CalDAV Mail Invitations: Accept/decline buttons missing for added attendees via macOS Calendar app

[xMartin]

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Steps to reproduce

1. Create a new event in Apple Calendar app with an attendee
2. Click "send"
3. Add another attendee to the same event
4. Click "send"

Expected behaviour

The added attendee should receive an email invitation with buttons to accept/decline.

Actual behaviour

The added attendee recveives an email invitation without buttons to accept/decline.
(The first attendee does get the buttons in their email.)

Server configuration

Operating system:
Linux

PHP version:
7.4

Nextcloud version: (see Nextcloud admin page)
20.0.1

Updated from an older Nextcloud/ownCloud or fresh install:
Quite a history of updates

Client configuration

Calendar app:
Apple macOS Calendar app 11.0 (2760.4.1)

Operating system:
macOS 10.15.7

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macOS Calendar app doesn't seem to add RSVP:TRUE for added attendees. It could be a bug on their side, but using Google calendar server with macOS Calendar app does show these buttons so they seem to work around this.

server/apps/dav/lib/CalDAV/Schedule/IMipPlugin.php

Line 250 in 0f7fb7c

if (($method == self::METHOD_REQUEST) && $this->getAttendeeRSVP($attendee)) {

checks for this. Is it really necessary?

[szaimen]

Duplicate of #21189