Fix log plugin benchmark and slightly improve performance (#3004)

* log: use ioutil.Discard as write buffer in benchmark

Using a buffer gives unrealistic stats and consumes a large amount of
memory.

* log: lazily check if a msg should be logged

* log: improve variable name

Change 'ok' to the more descriptive 'shouldLog'.

* log: code comments: don't reuse variable

plugin/log/log.go

@@ -26,20 +26,24 @@ type Logger struct {
 // ServeDNS implements the plugin.Handler interface.
 func (l Logger) ServeDNS(ctx context.Context, w dns.ResponseWriter, r *dns.Msg) (int, error) {
 	state := request.Request{W: w, Req: r}
+	name := state.Name()
 	for _, rule := range l.Rules {
-		if !plugin.Name(rule.NameScope).Matches(state.Name()) {
+		if !plugin.Name(rule.NameScope).Matches(name) {
 			continue
 		}
 
 		rrw := dnstest.NewRecorder(w)
 		rc, err := plugin.NextOrFailure(l.Name(), l.Next, ctx, rrw, r)
 
-		tpe, _ := response.Typify(rrw.Msg, time.Now().UTC())
-		class := response.Classify(tpe)
 		// If we don't set up a class in config, the default "all" will be added
 		// and we shouldn't have an empty rule.Class.
 		_, ok := rule.Class[response.All]
-		_, ok1 := rule.Class[class]
+		var ok1 bool
+		if !ok {
+			tpe, _ := response.Typify(rrw.Msg, time.Now().UTC())
+			class := response.Classify(tpe)
+			_, ok1 = rule.Class[class]
+		}
 		if ok || ok1 {
 			logstr := l.repl.Replace(ctx, state, rrw, rule.Format)
 			clog.Infof(logstr)

plugin/log/log_test.go

@@ -3,6 +3,7 @@ package log
 import (
 	"bytes"
 	"context"
+	"io/ioutil"
 	"log"
 	"strings"
 	"testing"
@@ -239,8 +240,7 @@ func TestLogged(t *testing.T) {
 }
 
 func BenchmarkLogged(b *testing.B) {
-	var f bytes.Buffer
+	log.SetOutput(ioutil.Discard)
-	log.SetOutput(&f)
 
 	rule := Rule{
 		NameScope: ".",