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

[John Hamelink <me@johnhame.link>]

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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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