Last snapshot of state machine

[nptai]

void raft_server::on_snapshot_completed
     ( ptr<snapshot>& s, bool result, ptr<std::exception>& err )
{
...
    {
        recur_lock(lock_);
        p_in("snapshot idx %lu log_term %lu created, "
             "compact the log store if needed",
             s->get_last_log_idx(), s->get_last_log_term());

       ptr<snapshot> new_snp = state_machine_->last_snapshot();
...
}

I found that state_machine_->last_snapshot() may change during doing commit_in_bg_exec(), that causes N19_bad_log_idx_for_term
Why don't use s?

[greensky00]

That is intended as new_snp is for calling set_last_snapshot and doing log compaction. As you mentioned, the last snapshot may change so that we always want to pick up the latest one under the lock_.

Can you please elaborate more on N19_bad_log_idx_for_term (log, stack trace)? How did it happen?