middleware/cache: fix race (#757)

While adding a parallel performance benchmark I stumbled on a race
condition (another reason to add performance benchmarks!), so this
PR makes sure the msg is created in a race free manor and adds the
parallel benchmark.

middleware/cache/cache_test.go

@@ -6,6 +6,8 @@ import (
 	"testing"
 	"time"
 
+	"golang.org/x/net/context"
+
 	"github.com/coredns/coredns/middleware"
 	"github.com/coredns/coredns/middleware/pkg/cache"
 	"github.com/coredns/coredns/middleware/pkg/response"
@@ -205,3 +207,45 @@ func TestCache(t *testing.T) {
 		}
 	}
 }
+
+func BenchmarkCacheResponse(b *testing.B) {
+	c := &Cache{Zones: []string{"."}, pcap: defaultCap, ncap: defaultCap, pttl: maxTTL, nttl: maxTTL}
+	c.pcache = cache.New(c.pcap)
+	c.ncache = cache.New(c.ncap)
+	c.prefetch = 1
+	c.duration = 1 * time.Second
+	c.Next = BackendHandler()
+
+	ctx := context.TODO()
+
+	reqs := make([]*dns.Msg, 5)
+	for i, q := range []string{"example1", "example2", "a", "b", "ddd"} {
+		reqs[i] = new(dns.Msg)
+		reqs[i].SetQuestion(q+".example.org.", dns.TypeA)
+	}
+
+	b.RunParallel(func(pb *testing.PB) {
+		i := 0
+		for pb.Next() {
+			req := reqs[i]
+			c.ServeDNS(ctx, &test.ResponseWriter{}, req)
+			i++
+			i = i % 5
+		}
+	})
+}
+
+func BackendHandler() middleware.Handler {
+	return middleware.HandlerFunc(func(ctx context.Context, w dns.ResponseWriter, r *dns.Msg) (int, error) {
+		m := new(dns.Msg)
+		m.SetReply(r)
+		m.Response = true
+		m.RecursionAvailable = true
+
+		owner := m.Question[0].Name
+		m.Answer = []dns.RR{test.A(owner + " 303 IN A 127.0.0.53")}
+
+		w.WriteMsg(m)
+		return dns.RcodeSuccess, nil
+	})
+}