nextcloud_server/apps/dav/lib/CalDAV/Schedule/Plugin.php

<?php

/**
 * SPDX-FileCopyrightText: 2016 Nextcloud GmbH and Nextcloud contributors
 * SPDX-License-Identifier: AGPL-3.0-or-later
 */
namespace OCA\DAV\CalDAV\Schedule;

use DateTimeZone;
use OCA\DAV\CalDAV\CalDavBackend;
use OCA\DAV\CalDAV\Calendar;
use OCA\DAV\CalDAV\CalendarHome;
use OCA\DAV\CalDAV\CalendarObject;
use OCA\DAV\CalDAV\DefaultCalendarValidator;
use OCA\DAV\CalDAV\TipBroker;
use OCP\IConfig;
use Psr\Log\LoggerInterface;
use Sabre\CalDAV\ICalendar;
use Sabre\CalDAV\ICalendarObject;
use Sabre\CalDAV\Schedule\ISchedulingObject;
use Sabre\DAV\Exception as DavException;
use Sabre\DAV\INode;
use Sabre\DAV\IProperties;
use Sabre\DAV\PropFind;
use Sabre\DAV\Server;
use Sabre\DAV\Xml\Property\LocalHref;
use Sabre\DAVACL\IACL;
use Sabre\DAVACL\IPrincipal;
use Sabre\HTTP\RequestInterface;
use Sabre\HTTP\ResponseInterface;
use Sabre\VObject\Component;
use Sabre\VObject\Component\VCalendar;
use Sabre\VObject\Component\VEvent;
use Sabre\VObject\DateTimeParser;
use Sabre\VObject\FreeBusyGenerator;
use Sabre\VObject\ITip;
use Sabre\VObject\ITip\SameOrganizerForAllComponentsException;
use Sabre\VObject\Parameter;
use Sabre\VObject\Property;
use Sabre\VObject\Reader;
use function Sabre\Uri\split;

class Plugin extends \Sabre\CalDAV\Schedule\Plugin {

	/** @var ITip\Message[] */
	private $schedulingResponses = [];

	/** @var string|null */
	private $pathOfCalendarObjectChange = null;

	public const CALENDAR_USER_TYPE = '{' . self::NS_CALDAV . '}calendar-user-type';
	public const SCHEDULE_DEFAULT_CALENDAR_URL = '{' . Plugin::NS_CALDAV . '}schedule-default-calendar-URL';

	/**
	 * @param IConfig $config
	 */
	public function __construct(
		private IConfig $config,
		private LoggerInterface $logger,
		private DefaultCalendarValidator $defaultCalendarValidator,
	) {
	}

	/**
	 * Initializes the plugin
	 *
	 * @param Server $server
	 * @return void
	 */
	public function initialize(Server $server) {
		parent::initialize($server);
		$server->on('propFind', [$this, 'propFindDefaultCalendarUrl'], 90);
		$server->on('afterWriteContent', [$this, 'dispatchSchedulingResponses']);
		$server->on('afterCreateFile', [$this, 'dispatchSchedulingResponses']);

		// We allow mutating the default calendar URL through the CustomPropertiesBackend
		// (oc_properties table)
		$server->protectedProperties = array_filter(
			$server->protectedProperties,
			static fn (string $property) => $property !== self::SCHEDULE_DEFAULT_CALENDAR_URL,
		);
	}

	/**
	 * Returns an instance of the iTip\Broker.
	 */
	protected function createITipBroker(): TipBroker {
		return new TipBroker();
	}

	/**
	 * Allow manual setting of the object change URL
	 * to support public write
	 *
	 * @param string $path
	 */
	public function setPathOfCalendarObjectChange(string $path): void {
		$this->pathOfCalendarObjectChange = $path;
	}

	/**
	 * This method handler is invoked during fetching of properties.
	 *
	 * We use this event to add calendar-auto-schedule-specific properties.
	 *
	 * @param PropFind $propFind
	 * @param INode $node
	 * @return void
	 */
	public function propFind(PropFind $propFind, INode $node) {
		if ($node instanceof IPrincipal) {
			// overwrite Sabre/Dav's implementation
			$propFind->handle(self::CALENDAR_USER_TYPE, function () use ($node) {
				if ($node instanceof IProperties) {
					$props = $node->getProperties([self::CALENDAR_USER_TYPE]);

					if (isset($props[self::CALENDAR_USER_TYPE])) {
						return $props[self::CALENDAR_USER_TYPE];
					}
				}

				return 'INDIVIDUAL';
			});
		}

		parent::propFind($propFind, $node);
	}

	/**
	 * Returns a list of addresses that are associated with a principal.
	 *
	 * @param string $principal
	 * @return array
	 */
	protected function getAddressesForPrincipal($principal) {
		$result = parent::getAddressesForPrincipal($principal);

		if ($result === null) {
			$result = [];
		}

		// iterate through items and html decode values
		foreach ($result as $key => $value) {
			$result[$key] = urldecode($value);
		}

		return $result;
	}

	/**
	 * @param RequestInterface $request
	 * @param ResponseInterface $response
	 * @param VCalendar $vCal
	 * @param mixed $calendarPath
	 * @param mixed $modified
	 * @param mixed $isNew
	 */
	public function calendarObjectChange(RequestInterface $request, ResponseInterface $response, VCalendar $vCal, $calendarPath, &$modified, $isNew) {
		// Save the first path we get as a calendar-object-change request
		if (!$this->pathOfCalendarObjectChange) {
			$this->pathOfCalendarObjectChange = $request->getPath();
		}

		try {

			// Do not generate iTip and iMip messages if scheduling is disabled for this message
			if ($request->getHeader('x-nc-scheduling') === 'false') {
				return;
			}

			if (!$this->scheduleReply($this->server->httpRequest)) {
				return;
			}

			/** @var Calendar $calendarNode */
			$calendarNode = $this->server->tree->getNodeForPath($calendarPath);
			// extract addresses for owner
			$addresses = $this->getAddressesForPrincipal($calendarNode->getOwner());
			// determine if request is from a sharee
			if ($calendarNode->isShared()) {
				// extract addresses for sharee and add to address collection
				$addresses = array_merge(
					$addresses,
					$this->getAddressesForPrincipal($calendarNode->getPrincipalURI())
				);
			}
			// determine if we are updating a calendar event
			if (!$isNew) {
				// retrieve current calendar event node
				/** @var CalendarObject $currentNode */
				$currentNode = $this->server->tree->getNodeForPath($request->getPath());
				// convert calendar event string data to VCalendar object
				/** @var \Sabre\VObject\Component\VCalendar $currentObject */
				$currentObject = Reader::read($currentNode->get());
			} else {
				$currentObject = null;
			}
			// process request
			$this->processICalendarChange($currentObject, $vCal, $addresses, [], $modified);

			if ($currentObject) {
				// Destroy circular references so PHP will GC the object.
				$currentObject->destroy();
			}

		} catch (SameOrganizerForAllComponentsException $e) {
			$this->handleSameOrganizerException($e, $vCal, $calendarPath);
		}
	}

	/**
	 * @inheritDoc
	 */
	public function beforeUnbind($path): void {
		try {
			parent::beforeUnbind($path);
		} catch (SameOrganizerForAllComponentsException $e) {
			$node = $this->server->tree->getNodeForPath($path);
			if (!$node instanceof ICalendarObject || $node instanceof ISchedulingObject) {
				throw $e;
			}

			/** @var VCalendar $vCal */
			$vCal = Reader::read($node->get());
			$this->handleSameOrganizerException($e, $vCal, $path);
		}
	}

	/**
	 * @inheritDoc
	 */
	public function scheduleLocalDelivery(ITip\Message $iTipMessage):void {
		/** @var VEvent|null $vevent */
		$vevent = $iTipMessage->message->VEVENT ?? null;

		// Strip VALARMs from incoming VEVENT
		if ($vevent && isset($vevent->VALARM)) {
			$vevent->remove('VALARM');
		}

		parent::scheduleLocalDelivery($iTipMessage);
		// We only care when the message was successfully delivered locally
		// Log all possible codes returned from the parent method that mean something went wrong
		// 3.7, 3.8, 5.0, 5.2
		if ($iTipMessage->scheduleStatus !== '1.2;Message delivered locally') {
			$this->logger->debug('Message not delivered locally with status: ' . $iTipMessage->scheduleStatus);
			return;
		}
		// We only care about request. reply and cancel are properly handled
		// by parent::scheduleLocalDelivery already
		if (strcasecmp($iTipMessage->method, 'REQUEST') !== 0) {
			return;
		}

		// If parent::scheduleLocalDelivery set scheduleStatus to 1.2,
		// it means that it was successfully delivered locally.
		// Meaning that the ACL plugin is loaded and that a principal
		// exists for the given recipient id, no need to double check
		/** @var \Sabre\DAVACL\Plugin $aclPlugin */
		$aclPlugin = $this->server->getPlugin('acl');
		$principalUri = $aclPlugin->getPrincipalByUri($iTipMessage->recipient);
		$calendarUserType = $this->getCalendarUserTypeForPrincipal($principalUri);
		if (strcasecmp($calendarUserType, 'ROOM') !== 0 && strcasecmp($calendarUserType, 'RESOURCE') !== 0) {
			$this->logger->debug('Calendar user type is room or resource, not processing further');
			return;
		}

		$attendee = $this->getCurrentAttendee($iTipMessage);
		if (!$attendee) {
			$this->logger->debug('No attendee set for scheduling message');
			return;
		}

		// We only respond when a response was actually requested
		$rsvp = $this->getAttendeeRSVP($attendee);
		if (!$rsvp) {
			$this->logger->debug('No RSVP requested for attendee ' . $attendee->getValue());
			return;
		}

		if (!$vevent) {
			$this->logger->debug('No VEVENT set to process on scheduling message');
			return;
		}

		// We don't support autoresponses for recurrencing events for now
		if (isset($vevent->RRULE) || isset($vevent->RDATE)) {
			$this->logger->debug('VEVENT is a recurring event, autoresponding not supported');
			return;
		}

		$dtstart = $vevent->DTSTART;
		$dtend = $this->getDTEndFromVEvent($vevent);
		$uid = $vevent->UID->getValue();
		$sequence = isset($vevent->SEQUENCE) ? $vevent->SEQUENCE->getValue() : 0;
		$recurrenceId = isset($vevent->{'RECURRENCE-ID'}) ? $vevent->{'RECURRENCE-ID'}->serialize() : '';

		$message = <<<EOF
BEGIN:VCALENDAR
PRODID:-//Nextcloud/Nextcloud CalDAV Server//EN
METHOD:REPLY
VERSION:2.0
BEGIN:VEVENT
ATTENDEE;PARTSTAT=%s:%s
ORGANIZER:%s
UID:%s
SEQUENCE:%s
REQUEST-STATUS:2.0;Success
%sEND:VEVENT
END:VCALENDAR
EOF;

		if ($this->isAvailableAtTime($attendee->getValue(), $dtstart->getDateTime(), $dtend->getDateTime(), $uid)) {
			$partStat = 'ACCEPTED';
		} else {
			$partStat = 'DECLINED';
		}

		$vObject = Reader::read(vsprintf($message, [
			$partStat,
			$iTipMessage->recipient,
			$iTipMessage->sender,
			$uid,
			$sequence,
			$recurrenceId
		]));

		$responseITipMessage = new ITip\Message();
		$responseITipMessage->uid = $uid;
		$responseITipMessage->component = 'VEVENT';
		$responseITipMessage->method = 'REPLY';
		$responseITipMessage->sequence = $sequence;
		$responseITipMessage->sender = $iTipMessage->recipient;
		$responseITipMessage->recipient = $iTipMessage->sender;
		$responseITipMessage->message = $vObject;

		// We can't dispatch them now already, because the organizers calendar-object
		// was not yet created. Hence Sabre/DAV won't find a calendar-object, when we
		// send our reply.
		$this->schedulingResponses[] = $responseITipMessage;
	}

	/**
	 * @param string $uri
	 */
	public function dispatchSchedulingResponses(string $uri):void {
		if ($uri !== $this->pathOfCalendarObjectChange) {
			return;
		}

		foreach ($this->schedulingResponses as $schedulingResponse) {
			$this->scheduleLocalDelivery($schedulingResponse);
		}
	}

	/**
	 * Always use the personal calendar as target for scheduled events
	 *
	 * @param PropFind $propFind
	 * @param INode $node
	 * @return void
	 */
	public function propFindDefaultCalendarUrl(PropFind $propFind, INode $node) {
		if ($node instanceof IPrincipal) {
			$propFind->handle(self::SCHEDULE_DEFAULT_CALENDAR_URL, function () use ($node) {
				/** @var \OCA\DAV\CalDAV\Plugin $caldavPlugin */
				$caldavPlugin = $this->server->getPlugin('caldav');
				$principalUrl = $node->getPrincipalUrl();

				$calendarHomePath = $caldavPlugin->getCalendarHomeForPrincipal($principalUrl);
				if (!$calendarHomePath) {
					return null;
				}

				$isResourceOrRoom = str_starts_with($principalUrl, 'principals/calendar-resources') ||
					str_starts_with($principalUrl, 'principals/calendar-rooms');

				if (str_starts_with($principalUrl, 'principals/users')) {
					[, $userId] = split($principalUrl);
					$uri = $this->config->getUserValue($userId, 'dav', 'defaultCalendar', CalDavBackend::PERSONAL_CALENDAR_URI);
					$displayName = CalDavBackend::PERSONAL_CALENDAR_NAME;
				} elseif ($isResourceOrRoom) {
					$uri = CalDavBackend::RESOURCE_BOOKING_CALENDAR_URI;
					$displayName = CalDavBackend::RESOURCE_BOOKING_CALENDAR_NAME;
				} else {
					// How did we end up here?
					// TODO - throw exception or just ignore?
					return null;
				}

				/** @var CalendarHome $calendarHome */
				$calendarHome = $this->server->tree->getNodeForPath($calendarHomePath);
				$currentCalendarDeleted = false;
				if (!$calendarHome->childExists($uri) || $currentCalendarDeleted = $this->isCalendarDeleted($calendarHome, $uri)) {
					// If the default calendar doesn't exist
					if ($isResourceOrRoom) {
						// Resources or rooms can't be in the trashbin, so we're fine
						$this->createCalendar($calendarHome, $principalUrl, $uri, $displayName);
					} else {
						// And we're not handling scheduling on resource/room booking
						$userCalendars = [];
						/**
						 * If the default calendar of the user isn't set and the
						 * fallback doesn't match any of the user's calendar
						 * try to find the first "personal" calendar we can write to
						 * instead of creating a new one.
						 * A appropriate personal calendar to receive invites:
						 * - isn't a calendar subscription
						 * - user can write to it (no virtual/3rd-party calendars)
						 * - calendar isn't a share
						 * - calendar supports VEVENTs
						 */
						foreach ($calendarHome->getChildren() as $node) {
							if (!($node instanceof Calendar)) {
								continue;
							}

							try {
								$this->defaultCalendarValidator->validateScheduleDefaultCalendar($node);
							} catch (DavException $e) {
								continue;
							}

							$userCalendars[] = $node;
						}

						if (count($userCalendars) > 0) {
							// Calendar backend returns calendar by calendarorder property
							$uri = $userCalendars[0]->getName();
						} else {
							// Otherwise if we have really nothing, create a new calendar
							if ($currentCalendarDeleted) {
								// If the calendar exists but is in the trash bin, we try to rename its uri
								// so that we can create the new one and still restore the previous one
								// otherwise we just purge the calendar by removing it before recreating it
								$calendar = $this->getCalendar($calendarHome, $uri);
								if ($calendar instanceof Calendar) {
									$backend = $calendarHome->getCalDAVBackend();
									if ($backend instanceof CalDavBackend) {
										// If the CalDAV backend supports moving calendars
										$this->moveCalendar($backend, $principalUrl, $uri, $uri . '-back-' . time());
									} else {
										// Otherwise just purge the calendar
										$calendar->disableTrashbin();
										$calendar->delete();
									}
								}
							}
							$this->createCalendar($calendarHome, $principalUrl, $uri, $displayName);
						}
					}
				}

				$result = $this->server->getPropertiesForPath($calendarHomePath . '/' . $uri, [], 1);
				if (empty($result)) {
					return null;
				}

				return new LocalHref($result[0]['href']);
			});
		}
	}

	/**
	 * Returns a list of addresses that are associated with a principal.
	 *
	 * @param string $principal
	 * @return string|null
	 */
	protected function getCalendarUserTypeForPrincipal($principal):?string {
		$calendarUserType = '{' . self::NS_CALDAV . '}calendar-user-type';
		$properties = $this->server->getProperties(
			$principal,
			[$calendarUserType]
		);

		// If we can't find this information, we'll stop processing
		if (!isset($properties[$calendarUserType])) {
			return null;
		}

		return $properties[$calendarUserType];
	}

	/**
	 * @param ITip\Message $iTipMessage
	 * @return null|Property
	 */
	private function getCurrentAttendee(ITip\Message $iTipMessage):?Property {
		/** @var VEvent $vevent */
		$vevent = $iTipMessage->message->VEVENT;
		$attendees = $vevent->select('ATTENDEE');
		foreach ($attendees as $attendee) {
			/** @var Property $attendee */
			if (strcasecmp($attendee->getValue(), $iTipMessage->recipient) === 0) {
				return $attendee;
			}
		}
		return null;
	}

	/**
	 * @param Property|null $attendee
	 * @return bool
	 */
	private function getAttendeeRSVP(?Property $attendee = null):bool {
		if ($attendee !== null) {
			$rsvp = $attendee->offsetGet('RSVP');
			if (($rsvp instanceof Parameter) && (strcasecmp($rsvp->getValue(), 'TRUE') === 0)) {
				return true;
			}
		}
		// RFC 5545 3.2.17: default RSVP is false
		return false;
	}

	/**
	 * @param VEvent $vevent
	 * @return Property\ICalendar\DateTime
	 */
	private function getDTEndFromVEvent(VEvent $vevent):Property\ICalendar\DateTime {
		if (isset($vevent->DTEND)) {
			return $vevent->DTEND;
		}

		if (isset($vevent->DURATION)) {
			$isFloating = $vevent->DTSTART->isFloating();
			/** @var Property\ICalendar\DateTime $end */
			$end = clone $vevent->DTSTART;
			$endDateTime = $end->getDateTime();
			$endDateTime = $endDateTime->add(DateTimeParser::parse($vevent->DURATION->getValue()));
			$end->setDateTime($endDateTime, $isFloating);
			return $end;
		}

		if (!$vevent->DTSTART->hasTime()) {
			$isFloating = $vevent->DTSTART->isFloating();
			/** @var Property\ICalendar\DateTime $end */
			$end = clone $vevent->DTSTART;
			$endDateTime = $end->getDateTime();
			$endDateTime = $endDateTime->modify('+1 day');
			$end->setDateTime($endDateTime, $isFloating);
			return $end;
		}

		return clone $vevent->DTSTART;
	}

	/**
	 * @param string $email
	 * @param \DateTimeInterface $start
	 * @param \DateTimeInterface $end
	 * @param string $ignoreUID
	 * @return bool
	 */
	private function isAvailableAtTime(string $email, \DateTimeInterface $start, \DateTimeInterface $end, string $ignoreUID):bool {
		// This method is heavily inspired by Sabre\CalDAV\Schedule\Plugin::scheduleLocalDelivery
		// and Sabre\CalDAV\Schedule\Plugin::getFreeBusyForEmail

		$aclPlugin = $this->server->getPlugin('acl');
		$this->server->removeListener('propFind', [$aclPlugin, 'propFind']);

		$result = $aclPlugin->principalSearch(
			['{http://sabredav.org/ns}email-address' => $this->stripOffMailTo($email)],
			[
				'{DAV:}principal-URL',
				'{' . self::NS_CALDAV . '}calendar-home-set',
				'{' . self::NS_CALDAV . '}schedule-inbox-URL',
				'{http://sabredav.org/ns}email-address',

			]
		);
		$this->server->on('propFind', [$aclPlugin, 'propFind'], 20);


		// Grabbing the calendar list
		$objects = [];
		$calendarTimeZone = new DateTimeZone('UTC');

		$homePath = $result[0][200]['{' . self::NS_CALDAV . '}calendar-home-set']->getHref();
		/** @var Calendar $node */
		foreach ($this->server->tree->getNodeForPath($homePath)->getChildren() as $node) {

			if (!$node instanceof ICalendar) {
				continue;
			}

			// Getting the list of object uris within the time-range
			$urls = $node->calendarQuery([
				'name' => 'VCALENDAR',
				'comp-filters' => [
					[
						'name' => 'VEVENT',
						'is-not-defined' => false,
						'time-range' => [
							'start' => $start,
							'end' => $end,
						],
						'comp-filters' => [],
						'prop-filters' => [],
					],
					[
						'name' => 'VEVENT',
						'is-not-defined' => false,
						'time-range' => null,
						'comp-filters' => [],
						'prop-filters' => [
							[
								'name' => 'UID',
								'is-not-defined' => false,
								'time-range' => null,
								'text-match' => [
									'value' => $ignoreUID,
									'negate-condition' => true,
									'collation' => 'i;octet',
								],
								'param-filters' => [],
							],
						]
					],
				],
				'prop-filters' => [],
				'is-not-defined' => false,
				'time-range' => null,
			]);

			foreach ($urls as $url) {
				$objects[] = $node->getChild($url)->get();
			}
		}

		$inboxProps = $this->server->getProperties(
			$result[0][200]['{' . self::NS_CALDAV . '}schedule-inbox-URL']->getHref(),
			['{' . self::NS_CALDAV . '}calendar-availability']
		);

		$vcalendar = new VCalendar();
		$vcalendar->METHOD = 'REPLY';

		$generator = new FreeBusyGenerator();
		$generator->setObjects($objects);
		$generator->setTimeRange($start, $end);
		$generator->setBaseObject($vcalendar);
		$generator->setTimeZone($calendarTimeZone);

		if (isset($inboxProps['{' . self::NS_CALDAV . '}calendar-availability'])) {
			$generator->setVAvailability(
				Reader::read(
					$inboxProps['{' . self::NS_CALDAV . '}calendar-availability']
				)
			);
		}

		$result = $generator->getResult();
		if (!isset($result->VFREEBUSY)) {
			return false;
		}

		/** @var Component $freeBusyComponent */
		$freeBusyComponent = $result->VFREEBUSY;
		$freeBusyProperties = $freeBusyComponent->select('FREEBUSY');
		// If there is no Free-busy property at all, the time-range is empty and available
		if (count($freeBusyProperties) === 0) {
			return true;
		}

		// If more than one Free-Busy property was returned, it means that an event
		// starts or ends inside this time-range, so it's not available and we return false
		if (count($freeBusyProperties) > 1) {
			return false;
		}

		/** @var Property $freeBusyProperty */
		$freeBusyProperty = $freeBusyProperties[0];
		if (!$freeBusyProperty->offsetExists('FBTYPE')) {
			// If there is no FBTYPE, it means it's busy
			return false;
		}

		$fbTypeParameter = $freeBusyProperty->offsetGet('FBTYPE');
		if (!($fbTypeParameter instanceof Parameter)) {
			return false;
		}

		return (strcasecmp($fbTypeParameter->getValue(), 'FREE') === 0);
	}

	/**
	 * @param string $email
	 * @return string
	 */
	private function stripOffMailTo(string $email): string {
		if (stripos($email, 'mailto:') === 0) {
			return substr($email, 7);
		}

		return $email;
	}

	private function getCalendar(CalendarHome $calendarHome, string $uri): INode {
		return $calendarHome->getChild($uri);
	}

	private function isCalendarDeleted(CalendarHome $calendarHome, string $uri): bool {
		$calendar = $this->getCalendar($calendarHome, $uri);
		return $calendar instanceof Calendar && $calendar->isDeleted();
	}

	private function createCalendar(CalendarHome $calendarHome, string $principalUri, string $uri, string $displayName): void {
		$calendarHome->getCalDAVBackend()->createCalendar($principalUri, $uri, [
			'{DAV:}displayname' => $displayName,
		]);
	}

	private function moveCalendar(CalDavBackend $calDavBackend, string $principalUri, string $oldUri, string $newUri): void {
		$calDavBackend->moveCalendar($oldUri, $principalUri, $principalUri, $newUri);
	}

	/**
	 * Try to handle the given exception gracefully or throw it if necessary.
	 *
	 * @throws SameOrganizerForAllComponentsException If the exception should not be ignored
	 */
	private function handleSameOrganizerException(
		SameOrganizerForAllComponentsException $e,
		VCalendar $vCal,
		string $calendarPath,
	): void {
		// This is very hacky! However, we want to allow saving events with multiple
		// organizers. Those events are not RFC compliant, but sometimes imported from major
		// external calendar services (e.g. Google). If the current user is not an organizer of
		// the event we ignore the exception as no scheduling messages will be sent anyway.

		// It would be cleaner to patch Sabre to validate organizers *after* checking if
		// scheduling messages are necessary. Currently, organizers are validated first and
		// afterwards the broker checks if messages should be scheduled. So the code will throw
		// even if the organizers are not relevant. This is to ensure compliance with RFCs but
		// a bit too strict for real world usage.

		if (!isset($vCal->VEVENT)) {
			throw $e;
		}

		$calendarNode = $this->server->tree->getNodeForPath($calendarPath);
		if (!($calendarNode instanceof IACL)) {
			// Should always be an instance of IACL but just to be sure
			throw $e;
		}

		$addresses = $this->getAddressesForPrincipal($calendarNode->getOwner());
		foreach ($vCal->VEVENT as $vevent) {
			if (in_array($vevent->ORGANIZER->getNormalizedValue(), $addresses, true)) {
				// User is an organizer => throw the exception
				throw $e;
			}
		}
	}
}