plugin/cache: unroll minTTL loop (#1773)

This allocates memory because of the append, just unroll the loop.
Also add benchmark.

Before:
goos: linux
goarch: amd64
pkg: github.com/coredns/coredns/plugin/cache
BenchmarkCacheResponse-4   	  100000	     11910 ns/op
BenchmarkMinMsgTTL-4       	 1000000	      1494 ns/op
PASS

After:
goos: linux
goarch: amd64
pkg: github.com/coredns/coredns/plugin/cache
BenchmarkCacheResponse-4   	  100000	     12016 ns/op
BenchmarkMinMsgTTL-4       	 2000000	       668 ns/op
PASS

2 files changed, 57 insertions, 2 deletions

diff --git a/plugin/cache/item.go b/plugin/cache/item.go
index 094b8bb2e..5761cdf87 100644
--- a/plugin/cache/item.go
+++ b/plugin/cache/item.go
@@ -99,7 +99,32 @@ func minMsgTTL(m *dns.Msg, mt response.Type) time.Duration {
 	}
 
 	minTTL := maxTTL
-	for _, r := range append(append(m.Answer, m.Ns...), m.Extra...) {
+	for _, r := range m.Answer {
+		switch mt {
+		case response.NameError, response.NoData:
+			if r.Header().Rrtype == dns.TypeSOA {
+				minTTL = time.Duration(r.(*dns.SOA).Minttl) * time.Second
+			}
+		case response.NoError, response.Delegation:
+			if r.Header().Ttl < uint32(minTTL.Seconds()) {
+				minTTL = time.Duration(r.Header().Ttl) * time.Second
+			}
+		}
+	}
+	for _, r := range m.Ns {
+		switch mt {
+		case response.NameError, response.NoData:
+			if r.Header().Rrtype == dns.TypeSOA {
+				minTTL = time.Duration(r.(*dns.SOA).Minttl) * time.Second
+			}
+		case response.NoError, response.Delegation:
+			if r.Header().Ttl < uint32(minTTL.Seconds()) {
+				minTTL = time.Duration(r.Header().Ttl) * time.Second
+			}
+		}
+	}
+
+	for _, r := range m.Extra {
 		if r.Header().Rrtype == dns.TypeOPT {
 			// OPT records use TTL field for extended rcode and flags
 			continue
@@ -107,7 +132,7 @@ func minMsgTTL(m *dns.Msg, mt response.Type) time.Duration {
 		switch mt {
 		case response.NameError, response.NoData:
 			if r.Header().Rrtype == dns.TypeSOA {
-				return time.Duration(r.(*dns.SOA).Minttl) * time.Second
+				minTTL = time.Duration(r.(*dns.SOA).Minttl) * time.Second
 			}
 		case response.NoError, response.Delegation:
 			if r.Header().Ttl < uint32(minTTL.Seconds()) {
diff --git a/plugin/cache/minttl_test.go b/plugin/cache/minttl_test.go
index 5ce4f2341..376c638a1 100644
--- a/plugin/cache/minttl_test.go
+++ b/plugin/cache/minttl_test.go
@@ -40,3 +40,33 @@ func TestMinMsgTTL(t *testing.T) {
 		t.Fatalf("Expected minttl duration to be %d, got %d", 3600, dur)
 	}
 }
+
+func BenchmarkMinMsgTTL(b *testing.B) {
+	m := new(dns.Msg)
+	m.SetQuestion("example.org.", dns.TypeA)
+	m.Ns = []dns.RR{
+		test.A("a.example.org. 	1800	IN	A 127.0.0.53"),
+		test.A("b.example.org. 	1900	IN	A 127.0.0.53"),
+		test.A("c.example.org. 	1600	IN	A 127.0.0.53"),
+		test.A("d.example.org. 	1100	IN	A 127.0.0.53"),
+		test.A("e.example.org. 	1000	IN	A 127.0.0.53"),
+	}
+	m.Extra = []dns.RR{
+		test.A("a.example.org. 	1800	IN	A 127.0.0.53"),
+		test.A("b.example.org. 	1600	IN	A 127.0.0.53"),
+		test.A("c.example.org. 	1400	IN	A 127.0.0.53"),
+		test.A("d.example.org. 	1200	IN	A 127.0.0.53"),
+		test.A("e.example.org. 	1100	IN	A 127.0.0.53"),
+	}
+
+	utc := time.Now().UTC()
+	mt, _ := response.Typify(m, utc)
+
+	b.ResetTimer()
+	for i := 0; i < b.N; i++ {
+		dur := minMsgTTL(m, mt)
+		if dur != 1000*time.Second {
+			b.Fatalf("Wrong minMsgTTL %d, expected %d", dur, 1000*time.Second)
+		}
+	}
+}