middleware/proxy: make it scale (#287)

* middleware/proxy

Use connection pooling for communicating with an upstream, instead of
opening a new socket every time.

This makes the proxy more efficient and allowed for some cleanups.

* Some cleanups

* Some fixes

* more

* Kill pool

* Add nil check

* remove pool

test/proxy_test.go

@@ -62,3 +62,41 @@ func TestLookupProxy(t *testing.T) {
 		t.Errorf("Expected 127.0.0.1, got: %s", resp.Answer[0].(*dns.A).A.String())
 	}
 }
+
+func BenchmarkLookupProxy(b *testing.B) {
+	t := new(testing.T)
+	name, rm, err := test.TempFile(t, ".", exampleOrg)
+	if err != nil {
+		t.Fatalf("failed to created zone: %s", err)
+	}
+	defer rm()
+
+	corefile := `example.org:0 {
+       file ` + name + `
+}
+`
+
+	i, err := CoreDNSServer(corefile)
+	if err != nil {
+		t.Fatalf("could not get CoreDNS serving instance: %s", err)
+	}
+
+	udp, _ := CoreDNSServerPorts(i, 0)
+	if udp == "" {
+		t.Fatalf("could not get udp listening port")
+	}
+	defer i.Stop()
+
+	log.SetOutput(ioutil.Discard)
+
+	p := proxy.New([]string{udp})
+	state := request.Request{W: &test.ResponseWriter{}, Req: new(dns.Msg)}
+
+	b.ResetTimer()
+	for i := 0; i < b.N; i++ {
+		_, err := p.Lookup(state, "example.org.", dns.TypeA)
+		if err != nil {
+			b.Fatal("Expected to receive reply, but didn't")
+		}
+	}
+}