From a4af4dd14e1115ec63796347be5278bf401bcc0c Mon Sep 17 00:00:00 2001
From: Daniel Morsing <daniel@cloudflare.com>
Date: Thu, 5 Feb 2015 13:07:07 +0000
Subject: [PATCH] Used shared backing array for Msg.Copy

If you have a system with large amounts of copies, these slice
allocations start stacking up. Use a shared slice and then subslice
them with a cap limit so that append works properly.

Also, add a benchmark and test for Msg.Copy

Benchcmp:
benchmark         old ns/op     new ns/op     delta
BenchmarkCopy     1880          1672          -11.06%

benchmark         old allocs     new allocs     delta
BenchmarkCopy     13             11             -15.38%

benchmark         old bytes     new bytes     delta
BenchmarkCopy     528           528           +0.00%
---
 dns_test.go | 42 ++++++++++++++++++++++++++++++++++++++++++
 msg.go      | 21 +++++++++++++++------
 2 files changed, 57 insertions(+), 6 deletions(-)

diff --git a/dns_test.go b/dns_test.go
index df1e99f3..0b30af6f 100644
--- a/dns_test.go
+++ b/dns_test.go
@@ -510,6 +510,48 @@ func TestCopy(t *testing.T) {
 	}
 }
 
+func TestMsgCopy(t *testing.T) {
+	m := new(Msg)
+	m.SetQuestion("miek.nl.", TypeA)
+	rr, _ := NewRR("miek.nl. 2311 IN A 127.0.0.1")
+	m.Answer = []RR{rr}
+	rr, _ = NewRR("miek.nl. 2311 IN NS 127.0.0.1")
+	m.Ns = []RR{rr}
+
+	m1 := m.Copy()
+	if m.String() != m1.String() {
+		t.Fatalf("Msg.Copy() failed %s != %s", m.String(), m1.String())
+	}
+
+	m1.Answer[0], _ = NewRR("somethingelse.nl. 2311 IN A 127.0.0.1")
+	if m.String() == m1.String() {
+		t.Fatalf("Msg.Copy() failed; change to copy changed template %s", m.String())
+	}
+
+	rr, _ = NewRR("miek.nl. 2311 IN A 127.0.0.2")
+	m1.Answer = append(m1.Answer, rr)
+	if m1.Ns[0].String() == m1.Answer[1].String() {
+		t.Fatalf("Msg.Copy() failed; append changed underlying array %s", m1.Ns[0].String())
+	}
+}
+
+func BenchmarkCopy(b *testing.B) {
+	b.ReportAllocs()
+	m := new(Msg)
+	m.SetQuestion("miek.nl.", TypeA)
+	rr, _ := NewRR("miek.nl. 2311 IN A 127.0.0.1")
+	m.Answer = []RR{rr}
+	rr, _ = NewRR("miek.nl. 2311 IN NS 127.0.0.1")
+	m.Ns = []RR{rr}
+	rr, _ = NewRR("miek.nl. 2311 IN A 127.0.0.1")
+	m.Extra = []RR{rr}
+
+	b.ResetTimer()
+	for i := 0; i < b.N; i++ {
+		m.Copy()
+	}
+}
+
 func TestPackIPSECKEY(t *testing.T) {
 	tests := []string{
 		"38.2.0.192.in-addr.arpa. 7200 IN     IPSECKEY ( 10 1 2 192.0.2.38 AQNRU3mG7TVTO2BkR47usntb102uFJtugbo6BSGvgqt4AQ== )",
diff --git a/msg.go b/msg.go
index 578fe924..aded9653 100644
--- a/msg.go
+++ b/msg.go
@@ -1912,25 +1912,34 @@ func (dns *Msg) Copy() *Msg {
 		copy(r1.Question, dns.Question) // TODO(miek): Question is an immutable value, ok to do a shallow-copy
 	}
 
+	rrArr := make([]RR, len(dns.Answer)+len(dns.Ns)+len(dns.Extra))
+	var rri int
+
 	if len(dns.Answer) > 0 {
-		r1.Answer = make([]RR, len(dns.Answer))
+		rrbegin := rri
 		for i := 0; i < len(dns.Answer); i++ {
-			r1.Answer[i] = dns.Answer[i].copy()
+			rrArr[rri] = dns.Answer[i].copy()
+			rri++
 		}
+		r1.Answer = rrArr[rrbegin:rri:rri]
 	}
 
 	if len(dns.Ns) > 0 {
-		r1.Ns = make([]RR, len(dns.Ns))
+		rrbegin := rri
 		for i := 0; i < len(dns.Ns); i++ {
-			r1.Ns[i] = dns.Ns[i].copy()
+			rrArr[rri] = dns.Ns[i].copy()
+			rri++
 		}
+		r1.Ns = rrArr[rrbegin:rri:rri]
 	}
 
 	if len(dns.Extra) > 0 {
-		r1.Extra = make([]RR, len(dns.Extra))
+		rrbegin := rri
 		for i := 0; i < len(dns.Extra); i++ {
-			r1.Extra[i] = dns.Extra[i].copy()
+			rrArr[rri] = dns.Extra[i].copy()
+			rri++
 		}
+		r1.Extra = rrArr[rrbegin:rri:rri]
 	}
 
 	return r1