dnstest: add multirecorder (#1326)

* dnstest: add multirecorder

This adds a new recorder that captures all messages written to it. This
can be useful when, for instance, testing AXFR which can write muliple
messages back to the client.

* docs

plugin/pkg/dnstest/multirecorder.go

@@ -0,0 +1,41 @@
+package dnstest
+
+import (
+	"time"
+
+	"github.com/miekg/dns"
+)
+
+// MultiRecorder is a type of ResponseWriter that captures all messages written to it.
+type MultiRecorder struct {
+	Len   int
+	Msgs  []*dns.Msg
+	Start time.Time
+	dns.ResponseWriter
+}
+
+// NewMultiRecorder makes and returns a new MultiRecorder.
+func NewMultiRecorder(w dns.ResponseWriter) *MultiRecorder {
+	return &MultiRecorder{
+		ResponseWriter: w,
+		Msgs:           make([]*dns.Msg, 0),
+		Start:          time.Now(),
+	}
+}
+
+// WriteMsg records the message and its length written to it and call the
+// underlying ResponseWriter's WriteMsg method.
+func (r *MultiRecorder) WriteMsg(res *dns.Msg) error {
+	r.Len += res.Len()
+	r.Msgs = append(r.Msgs, res)
+	return r.ResponseWriter.WriteMsg(res)
+}
+
+// Write is a wrapper that records the length of the messages that get written to it.
+func (r *MultiRecorder) Write(buf []byte) (int, error) {
+	n, err := r.ResponseWriter.Write(buf)
+	if err == nil {
+		r.Len += n
+	}
+	return n, err
+}