/threads includes thread roots that have no thread attached to it anymore

[justjanne]

Description

/threads includes thread roots that have no thread attached to it anymore due to redactions.

Steps to reproduce

1. create a thread
2. write replies
3. redact all replies

Expected Behavior

The /threads endpoint stops including this thread in its response once all responses have been redacted

Actual Behavior

The /threads endpoint still includes the now empty threads

Additional Information

The homeserver correctly removed the bundled relationship from the threads, yet it still includes the roots in the /threads response.

This issue is likely related to the MSC3856 implementation

[clokep]

In #13394 we changed from looking at the "live" event information to a separate table, I suspect we're not tracking event redactions properly in that table. Things to double check:

• Redacting the "last" event in a thread should replace itself with the (now) last event.
• If there is no events left, the thread should be removed.

[clokep]

This will be fixed in 1.70.0rc2.