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.go

@@ -113,7 +113,6 @@ func (w *ResponseWriter) WriteMsg(res *dns.Msg) error {
 		cacheSize.WithLabelValues(Denial).Set(float64(w.ncache.Len()))
 	}
 
-	setMsgTTL(res, uint32(duration.Seconds()))
 	if w.prefetch {
 		return nil
 	}

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
+	})
+}

middleware/cache/handler.go

@@ -29,6 +29,7 @@ func (c *Cache) ServeDNS(ctx context.Context, w dns.ResponseWriter, r *dns.Msg)
 	i, ttl := c.get(now, qname, qtype, do)
 	if i != nil && ttl > 0 {
 		resp := i.toMsg(r)
+
 		state.SizeAndDo(resp)
 		resp, _ = state.Scrub(resp)
 		w.WriteMsg(resp)

middleware/cache/item.go

@@ -63,39 +63,35 @@ func (i *item) toMsg(m *dns.Msg) *dns.Msg {
 	m1.Rcode = i.Rcode
 	m1.Compress = true
 
-	m1.Answer = i.Answer
-	m1.Ns = i.Ns
-	m1.Extra = i.Extra
+	m1.Answer = make([]dns.RR, len(i.Answer))
+	m1.Ns = make([]dns.RR, len(i.Ns))
+	m1.Extra = make([]dns.RR, len(i.Extra))
 
-	ttl := int(i.origTTL) - int(time.Now().UTC().Sub(i.stored).Seconds())
-	setMsgTTL(m1, uint32(ttl))
-	return m1
-}
-
-func (i *item) ttl(now time.Time) int {
-	ttl := int(i.origTTL) - int(now.UTC().Sub(i.stored).Seconds())
-	return ttl
-}
-
-// setMsgTTL sets the ttl on all RRs in all sections. If ttl is smaller than minTTL
-// that value is used.
-func setMsgTTL(m *dns.Msg, ttl uint32) {
+	ttl := uint32(i.ttl(time.Now()))
 	if ttl < minTTL {
 		ttl = minTTL
 	}
 
-	for _, r := range m.Answer {
-		r.Header().Ttl = ttl
+	for j, r := range i.Answer {
+		m1.Answer[j] = dns.Copy(r)
+		m1.Answer[j].Header().Ttl = ttl
 	}
-	for _, r := range m.Ns {
-		r.Header().Ttl = ttl
+	for j, r := range i.Ns {
+		m1.Ns[j] = dns.Copy(r)
+		m1.Ns[j].Header().Ttl = ttl
 	}
-	for _, r := range m.Extra {
-		if r.Header().Rrtype == dns.TypeOPT {
-			continue
+	for j, r := range i.Extra {
+		m1.Extra[j] = dns.Copy(r)
+		if m1.Extra[j].Header().Rrtype != dns.TypeOPT {
+			m1.Extra[j].Header().Ttl = ttl
 		}
-		r.Header().Ttl = ttl
 	}
+	return m1
+}
+
+func (i *item) ttl(now time.Time) int {
+	ttl := int(i.origTTL) - int(now.UTC().Sub(i.stored).Seconds())
+	return ttl
 }
 
 func minMsgTTL(m *dns.Msg, mt response.Type) time.Duration {