Fix two issues with user Calendar event availability cache display

Summary:
Ref T11816. Two minor issues:

  - We used `$event`, not `$next_event`, as the event providing the PHID for "Busy at <event name>". This rendered "Busy at <most future event>" on the profile instead of "Busy at <next upcoming event".
  - The TTL computation used the event start, not the event end, so we could end up rebuilding the cache too often for users busy at an event.

Test Plan:
  - Attended an event in the near future and one later on.
  - Saw profile now say "busy at <near future event>" correctly.
  - In DarkConsole "Services" tab, no longer saw unnecessary cache refills while attending an event.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T11816

Differential Revision: https://secure.phabricator.com/D17643

src/applications/calendar/storage/PhabricatorCalendarEvent.php

@@ -391,6 +391,10 @@ final class PhabricatorCalendarEvent extends PhabricatorCalendarDAO
     return ($epoch - $window);
   }
 
+  public function getEndDateTimeEpochForCache() {
+    return $this->getEndDateTimeEpoch();
+  }
+
   protected function getConfiguration() {
     return array(
       self::CONFIG_AUX_PHID => true,

src/applications/people/query/PhabricatorPeopleQuery.php

@@ -465,7 +465,7 @@ final class PhabricatorPeopleQuery
         while (true) {
           foreach ($events as $event) {
             $from = $event->getStartDateTimeEpochForCache();
-            $to = $event->getEndDateTimeEpoch();
+            $to = $event->getEndDateTimeEpochForCache();
             if (($from <= $cursor) && ($to > $cursor)) {
               $cursor = $to;
               if (!$next_event) {
@@ -483,7 +483,7 @@ final class PhabricatorPeopleQuery
         $availability_type = $invitee->getDisplayAvailability($next_event);
         $availability = array(
           'until' => $cursor,
-          'eventPHID' => $event->getPHID(),
+          'eventPHID' => $next_event->getPHID(),
           'availability' => $availability_type,
         );
 
@@ -496,7 +496,7 @@ final class PhabricatorPeopleQuery
         // simultaneously we should accommodate that. However, it's complex
         // to compute, rare, and probably not confusing most of the time.
 
-        $availability_ttl = $next_event->getStartDateTimeEpochForCache();
+        $availability_ttl = $next_event->getEndDateTimeEpochForCache();
       } else {
         $availability = array(
           'until' => null,

src/applications/people/storage/PhabricatorUser.php

@@ -1082,7 +1082,7 @@ final class PhabricatorUser
       'UPDATE %T SET availabilityCache = %s, availabilityCacheTTL = %nd
         WHERE id = %d',
       $this->getTableName(),
-      json_encode($availability),
+      phutil_json_encode($availability),
       $ttl,
       $this->getID());
     unset($unguarded);