CopyFrom pattern for DNS and use it for Cookie

dns/client.go

@@ -91,6 +91,18 @@ func (c *Client) isClosed() bool {
 	return c.state == dnsClosed || c.state == dnsAborted
 }
 
+func (c *Client) MessageCopyTo(dst *Message) (done bool, err error) {
+	if !c.respFlags.IsResponse() {
+		return false, nil
+	}
+	dst.CopyFrom(c.msg)
+	rcode := c.respFlags.ResponseCode()
+	if rcode != 0 {
+		return true, rcode
+	}
+	return true, nil
+}
+
 func (c *Client) Answers() []Resource {
 	if c.state != dnsDone {
 		return nil

dns/definitions.go

@@ -9,6 +9,7 @@ import (
 
 // common errors. Taken from golang.org/x/net/dns/dnsmessage module.
 var (
+	errNoResponse         = errors.New("no DNS response")
 	errNameTooLong        = errors.New("DNS name exceeds maximum length")
 	errNoNullTerm         = errors.New("DNS name missing null terminator")
 	errCalcLen            = errors.New("DNS calculated name label length exceeds remaining buffer length")
@@ -141,6 +142,10 @@ func (flags HeaderFlags) String() string {
 	return string(flags.appendF(buf))
 }
 
+func (flags RCode) Error() string {
+	return flags.String()
+}
+
 func (flags HeaderFlags) appendF(buf []byte) []byte {
 	writeBit := func(b bool, s string) {
 		if b {

dns/dns.go

@@ -245,7 +245,7 @@ func (m *Message) AddQuestions(questions []Question) {
 	m.Questions = slices.Grow(m.Questions, len(questions))
 	m.Questions = m.Questions[:qoff+len(questions)]
 	for i := range questions {
-		m.Questions[qoff+i].Name.CloneFrom(questions[i].Name)
+		m.Questions[qoff+i].Name.CopyFrom(questions[i].Name)
 		m.Questions[qoff+i].Type = questions[i].Type
 		m.Questions[qoff+i].Class = questions[i].Class
 	}
@@ -418,7 +418,7 @@ func (n *Name) Len() uint16 {
 	return uint16(len(n.data))
 }
 
-func (n *Name) CloneFrom(ex Name) {
+func (n *Name) CopyFrom(ex Name) {
 	n.data = append(n.data[:0], ex.data...)
 }
 
@@ -573,3 +573,48 @@ LOOP:
 	}
 	return newOff, nil
 }
+
+func (dst *Message) CopyFrom(m Message) {
+	reuseGrowSlice(&dst.Questions, len(m.Questions))
+	reuseGrowSlice(&dst.Answers, len(m.Answers))
+	reuseGrowSlice(&dst.Authorities, len(m.Authorities))
+	reuseGrowSlice(&dst.Additionals, len(m.Additionals))
+	for i := range dst.Questions {
+		dst.Questions[i].CopyFrom(m.Questions[i])
+	}
+	for i := range dst.Answers {
+		dst.Answers[i].CopyFrom(m.Answers[i])
+	}
+	for i := range dst.Answers {
+		dst.Authorities[i].CopyFrom(m.Authorities[i])
+	}
+	for i := range dst.Answers {
+		dst.Additionals[i].CopyFrom(m.Additionals[i])
+	}
+}
+
+func (dst *Question) CopyFrom(q Question) {
+	dst.Name.CopyFrom(q.Name)
+	dst.Class = q.Class
+	dst.Type = q.Type
+}
+
+func (dst *Resource) CopyFrom(r Resource) {
+	dst.Header.CopyFrom(r.Header)
+	dst.data = append(dst.data[:0], r.data...)
+}
+
+func (dst *ResourceHeader) CopyFrom(rh ResourceHeader) {
+	dst.Name.CopyFrom(rh.Name)
+	dst.Type = rh.Type
+	dst.Class = rh.Class
+	dst.TTL = rh.TTL
+	dst.Length = rh.Length
+}
+
+func reuseGrowSlice[T any](dst *[]T, n int) {
+	if n == 0 {
+		return
+	}
+	*dst = slices.Grow(*dst, n)[:n]
+}