Add negative cache

Cache negative results, too, but only with half TTL, as configured.

Plus get rid of "mesg.Id = req.Id" data race (build with -race flag)
at once without locking.

Found another data race: delete must be protected with Lock,
RLock is not enough.

cache.go

@@ -58,7 +58,7 @@ type MemoryCache struct {
 	Backend  map[string]Mesg
 	Expire   time.Duration
 	Maxcount int
-	mu       *sync.RWMutex
+	mu       sync.RWMutex
 }
 
 func (c *MemoryCache) Get(key string) (*dns.Msg, error) {
@@ -92,9 +92,9 @@ func (c *MemoryCache) Set(key string, msg *dns.Msg) error {
 }
 
 func (c *MemoryCache) Remove(key string) {
-	c.mu.RLock()
+	c.mu.Lock()
 	delete(c.Backend, key)
-	c.mu.RUnlock()
+	c.mu.Unlock()
 }
 
 func (c *MemoryCache) Exists(key string) bool {

handler.go

@@ -1,7 +1,6 @@
 package main
 
 import (
-	"sync"
 	"time"
 
 	"github.com/miekg/dns"
@@ -24,18 +23,18 @@ func (q *Question) String() string {
 }
 
 type GODNSHandler struct {
-	resolver *Resolver
-	cache    Cache
-	hosts    Hosts
+	resolver        *Resolver
+	cache, negCache Cache
+	hosts           Hosts
 }
 
 func NewHandler() *GODNSHandler {
 
 	var (
-		clientConfig *dns.ClientConfig
-		cacheConfig  CacheSettings
-		resolver     *Resolver
-		cache        Cache
+		clientConfig    *dns.ClientConfig
+		cacheConfig     CacheSettings
+		resolver        *Resolver
+		cache, negCache Cache
 	)
 
 	resolvConfig := settings.ResolvConfig
@@ -52,10 +51,14 @@ func NewHandler() *GODNSHandler {
 	switch cacheConfig.Backend {
 	case "memory":
 		cache = &MemoryCache{
-			Backend:  make(map[string]Mesg),
+			Backend:  make(map[string]Mesg, cacheConfig.Maxcount),
 			Expire:   time.Duration(cacheConfig.Expire) * time.Second,
 			Maxcount: cacheConfig.Maxcount,
-			mu:       new(sync.RWMutex),
+		}
+		negCache = &MemoryCache{
+			Backend:  make(map[string]Mesg),
+			Expire:   time.Duration(cacheConfig.Expire) * time.Second / 2,
+			Maxcount: cacheConfig.Maxcount,
 		}
 	case "redis":
 		// cache = &MemoryCache{
@@ -72,7 +75,7 @@ func NewHandler() *GODNSHandler {
 
 	hosts := NewHosts(settings.Hosts, settings.Redis)
 
-	return &GODNSHandler{resolver, cache, hosts}
+	return &GODNSHandler{resolver, cache, negCache, hosts}
 }
 
 func (h *GODNSHandler) do(Net string, w dns.ResponseWriter, req *dns.Msg) {
@@ -123,11 +126,19 @@ func (h *GODNSHandler) do(Net string, w dns.ResponseWriter, req *dns.Msg) {
 	if IPQuery > 0 {
 		mesg, err := h.cache.Get(key)
 		if err != nil {
-			Debug("%s didn't hit cache: %s", Q.String(), err)
+			if mesg, err = h.negCache.Get(key); err != nil {
+				Debug("%s didn't hit cache: %s", Q.String(), err)
+			} else {
+				Debug("%s hit negative cache", Q.String())
+				dns.HandleFailed(w, req)
+				return
+			}
 		} else {
 			Debug("%s hit cache", Q.String())
-			mesg.Id = req.Id
-			w.WriteMsg(mesg)
+			// we need this copy against concurrent modification of Id
+			msg := *mesg
+			msg.Id = req.Id
+			w.WriteMsg(&msg)
 			return
 		}
 	}
@@ -137,6 +148,11 @@ func (h *GODNSHandler) do(Net string, w dns.ResponseWriter, req *dns.Msg) {
 	if err != nil {
 		Debug("%s", err)
 		dns.HandleFailed(w, req)
+
+		// cache the failure, too!
+		if err = h.negCache.Set(key, nil); err != nil {
+			Debug("Set %s negative cache failed: %v", Q.String(), err)
+		}
 		return
 	}