Function required_server_signatures_to_verify_event 

pub fn required_server_signatures_to_verify_event(
    object: &BTreeMap<String, CanonicalJsonValue>,
    rules: &SignaturesRules,
) -> Result<BTreeSet<OwnedServerName>, VerificationError>

Available on crate feature signatures only.

Get the list of servers whose signature must be checked to verify the given event.

Applies the rules for validating signatures on received events for populating the list:

• Add the server of the sender, except if it’s an invite event that results from a third-party invite.
• For room versions 1 and 2, add the server of the event_id.
• For room versions that support restricted join rules, if it’s a join event with a join_authorised_via_users_server, add the server of that user.