Adds the dnstap I/O routines and should fix some issues (#1083)

* adds the dnstap I/O thread and should fix a lot of mistakes

* docs

* -race test

* oops

* docs

plugin/dnstap/dnstapio/io.go

@@ -0,0 +1,69 @@
+package dnstapio
+
+import (
+	"fmt"
+	"io"
+
+	tap "github.com/dnstap/golang-dnstap"
+	"github.com/golang/protobuf/proto"
+)
+
+// DnstapIO wraps the dnstap I/O routine.
+type DnstapIO struct {
+	writer io.WriteCloser
+	queue  chan tap.Dnstap
+	stop   chan bool
+}
+
+// Protocol is either `out.TCP` or `out.Socket`.
+type Protocol interface {
+	// Write takes a single frame at once.
+	Write([]byte) (int, error)
+
+	Close() error
+}
+
+// New dnstap I/O routine from Protocol.
+func New(w Protocol) *DnstapIO {
+	dio := DnstapIO{}
+	dio.writer = w
+	dio.queue = make(chan tap.Dnstap, 10)
+	dio.stop = make(chan bool)
+	go dio.serve()
+	return &dio
+}
+
+// Dnstap enqueues the payload for log.
+func (dio *DnstapIO) Dnstap(payload tap.Dnstap) {
+	select {
+	case dio.queue <- payload:
+	default:
+		fmt.Println("[WARN] Dnstap payload dropped.")
+	}
+}
+
+func (dio *DnstapIO) serve() {
+	for {
+		select {
+		case payload := <-dio.queue:
+			frame, err := proto.Marshal(&payload)
+			if err == nil {
+				dio.writer.Write(frame)
+			} else {
+				fmt.Println("[ERROR] Invalid dnstap payload dropped.")
+			}
+		case <-dio.stop:
+			close(dio.queue)
+			dio.stop <- true
+			return
+		}
+	}
+}
+
+// Close waits until the I/O routine is finished to return.
+func (dio DnstapIO) Close() error {
+	dio.stop <- true
+	<-dio.stop
+	close(dio.stop)
+	return dio.writer.Close()
+}