Custom iCalendar data in Kronolith & Nag

The iCalendar exchange format is everywhere in Horde’s calendar (kronolith) and tasks (nag) apps. It is offered for manual import/export. It is the centerpiece of the CalDAV synchronisation protocol and various APIs of these apps. The format also plays a role in email invitiations and updates sent via iTip. It is a very powerful and well-accepted format. This is, unfortunately, a little bit of a problem. Fortunately, we also have a solution

The iCalendar format was originally defined by the IETF. They released RFC 2445 back in 1998. While Horde still understands that format, a newer version 2.0 was defined in RFC 5545 in 2009. An extension to the 2.0 standard was released in 2016: RFC 7986 defines additional attributes needed to describe related conferencing software resources or other technical aspects.

Server and client software has always been free to add custom attributes to the different components of the standard. For example, Horde Kronolith stores and reads a special attribute X-HORDE-ATTENDEE to mark event attendees who have a horde user attached. Mozilla uses attributes like X-MOZ-LASTACK and MS Exchange comes with a long list of custom attributes, among them X-CALEND. Understanding these attributes when processing a icalendar file is optional. However, servers are expected not to silently drop attributes they do not understand.

Up until now, both Kronolith and Nag did support a wide range of attributes, but far from all. Events are reconstructed from calendar backend contents. Unsupported data simply gets lost. As it is technically impossible to know all attributes anybody out there may use, the opposite definition was needed: An attribute is an “other” attribute if it is not in the list of attributes the app already handles. Both apps got a catalog of attributes they understand and a property for storing everything else. Support for actually storing and retrieving this data is currently limited to the SQL backend. I do not currently invest time in Kolab support, maybe I will never do that.

Also, while I took care to ensure that normal editing via the UI will not break the “other” data, I did not cross check with scenarios where ActiveSync is involved. Please report bugs as you find them.

CalDAV event Storage

For kronolith, an optional storage of unmodified caldav events as received has been added during development. I am not sure if I will keep this or remove it again. It is useful for debugging various support scenarios but under normal operation, it is not required. It may be useful for creating a cache of external caldav calendars though.

Extend to Turba Addressbook

Turba Addressbook uses a similar data format vcard and implements the sister protocol CardDAV. Things are indeed a little more complicated. There are three relevant versions of vcard 2.1, 3.0 and 4.0 and multiple extension RFCs with additional contact attributes exist. Worse, there is a host of X- attributes from popular desktop addressbook vendors, Evolution, Kontact, Thunderbird, Android, MS Exchange/Outlook… There are at least two different ways to represent contact groups and the whole thing is a little messy. But the real problem lies with Turba’s highly configurable nature. At least two backends need to be upgraded: Both SQL storage and LDAP play a key role in typical Turba use cases. The exact layout of each addressbook can be different even inside the same installation. The list of vcard attributes considered as natively supported needs to be calculated for each addressbook and additional attributes maybe need to be stored outside of the specific backend. This will take some time and consideration.

Still, Turba’s tendency to forget what it does not care for needs to be addressed. Otherwise users risk losing attributes on sync.

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