[Bug]: Cannot reshare calendar to group to user after user has unshared

⚠️ This issue respects the following points: ⚠️

• This is a bug, not a question or a configuration/webserver/proxy issue.
• This issue is not already reported on Github OR Nextcloud Community Forum (I've searched it).
• Nextcloud Server is up to date. See Maintenance and Release Schedule for supported versions.
• I agree to follow Nextcloud's Code of Conduct.

Bug description

Cannot reshare calendar to group to user after user has unshared

Steps to reproduce

1. Add a user to a group
2. Share calendar to the group
3. For user in group select unshare calendar
4. Remove group from share
5. Add group back to sharing for the calendar

Expected behavior

After the calendar is reshared to the group it still does not appear to the user that originally unshared the calendar.

The use case here is that a user either inadvertently unshares themself from the shared calendar or unshared and then decided they need to see the calendar again. I thought that removing the group and adding back would be a work around but it seems that when the calendar is unshared by the user its permanent.,

Nextcloud Server version

29

Operating system

Other

PHP engine version

None

Web server

None

Database engine version

None

Is this bug present after an update or on a fresh install?

None

Are you using the Nextcloud Server Encryption module?

None

What user-backends are you using?

• Default user-backend (database)
• LDAP/ Active Directory
• SSO - SAML
• Other

Configuration report

List of activated Apps

Nextcloud Signing status

Nextcloud Logs

Additional info

This is related to https://github.com/nextcloud/server/issues/1653 which was closed. Originally Unsharing was not allowed which I think is better behavior but if that is something users want then there should be a means to atleast force the calendar to be reseen by the user that is in the group.

No source files or tests are identified in the issue. Start by reproducing the six-step group-sharing sequence on Nextcloud Server 29, then review the related issue #1653 and trace the calendar-sharing behavior. Done means a user who previously unshared the calendar can see it again after the group is removed and reshared, with a regression test covering that sequence.