Perform query inflight deduplication

Taking a queue from Go tip, I copied over the new code and tweaked
it a bit for use in Go DNS.

client.go

@@ -36,6 +36,41 @@ type Client struct {
 	WriteTimeout time.Duration     // the net.Conn.SetWriteTimeout value for new connections (ns), defaults to 2 * 1e9
 	TsigSecret   map[string]string // secret(s) for Tsig map[<zonename>]<base64 secret>, zonename must be fully qualified
 	Inflight     bool              // if true suppress multiple outstanding queries for the same Qname, Qtype and Qclass
+	group        singleflight
+}
+
+func (c *Client) exchangeMerge(m *Msg, a string, s net.Conn) (r *Msg, rtt time.Duration, err error) {
+	if !c.Inflight {
+		if s == nil {
+			return c.exchange(m, a)
+		}
+		return c.exchangeConn(m, s)
+	}
+	// This adds a bunch of garbage, TODO(miek).
+	t := "nop"
+	if t1, ok := TypeToString[r.Question[0].Qtype]; ok {
+		t = t1
+	}
+	cl := "nop"
+	if cl1, ok := ClassToString[r.Question[0].Qclass]; ok {
+		cl = cl1
+	}
+	key := r.Question[0].Name + t + cl
+	r, rtt, err, shared := c.group.Do(key, func() (*Msg, time.Duration, error) {
+		if s == nil {
+			return c.exchange(m, a)
+		}
+		return c.exchangeConn(m, s)
+	})
+	if err != nil {
+		return r, rtt, err
+	}
+	if shared {
+		r1 := r.copy()
+		r1.Id = r.Id // Copy Id!
+		r = r1
+	}
+	return r, rtt, nil
 }
 
 // Exchange performs an synchronous query. It sends the message m to the address
@@ -45,6 +80,10 @@ type Client struct {
 //	in, rtt, err := c.Exchange(message, "127.0.0.1:53")
 //
 func (c *Client) Exchange(m *Msg, a string) (r *Msg, rtt time.Duration, err error) {
+	return c.exchangeMerge(m, a, nil)
+}
+
+func (c *Client) exchange(m *Msg, a string) (r *Msg, rtt time.Duration, err error) {
 	w := &reply{client: c, addr: a}
 	if err = w.dial(); err != nil {
 		return nil, 0, err
@@ -60,6 +99,10 @@ func (c *Client) Exchange(m *Msg, a string) (r *Msg, rtt time.Duration, err erro
 // ExchangeConn performs an synchronous query. It sends the message m trough the
 // connection s and waits for a reply.
 func (c *Client) ExchangeConn(m *Msg, s net.Conn) (r *Msg, rtt time.Duration, err error) {
+	return c.exchangeMerge(m, "", s)
+}
+
+func (c *Client) exchangeConn(m *Msg, s net.Conn) (r *Msg, rtt time.Duration, err error) {
 	w := &reply{client: c, conn: s}
 	if err = w.send(m); err != nil {
 		return nil, 0, err