Soft failed events cause extremities to build up

[erikjohnston]

This affects v0.99.5 and was introduced in #5146, though that was fixing otherwise broken behaviour.

Broadly, when we receive an event that we soft-fail we do not update the forward extremities with the new event nor remove events that it references. This ensures that the server does not point to the soft failed event when building new events.

However, if another server sends an event that references the soft failed event (typically the original server) that is not soft-failed, then we update the forward extremities table with the new event but do not remove the original extremity referenced by the soft failed event.

The fix to this is to correctly delete the original extremity when the new non-soft-failed event is received.

[erikjohnston]

A query to find the the number of extremities whose parents have been soft failed is:

SELECT
	CASE WHEN internal_metadata IS NOT NULL THEN COALESCE(internal_metadata::json->>'soft_failed', 'false') ELSE '<not referenced>' END AS "Soft Failed",
	COUNT(*) AS "Count"
FROM event_forward_extremities AS f
LEFT JOIN event_edges
	ON prev_event_id = f.event_id
LEFT JOIN event_json
	ON event_edges.event_id = event_json.event_id
WHERE
	f.room_id = <room_id>
GROUP BY CASE WHEN internal_metadata IS NOT NULL THEN COALESCE(internal_metadata::json->>'soft_failed', 'false') ELSE '<not referenced>' END;

Though this double counts if extremities have multiple parents.