raft/read_only.go
func (ro *readOnly) recvAck(m pb.Message) int {
	rs, ok := ro.pendingReadIndex[string(m.Context)]
	if !ok {
		return 0
	}

	rs.acks[m.From] = struct{}{}
	// add one to include an ack from local node
	return len(rs.acks) + 1
}
func (ro *readOnly) recvAck(m pb.Message) int {
	rs, ok := ro.pendingReadIndex[string(m.Context)]
	if !ok {
		return 0
	}

	rs.acks[m.From] = struct{}{}
	// add one to include an ack from local node
	return len(rs.acks) + 1
}
// recvAck notifies the readonly struct that the raft state machine received
// an acknowledgment of the heartbeat that attached with the read only request
// context.