Hello Philip!
I think there is an issue with this part of the code of rqlite (store/store.go).

func (s *Store) Database(leader bool) ([]byte, error) {
	if leader && s.raft.State() != raft.Leader {
		return nil, ErrNotLeader
	}
	// Ensure only one snapshot can take place at once, and block all queries.
	s.mu.Lock()
	defer s.mu.Unlock()

	f, err := ioutil.TempFile("", "rqlilte-snap-")
	if err != nil {
		return nil, err
	}
	f.Close()
	defer os.Remove(f.Name())

	if err := s.db.Backup(f.Name()); err != nil {
		return nil, err
	}

	return ioutil.ReadFile(f.Name())
}

As far as I understood from your code it's a support function for creating a snapshot of the database. However, this implementation reads the whole database into memory, in your particular case in the variable fsm.database, err = s.Database(false), so if the size of your database is larger than RAM available, I guess the program would crash.

I guess it's better to read the database in buffers or send chunks of the database via a channel when writing it to the so called SnapshotSink.

Thanks!