raft/log_unstable.go
func (u *unstable) maybeFirstIndex() (uint64, bool) {
	if u.snapshot != nil {
		return u.snapshot.Metadata.Index + 1, true
	}
	return 0, false
}
func (u *unstable) maybeFirstIndex() (uint64, bool) {
	if u.snapshot != nil {
		return u.snapshot.Metadata.Index + 1, true
	}
	return 0, false
}
// maybeFirstIndex returns the index of the first possible entry in entries
// if it has a snapshot.