Add functions to enable/disable periodic_event_t objects.

src/common/compat_libevent.c

@@ -253,10 +253,39 @@ periodic_timer_new(struct event_base *base,
   }
   timer->cb = cb;
   timer->data = data;
-  event_add(timer->ev, (struct timeval *)tv); /*drop const for old libevent*/
+  periodic_timer_launch(timer, tv);
   return timer;
 }
 
+/**
+ * Launch the timer <b>timer</b> to run at <b>tv</b> from now, and every
+ * <b>tv</b> thereafter.
+ *
+ * If the timer is already enabled, this function does nothing.
+ */
+void
+periodic_timer_launch(periodic_timer_t *timer, const struct timeval *tv)
+{
+  tor_assert(timer);
+  if (event_pending(timer->ev, EV_TIMEOUT, NULL))
+    return;
+  event_add(timer->ev, (struct timeval *)tv); /*drop const for old libevent*/
+}
+
+/**
+ * Disable the provided <b>timer</b>, but do not free it.
+ *
+ * You can reenable the same timer later with periodic_timer_launch.
+ *
+ * If the timer is already disabled, this function does nothing.
+ */
+void
+periodic_timer_disable(periodic_timer_t *timer)
+{
+  tor_assert(timer);
+  event_del(timer->ev);
+}
+
 /** Stop and free a periodic timer */
 void
 periodic_timer_free_(periodic_timer_t *timer)

src/common/compat_libevent.h

@@ -31,6 +31,8 @@ periodic_timer_t *periodic_timer_new(struct event_base *base,
              void (*cb)(periodic_timer_t *timer, void *data),
              void *data);
 void periodic_timer_free_(periodic_timer_t *);
+void periodic_timer_launch(periodic_timer_t *, const struct timeval *tv);
+void periodic_timer_disable(periodic_timer_t *);
 #define periodic_timer_free(t) \
   FREE_AND_NULL(periodic_timer_t, periodic_timer_free_, (t))