events: Clarified return values, proper checks for event plan.

src/knot/zone/events.c

@@ -367,7 +367,7 @@ static int event_xfer(zone_t *zone)
 		/* Sync zonefile immediately if configured. */
 		if (zone->conf->dbsync_timeout == 0) {
 			zone_events_schedule(zone, ZONE_EVENT_FLUSH, ZONE_EVENT_NOW);
-		} else if (zone_events_get_time(zone, ZONE_EVENT_FLUSH) == ZONE_EVENT_NOW) {
+		} else if (zone_events_get_time(zone, ZONE_EVENT_FLUSH) <= ZONE_EVENT_NOW) {
 			/* Plan sync if not previously planned. */
 			zone_events_schedule(zone, ZONE_EVENT_FLUSH, zone->conf->dbsync_timeout);
 		}
@@ -611,7 +611,7 @@ static void replan_flush(zone_t *zone, const zone_t *old_zone)
 	}
 
 	const time_t flush_time = zone_events_get_time(old_zone, ZONE_EVENT_FLUSH);
-	if (flush_time == ZONE_EVENT_NOW) {
+	if (flush_time <= ZONE_EVENT_NOW) {
 		// Not scheduled previously.
 		zone_events_schedule(zone, ZONE_EVENT_FLUSH, zone->conf->dbsync_timeout);
 		return;

src/knot/zone/events.h

@@ -145,7 +145,8 @@ void zone_events_start(struct zone_t *zone);
  * \param type  Event type.
  *
  * \retval time of the event when event found
- * \retval negative value if event was not found
+ * \retval 0 when the event is not planned
+ * \retval negative value if event is invalid
  */
 time_t zone_events_get_time(const struct zone_t *zone, zone_event_type_t type);