Function metadata_log_nonempty 

pub fn metadata_log_nonempty(dir: &Path) -> bool

True when the metadata partition directory dir already holds durable raft state, which a node that ran before leaves. A quorum-state file or a log segment that holds bytes is such state.

dir is the metadata partition directory, <metadata.log.dir>/__cluster_metadata-0. The broker binary’s detect_bootstrap_mode calls this so its Bootstrap/Rejoin choice can never disagree with Controller::start_with_listener’s mode validation. KraftController::open creates an empty active segment before the first election, so an empty segment is not state: a node killed mid-election, with that segment but no quorum-state yet, reads as un-formatted and re-Bootstraps rather than dying with “Rejoin requires non-empty raft log”.