middleware/proxy: dnstap (#786)

* experimental dnstap support into proxy

* proxy reports dnstap errors

* refactoring

* add a message builder for less dnstap code

* msg lint

* context

* proxy by DNS: dnstap comments

* TapBuilder

* resolves conflict

* dnstap into ServeDNS

* testing

* more tests

* `go lint`

* doc update

middleware/dnstap/test/helpers.go

@@ -7,11 +7,18 @@ import (
 	"github.com/coredns/coredns/middleware/dnstap/msg"
 
 	tap "github.com/dnstap/golang-dnstap"
+	"golang.org/x/net/context"
 )
 
+// Context is a message trap.
+type Context struct {
+	context.Context
+	TrapTapper
+}
+
+// TestingData returns the Data matching coredns/test.ResponseWriter.
 func TestingData() (d *msg.Data) {
 	d = &msg.Data{
-		Type:        tap.Message_CLIENT_RESPONSE,
 		SocketFam:   tap.SocketFamily_INET,
 		SocketProto: tap.SocketProtocol_UDP,
 		Address:     net.ParseIP("10.240.0.1"),
@@ -50,15 +57,24 @@ func toComp(m *tap.Message) comp {
 	}
 }
 
+// MsgEqual compares two dnstap messages ignoring timestamps.
 func MsgEqual(a, b *tap.Message) bool {
 	return reflect.DeepEqual(toComp(a), toComp(b))
 }
 
-type TrapTaper struct {
+// TrapTapper traps messages.
+type TrapTapper struct {
 	Trap []*tap.Message
+	Full bool
 }
 
-func (t *TrapTaper) TapMessage(m *tap.Message) error {
+// TapMessage adds the message to the trap.
+func (t *TrapTapper) TapMessage(m *tap.Message) error {
 	t.Trap = append(t.Trap, m)
 	return nil
 }
+
+// TapBuilder returns a test msg.Builder.
+func (t *TrapTapper) TapBuilder() msg.Builder {
+	return msg.Builder{Full: t.Full}
+}