Add a call to predict the next event for a trigger

Summary: Ref T6881. This is useful to show a "Next backup: 2:30 AM" sort of thing without requring callers to know how triggers work internally.

Test Plan: Showed that kind of thing in Instances.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T6881

Differential Revision: https://secure.phabricator.com/D11437

src/infrastructure/daemon/workers/storage/PhabricatorWorkerTrigger.php

@@ -130,6 +130,26 @@ final class PhabricatorWorkerTrigger
   }
 
 
+  /**
+   * Predict the epoch at which this trigger will next fire.
+   *
+   * @return int|null  Epoch when the event will next fire, or `null` if it is
+   *   not planned to trigger.
+   */
+  public function getNextEventPrediction() {
+    // NOTE: We're basically echoing the database state here, so this won't
+    // necessarily be accurate if the caller just updated the object but has
+    // not saved it yet. That's a weird use case and would require more
+    // gymnastics, so don't bother trying to get it totally correct for now.
+
+    if ($this->getEvent()) {
+      return $this->getEvent()->getNextEpoch();
+    } else {
+      return $this->getNextEventEpoch(null, $is_reschedule = false);
+    }
+  }
+
+
 /* -(  PhabricatorDestructibleInterface  )----------------------------------- */