Update attendence for external users

For local users it's possible to select their calendar via the principal url and first update their own attendance status.
External users have no calendar event hence the recipient is the organizer.

Signed-off-by: Daniel Kesselberg <mail@danielkesselberg.de>

apps/dav/lib/CalDAV/InvitationResponse/InvitationResponseServer.php

@@ -121,4 +121,10 @@ public function handleITipMessage(Message $iTipMessage) {
 		$schedulingPlugin = $this->server->getPlugin('caldav-schedule');
 		$schedulingPlugin->scheduleLocalDelivery($iTipMessage);
 	}
+
+	public function isExternalAttendee(string $principalUri): bool {
+		/** @var \Sabre\DAVACL\Plugin $aclPlugin */
+		$aclPlugin = $this->server->getPlugin('acl');
+		return $aclPlugin->getPrincipalByUri($principalUri) === null;
+	}
 }

apps/dav/lib/Controller/InvitationResponseController.php

@@ -198,7 +198,12 @@ private function buildITipResponse(array $row, string $partStat, int $guests = n
 		$iTipMessage->method = 'REPLY';
 		$iTipMessage->sequence = $row['sequence'];
 		$iTipMessage->sender = $row['attendee'];
-		$iTipMessage->recipient = $row['attendee'];
+
+		if ($this->responseServer->isExternalAttendee($row['attendee'])) {
+			$iTipMessage->recipient = $row['organizer'];
+		} else {
+			$iTipMessage->recipient = $row['attendee'];
+		}
 
 		$message = <<<EOF
 BEGIN:VCALENDAR