do NOT push participants of type email for privacy reasons

api/src/Link.php

@@ -92,7 +92,8 @@ namespace EGroupware\Api;
  *			'fetch'	=>	'app.class.method',			// method to return entry data for a given id. the method called should support id, and expected mime-type
  *													// basically you should return something like array(id, title, mimetype, body, linked-files)
  *
- *          'push_data' => "key" | ["key1", ...]    // keys of ACL relevant and privacy save data needed for push of changes to client
+ *          'push_data' => <callable> | "key" | ["key1", ...]    // keys of ACL relevant and privacy save data needed for push of changes to client
+ *                                                  // or callable to do the cleaning eg. used in calendar
  *
  *			'additional' => array(					// allow one app to register sub-types,
  *				'app-sub' => array(					// different from 'types' approach above

calendar/inc/class.calendar_hooks.inc.php

@@ -68,10 +68,30 @@ class calendar_hooks
 			'merge' => true,
 			'entry' => 'Event',
 			'entries' => 'Events',
-			'push_data'  => ['id','owner','participants','start','end']
+			'push_data'  => self::class.'::prepareEventPush',
 		);
 	}
 
+
+	/**
+	 * Prepare event to be pushed via Link::notify_update()
+	 *
+	 * Remove privacy sensitive data:
+	 * - participants of type email
+	 *
+	 * @param $event
+	 * @return array
+	 */
+	static public function prepareEventPush($event)
+	{
+		$event = array_intersect_key($event, array_flip(['id','owner','participants','start','end']));
+		foreach($event['participants'] as $uid => $status)
+		{
+			if ($uid[0] === 'e') unset($event['participants'][$uid]);
+		}
+		return $event;
+	}
+
 	/**
 	 * Hook called to retrieve a app specific exportLimit
 	 *