bug#64023: 29.0.91; gnus-icalendar does not update timestamp when description is empty

[Reindert-Jan Ekker]

I use mu4e for my email and org-mode for managing my calendar.

When I accept a calendar invitation that has *no description* using
gnus-icalendar, and later I receive an update for that invitation, the
code that updates the corresponding org element will remove the
timestamp completely.

This is quite common for invitations from e.g. thunderbird, that have
only a summary but where the description field has been left empty.

To show this in action we would need a complete mu4e and gnus-icalendar
setup, but we can recreate the effect as follows:

we have an org file in "~/test.org" containing an event exported from an
icalendar invitation:

----- contents of test.org -----
* Test
:PROPERTIES:
:ICAL_EVENT: t
:ID:       xxx-11-yyy
:ORGANIZER: Me
:LOCATION:
:PARTICIPATION_TYPE: non-participant
:REQ_PARTICIPANTS:
:OPT_PARTICIPANTS:
:RRULE:
:REPLY:    Not replied yet
:END:

  <2023-08-01 01:00-01:00>
----- end of test.org -----

We can now call the function to update this event to a new date as
follows (this works with emacs -Q):


(require 'gnus-icalendar)
(let ((ev (gnus-icalendar-event :organizer "Me" :summary "Test"
                                :start-time (encode-time '(0 0 0 1 9 2023 nil 
nil nil))
                                :end-time (encode-time '(0 0 0 1 9 2023 nil nil 
nil))
                                :uid "xxx-11-yyy"
                                :recur nil
                                :description nil)))
  (gnus-icalendar--update-org-event ev nil (list (expand-file-name 
"~/test.org"))))

This results in the following:

----- new contents of test.org -----
* [#C] Test ()
:PROPERTIES:
:ICAL_EVENT: t
:ID:       xxx-11-yyy
:ORGANIZER: Me
:PARTICIPATION_TYPE: non-participant
:REPLY:    Not replied yet
:END:
----- end of test.org -----

Note how the timestamp has not been updated but completely erased.

This is because in gnus-icalendar--update-org-event, the code for
updating the timestamp is inside "(when description ...)", causing it to
only be run when the description is not empty.

-----

In GNU Emacs 29.0.91 (build 1, aarch64-apple-darwin21.6.0, NS
 appkit-2113.60 Version 12.6.5 (Build 21G531)) of 2023-05-31 built on
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Windowing system distributor 'Apple', version 10.3.2299
System Description:  macOS 13.4

Configured using:
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-- 
Reindert-Jan Ekker
Code Sensei