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/handler.go

@@ -4,22 +4,40 @@ import (
 	"fmt"
 	"io"
 
-	"golang.org/x/net/context"
-
 	"github.com/coredns/coredns/middleware"
 	"github.com/coredns/coredns/middleware/dnstap/msg"
 	"github.com/coredns/coredns/middleware/dnstap/taprw"
 
 	tap "github.com/dnstap/golang-dnstap"
 	"github.com/miekg/dns"
+	"golang.org/x/net/context"
 )
 
+// Dnstap is the dnstap handler.
 type Dnstap struct {
 	Next middleware.Handler
 	Out  io.Writer
 	Pack bool
 }
 
+type (
+	// Tapper is implemented by the Context passed by the dnstap handler.
+	Tapper interface {
+		TapMessage(*tap.Message) error
+		TapBuilder() msg.Builder
+	}
+	tapContext struct {
+		context.Context
+		Dnstap
+	}
+)
+
+// TapperFromContext will return a Tapper if the dnstap middleware is enabled.
+func TapperFromContext(ctx context.Context) (t Tapper) {
+	t, _ = ctx.(Tapper)
+	return
+}
+
 func tapMessageTo(w io.Writer, m *tap.Message) error {
 	frame, err := msg.Marshal(m)
 	if err != nil {
@@ -29,15 +47,22 @@ func tapMessageTo(w io.Writer, m *tap.Message) error {
 	return err
 }
 
+// TapMessage implements Tapper.
 func (h Dnstap) TapMessage(m *tap.Message) error {
 	return tapMessageTo(h.Out, m)
 }
 
+// TapBuilder implements Tapper.
+func (h Dnstap) TapBuilder() msg.Builder {
+	return msg.Builder{Full: h.Pack}
+}
+
+// ServeDNS logs the client query and response to dnstap and passes the dnstap Context.
 func (h Dnstap) ServeDNS(ctx context.Context, w dns.ResponseWriter, r *dns.Msg) (int, error) {
-	rw := &taprw.ResponseWriter{ResponseWriter: w, Taper: &h, Query: r, Pack: h.Pack}
+	rw := &taprw.ResponseWriter{ResponseWriter: w, Tapper: &h, Query: r}
 	rw.QueryEpoch()
 
-	code, err := middleware.NextOrFailure(h.Name(), h.Next, ctx, rw, r)
+	code, err := middleware.NextOrFailure(h.Name(), h.Next, tapContext{ctx, h}, rw, r)
 	if err != nil {
 		// ignore dnstap errors
 		return code, err
@@ -49,4 +74,6 @@ func (h Dnstap) ServeDNS(ctx context.Context, w dns.ResponseWriter, r *dns.Msg)
 
 	return code, nil
 }
+
+// Name returns dnstap.
 func (h Dnstap) Name() string { return "dnstap" }