERVAL;
 }
 
+/** Callback to write heartbeat message in the logs. */
 static int
 heartbeat_callback(time_t now, const or_options_t *options)
 {
   static int first = 1;
-  /* 12. write the heartbeat message */
+
+  /* Check if heartbeat is disabled */
+  if (!options->HeartbeatPeriod) {
+    return PERIODIC_EVENT_NO_UPDATE;
+  }
+
+  /* Write the heartbeat message */
   if (first) {
     first = 0; /* Skip the first one. */
   } else {
     log_heartbeat(now);
   }
-  /* XXXX This isn't such a good way to handle possible changes in the
-   * callback event */
+
   return options->HeartbeatPeriod;
 }