CalDAV imposes a set of constraints on iCal Calendar objects. They are
spelled out in RFC 4791, section 4.1 [1]. Add an exported function to
validate a calendar according to those constraints, and return data that
is necessary for further CalDAV processing and which can only be
extracted if the calendar meets these constraints.
[1]: https://datatracker.ietf.org/doc/html/rfc4791#section-4.1
Currently, the user principal path and the home set path are both
hardcoded to "/", for both CalDAV and CardDAV. This poses a challenge if
one wishes to run a CardDAV and CalDAV server in the same server.
This commit introduces the concept of a UserPrincipalBackend. This
backend must provide the path of the current user's principal URL from
the given request context.
The CalDAV and CardDAV backends are extended to also function as
UserPrincipalBackend. In addition, they are required to supply the path
of the respective home set (`calendar-home-set` and
`addressbook-home-set`). The CardDAV and CalDAV servers act accordingly.
The individual servers will continue to work as before (including the
option of keeping everything at "/"). If one wishes to run CardDAV and
CalDAV in parallel, the new `webdav.ServeUserPrincipal()` can be used as
a convenience function to serve a common user principal URL for both
servers. The input for this function can be easily computed by the
application by getting the home set paths from the backends and using
`caldav.NewCalendarHomeSet()` and `carddav.NewAddressbookHomeSet()` to
create the home sets.
Note that the storage backend will have to know about these paths as
well. For any non-trivial use case, a storage backend should probably
have access to the same UserPrincipalBackend. That is, however, an
implementation detail and doesn't have to be reflected in the
interfaces.
The basic types related to queries and filtering are missing some
features specified in the RFC (as also noted in the TODO comments). This
adds several of the missing elements, working towards being able to
handle all RFC-compliant queries.
The work is not fully done, e.g. the collation for text-match is still
not handled, but it's getting pretty close.
