bug#54939: 29.0.50; icalendar cannot infer the DTEND from DTSTART + DURATION

[John Hamelink]

Hi there,

As part of my ongoing efforts to move entirely to Emacs for email, I've
discovered an incompatibility with calendar invites sent by Fastmail.

After triggering `mu4e-headers-view-message' from `mu4e-headers-mode', I
receive the following:

   Debugger entered--Lisp error: (wrong-type-argument consp nil)
   gnus-icalendar-event--decode-datefield((VEVENT nil ((UID nil
   "064fa1da-e6f1-4463-9f5d-97217cde2c17") (SEQUENCE nil "1") (DTSTAMP
   nil "20220414T130209Z") (CREATED nil "20220414T130209Z") (DTSTART
   (TZID "Europe/London") "20220415T090000") (DURATION nil "PT1H")
   (PRIORITY nil "0") (SUMMARY nil "Demo Event") (STATUS nil "CONFIRMED")
   (TRANSP nil "OPAQUE") (ORGANIZER (X-JMAP-ID "bWVAam9obmhhbWUubGluaw"
   CN "John Hamelink" EMAIL "me@johnhame.link")
   "mailto:me@johnhame.link";) (ATTENDEE (X-JMAP-ID
   "bWVAam9obmhhbWUubGluaw" CN "John Hamelink" EMAIL "me@johnhame.link"
   CUTYPE "INDIVIDUAL" X-JMAP-ROLE "owner" X-JMAP-ROLE "attendee"
   PARTSTAT "ACCEPTED" RSVP "FALSE") "mailto:me@johnhame.link";) (ATTENDEE
   (X-JMAP-ID "am9obkBtZWRpY2NyZWF0aW9ucy5jb20" CN "John Hamelink" EMAIL
   "john@work.com" CUTYPE "INDIVIDUAL" X-JMAP-ROLE "attendee" PARTSTAT
   "NEEDS-ACTION" RSVP "TRUE") "mailto:john@work.com";)) nil) DTEND
   (("Europe/London"
   . "STD-00:00DST-01:00,M3.5.0/01:00:00,M10.5.0/02:00:00")))

This makes sense, because on inspection of the ics file, we see that
DTEND is not part of the file (I added indentation to clarify scope):

   BEGIN:VCALENDAR
   VERSION:2.0
   PRODID:-//CyrusIMAP.org/Cyrus=20
   3.7.0-alpha0-387-g7ea99c4045-fm-20220413.002-g7ea99c40//EN
   METHOD:REQUEST
   CALSCALE:GREGORIAN
   BEGIN:VTIMEZONE
       TZID:Europe/London
       LAST-MODIFIED:20210816T175139Z
       X-LIC-LOCATION:Europe/London
       TZUNTIL:20220415T090000Z
       BEGIN:DAYLIGHT
           TZNAME:BST
           TZOFFSETFROM:+0000
           TZOFFSETTO:+0100
           DTSTART:19810329T010000
           RRULE:FREQ=3DYEARLY;BYMONTH=3D3;BYDAY=3D-1SU
       END:DAYLIGHT
       BEGIN:STANDARD
           TZNAME:GMT
           TZOFFSETFROM:+0100
           TZOFFSETTO:+0000
           DTSTART:19961027T020000
           RRULE:FREQ=3DYEARLY;BYMONTH=3D10;BYDAY=3D-1SU
       END:STANDARD
   END:VTIMEZONE
   BEGIN:VEVENT
       UID:064fa1da-e6f1-4463-9f5d-97217cde2c17
       SEQUENCE:1
       DTSTAMP:20220414T130209Z
       CREATED:20220414T130209Z
       DTSTART;TZID=3DEurope/London:20220415T090000
       DURATION:PT1H
       PRIORITY:0
       SUMMARY:Demo Event
       STATUS:CONFIRMED
       TRANSP:OPAQUE
       ORGANIZER;X-JMAP-ID=3DbWVAam9obmhhbWUubGluaw;CN=3DJohn Hamelink;
       EMAIL=3Dme@johnhame.link:mailto:me@johnhame.link
       ATTENDEE;X-JMAP-ID=3DbWVAam9obmhhbWUubGluaw;CN=3DJohn Hamelink;
       EMAIL=3Dme@johnhame.link;CUTYPE=3DINDIVIDUAL;X-JMAP-ROLE=3Downer;
       X-JMAP-ROLE=3Dattendee;PARTSTAT=3DACCEPTED;RSVP=3DFALSE:mailto:
       me@johnhame.link
       ATTENDEE;X-JMAP-ID=3Dam9obkBtZWRpY2NyZWF0aW9ucy5jb20;CN=3DJohn Hamelink;
       EMAIL=3Djohn@work.com;CUTYPE=3DINDIVIDUAL;X-JMAP-ROLE=3Dattendee;
       PARTSTAT=3DNEEDS-ACTION;RSVP=3DTRUE:mailto:john@work.com
   END:VEVENT
   END:VCALENDAR

However, it seems that from the ics file we did receive, we should be
able to infer the DTEND value by simply adding DURATION to DTSTART.

I've been able to setup a minimal configuration which replicates the
problem using the icalendar file above (and without involving mu4e):

   (package-install 'gnus)
   (package-install 'org)

   (require 'gnus)
   (require 'gnus-icalendar)

   (setq-default gnus-icalendar-org-capture-file "/tmp/agenda.org"
               gnus-icalendar-org-capture-headline '("Calendar"))

   (gnus-icalendar-org-setup)

   (gnus-icalendar-event-from-ical
   '((VCALENDAR nil
           ((VERSION nil "2.0")
           (PRODID nil "-//CyrusIMAP.org/Cyrus 
3.7.0-alpha0-387-g7ea99c4045-fm-20220413.002-g7ea99c40//EN")
           (METHOD nil "REQUEST")
           (CALSCALE nil "GREGORIAN"))
           ((VTIMEZONE nil
                       ((TZID nil "Europe/London")
                       (LAST-MODIFIED nil "20210816T175139Z")
                       (X-LIC-LOCATION nil "Europe/London")
                       (TZUNTIL nil "20220415T090000Z"))
                       ((DAYLIGHT nil (... ... ... ... ...) nil)
                       (STANDARD nil (... ... ... ... ...) nil)))
           (VEVENT nil (
                   (UID nil "064fa1da-e6f1-4463-9f5d-97217cde2c17")
                   (SEQUENCE nil "1")
                   (DTSTAMP nil "20220414T130209Z")
                   (CREATED nil "20220414T130209Z")
                   (DTSTART (TZID "Europe/London") "20220415T090000")
                   (DURATION nil "PT1H")
                   (PRIORITY nil "0")
                   (SUMMARY nil "Demo Event")
                   (STATUS nil "CONFIRMED")
                   (TRANSP nil "OPAQUE")
                   (ORGANIZER
                       (X-JMAP-ID "bWVAam9obmhhbWUubGluaw"
                        CN "John Hamelink"
                        EMAIL "me@johnhame.link") "mailto:me@johnhame.link";)
                   (ATTENDEE
                       (X-JMAP-ID "bWVAam9obmhhbWUubGluaw"
                        CN "John Hamelink"
                        EMAIL "me@johnhame.link"
                        CUTYPE "INDIVIDUAL"
                        X-JMAP-ROLE "owner"
                        X-JMAP-ROLE "attendee"
                        PARTSTAT "ACCEPTED"
                        RSVP "FALSE") "mailto:me@johnhame.link";)
                   (ATTENDEE
                       (X-JMAP-ID "am9obkBtZWRpY2NyZWF0aW9ucy5jb20"
                        CN "John Hamelink"
                        EMAIL "john@work.com"
                        CUTYPE "INDIVIDUAL"
                        X-JMAP-ROLE "attendee"
                        PARTSTAT "NEEDS-ACTION"
                        RSVP "TRUE") "mailto:john@work.com";)) nil)))))


The part of the code which handles the end time is gnus-icalendar.el
line 246, where the code simply assumes that the DTEND property exists:

   :end-time (gnus-icalendar-event--decode-datefield event 'DTEND zone-map)

RFC5545 Section 3.6.1 (the iCalendar spec) says:

   ; Either 'dtend' or 'duration' MAY appear in
   ; a 'eventprop', but 'dtend' and 'duration'
   ; MUST NOT occur in the same 'eventprop'.

And further on:

   For cases where a "VEVENT" calendar component
   specifies a "DTSTART" property with a DATE value type but no
   "DTEND" nor "DURATION" property, the event's duration is taken to
   be one day.  For cases where a "VEVENT" calendar component
   specifies a "DTSTART" property with a DATE-TIME value type but no
   "DTEND" property, the event ends on the same calendar date and
   time of day specified by the "DTSTART" property.

So, I propose we build a function that either returns the end-time using
DTEND as is currently the case, or else to add DTSTART + DURATION and
use that as DTEND if one doesn't exist, or else to use DTSTART + 1 Day
if DTSTART is a date, or else to use DTSTART if DTSTART is a datetime.

How does that sound?

Best,
JH

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