[Bug]: GroupPrincipalBackend ignores IHideFromCollaborationBackend

[kesselb]

Bug description

The IHideFromCollaborationBackend interface is used by the guests app (but can be implemented by any group backend) to hide the guest group from auto-completion dialogs. When sharing a calendar or address book, a report request is sent to perform a principal-property-search. The group principal backend currently does not check if the backend implements IHideFromCollaborationBackend and where the hideGroup method may return true.

Steps to reproduce

1. Have guests app
2. shareapi_only_share_with_group_members = no
3. Visit calendar app
4. Open settings for a calendar and type "Guests"
5. See the guests group suggested

Expected behavior

Guests group should be hidden

Nextcloud Server version

master

[kesselb]

I did some debugging today to understand the impact of hiding the guest group from the group’s principal backend. It seems reasonable and would also align with the initial implementation: #14347 and #27761.

However, sharing a calendar with the guest group can also be a useful feature. It works well and seems perfectly fine if done intentionally. Therefore, I’m considering adding a feature flag to the guests app to make hideGroup configurable.

That said, this change would affect every part of the system where we already check for hideGroup, and it would make the group more invisible than it currently is.

cc @icewind1991 @skjnldsv (as app owners)
cc @SebastianKrupinski @st3iny (for caldav)