Log flooded by "This recurrence rule does not generate any valid instances"

[tchernobog]

Steps to reproduce

1. Subscribe to this public calendar which exhibits the issue
2. Wait for a bit and look at the log in the admin settings
3. I see several instances of this every 10 minutes or so, from different subscriptions (seems I have multiple users subscribing to this calendar or some other calendar with the same issue)

Expected behavior

Log is not flodded with messages about this, the calendar seems good to me when I read it e.g. with my mail client.

Actual behavior

Several dozen lines every attempted calendar sync land in the log with "Forbidden" in front

Calendar app version

5.0.10

CalDAV-clients used

DAVx5, Evolution

Browser

Firefox 135.0

Client operating system

Debian 12

Server operating system

Debian 12

Web server

Nginx

Database engine version

PostgreSQL

PHP engine version

PHP 8.0

Nextcloud version

30.0.6

Updated from an older installed version or fresh install

Updated from an older version

List of activated apps

Nextcloud configuration

Web server error log

Log file

Browser log

Additional info

Raw details of one entry:

{
    "reqId": "1oAqWLIzknF2uRJPIU6y",
    "level": 2,
    "time": "2025-02-15T16:41:31+00:00",
    "remoteAddr": "",
    "user": "--",
    "app": "dav",
    "method": "",
    "url": "--",
    "message": "Unable to denormalize calendar object from subscription 28",
    "userAgent": "--",
    "version": "30.0.6.2",
    "exception": {
        "Exception": "Sabre\\DAV\\Exception\\Forbidden",
        "Message": "This recurrence rule does not generate any valid instances",
        "Code": 0,
        "Trace": [
            {
                "file": "/var/www/montecristosoftware/nextcloud/apps/dav/lib/CalDAV/WebcalCaching/RefreshWebcalService.php",
                "line": 104,
                "function": "getDenormalizedData",
                "class": "OCA\\DAV\\CalDAV\\CalDavBackend",
                "type": "->"
            },
            {
                "file": "/var/www/montecristosoftware/nextcloud/apps/dav/lib/BackgroundJob/RefreshWebcalJob.php",
                "line": 73,
                "function": "refreshSubscription",
                "class": "OCA\\DAV\\CalDAV\\WebcalCaching\\RefreshWebcalService",
                "type": "->"
            },
            {
                "file": "/var/www/montecristosoftware/nextcloud/lib/public/BackgroundJob/Job.php",
                "line": 61,
                "function": "run",
                "class": "OCA\\DAV\\BackgroundJob\\RefreshWebcalJob",
                "type": "->"
            },
            {
                "file": "/var/www/montecristosoftware/nextcloud/apps/dav/lib/BackgroundJob/RefreshWebcalJob.php",
                "line": 66,
                "function": "start",
                "class": "OCP\\BackgroundJob\\Job",
                "type": "->"
            },
            {
                "file": "/var/www/montecristosoftware/nextcloud/lib/public/BackgroundJob/Job.php",
                "line": 46,
                "function": "start",
                "class": "OCA\\DAV\\BackgroundJob\\RefreshWebcalJob",
                "type": "->"
            },
            {
                "file": "/var/www/montecristosoftware/nextcloud/cron.php",
                "line": 162,
                "function": "execute",
                "class": "OCP\\BackgroundJob\\Job",
                "type": "->"
            }
        ],
        "File": "/var/www/montecristosoftware/nextcloud/apps/dav/lib/CalDAV/CalDavBackend.php",
        "Line": 3044,
        "message": "Unable to denormalize calendar object from subscription {subscriptionId}",
        "exception": [],
        "source": "https://calendar.google.com/calendar/ical/volteuropa.org_3qtptk1l0mfg76gq9nfqq1h4mg%40group.calendar.google.com/public/basic.ics",
        "CustomMessage": "Unable to denormalize calendar object from subscription 28"
    },
    "id": "67b0c49348908"
}

[SebastianKrupinski]

Hi @tchernobog

I think I found the issue, I have come across this issue in the past.

I believe the issue is with the library we use for parsing the events, and RRULES that are missing a BYxxxxx parameter on weekly occurrences.

RRULE:FREQ=WEEKLY;UNTIL=20240612T215959Z

This should contain a BYDAY parameter.

RRULE:FREQ=WEEKLY;UNTIL=20240612T215959Z;BYDAY=WE

[tchernobog]

Thanks @SebastianKrupinski ! I think it is a bug then, right? https://www.rfc-editor.org/rfc/rfc5545 explicitly allows for RRULEs without BYxxxx parameters:

  Weekly until December 24, 1997:

   DTSTART;TZID=America/New_York:19970902T090000
   RRULE:FREQ=WEEKLY;UNTIL=19971224T000000Z

   ==> (1997 9:00 AM EDT) September 2,9,16,23,30;
                          October 7,14,21
       (1997 9:00 AM EST) October 28;
                          November 4,11,18,25;
                          December 2,9,16,23

Would this then be an issue against https://github.com/kewisch/ical.js ?

[SebastianKrupinski]

Thanks @SebastianKrupinski ! I think it is a bug then, right? rfc-editor.org/rfc/rfc5545 explicitly allows for RRULEs without BYxxxx parameters:

Yes its a bug.

Would this then be an issue against kewisch/ical.js ?

Its in the SabreDAV VOBJECT Reader or Event Iterator... It's somewhere in this block of code...

https://github.com/nextcloud/server/blob/509a53c36c485f8232b1d173f7aa0f7e8bfe9eb2/apps/dav/lib/CalDAV/CalDavBackend.php#L3021

Most likely this line... As this is the line that tries to figure out what the last occurrence of the event is...

https://github.com/nextcloud/server/blob/509a53c36c485f8232b1d173f7aa0f7e8bfe9eb2/apps/dav/lib/CalDAV/CalDavBackend.php#L3038

More deeply the issue is most likely in this function, that parses the RRULE...

https://github.com/sabre-io/vobject/blob/fc1c75526bb9cd4f45be9c6908e3e8eac596e037/lib/Recur/RRuleIterator.php#L751