Function gc 

pub async fn gc<T: Transport, C: Clock>(
    engine: &Engine<T, C>,
    probe: &dyn ProcessProbe,
    now_ms: i64,
    before_delete: Option<&dyn Fn()>,
) -> GcReport

The sweep, then the record loop (skipped when session.active cannot be read). before_delete is the test failpoint, called after the grace has elapsed and before the liveness is re-read for the deletion decision; every real caller passes None.