Add a function to schedule a periodic event once, then disable it

src/core/mainloop/mainloop.c

@@ -1600,7 +1600,11 @@ rescan_periodic_events(const or_options_t *options)
       periodic_event_enable(item);
     } else {
       log_debug(LD_GENERAL, "Disabling periodic event %s", item->name);
-      periodic_event_disable(item);
+      if (item->flags & PERIODIC_EVENT_FLAG_FLUSH_ON_DISABLE) {
+        periodic_event_flush_and_disable(item);
+      } else {
+        periodic_event_disable(item);
+      }
     }
   }
 }

src/core/mainloop/periodic.c

@@ -45,10 +45,6 @@ periodic_event_dispatch(mainloop_event_t *ev, void *data)
   periodic_event_item_t *event = data;
   tor_assert(ev == event->ev);
 
-  if (BUG(!periodic_event_is_enabled(event))) {
-    return;
-  }
-
   time_t now = time(NULL);
   update_current_time(now);
   const or_options_t *options = get_options();
@@ -57,7 +53,7 @@ periodic_event_dispatch(mainloop_event_t *ev, void *data)
   int next_interval = 0;
 
   if (!periodic_event_is_enabled(event)) {
-    /* The event got disabled from inside its callback; no need to
+    /* The event got disabled from inside its callback, or before: no need to
      * reschedule. */
     return;
   }
@@ -172,3 +168,19 @@ periodic_event_disable(periodic_event_item_t *event)
   mainloop_event_cancel(event->ev);
   event->enabled = 0;
 }
+
+/**
+ * Disable an event, then schedule it to run once.
+ * Do nothing if the event was already disabled.
+ */
+void
+periodic_event_flush_and_disable(periodic_event_item_t *event)
+{
+  tor_assert(event);
+  if (!periodic_event_is_enabled(event))
+    return;
+
+  periodic_event_disable(event);
+
+  mainloop_event_activate(event->ev);
+}

src/core/mainloop/periodic.h

@@ -39,6 +39,11 @@
  * the net_is_disabled() check. */
 #define PERIODIC_EVENT_FLAG_NEED_NET  (1U << 0)
 
+/* Indicate that it the event is enabled, it event needs to be run once before
+ * it becomes disabled.
+ */
+#define PERIODIC_EVENT_FLAG_FLUSH_ON_DISABLE  (1U << 1)
+
 /** Callback function for a periodic event to take action.  The return value
 * influences the next time the function will get called.  Return
 * PERIODIC_EVENT_NO_UPDATE to not update <b>last_action_time</b> and be polled
@@ -83,6 +88,6 @@ void periodic_event_destroy(periodic_event_item_t *event);
 void periodic_event_reschedule(periodic_event_item_t *event);
 void periodic_event_enable(periodic_event_item_t *event);
 void periodic_event_disable(periodic_event_item_t *event);
+void periodic_event_flush_and_disable(periodic_event_item_t *event);
 
 #endif /* !defined(TOR_PERIODIC_H) */
-