Complete multicast requests on first response

This is the simplest thing that works, is consistent with what happens
if there's a local answer, and avoids having all multicast requests
last 250ms or more.

Since we're no longer waiting *at least* the timeout period, I've
bumped the multicast response timeout up to half a second.

There was also a bug whereby the query would be entered into the
inflight map even if the socket write had failed (and the channel had
been closed).

server/mdns_client.go

@@ -14,9 +14,9 @@ const (
 	mdnsPort = 5353          // mDNS assigned port
 	// We wait this long to hear responses from other mDNS servers on the network.
 	// TODO: introduce caching so we don't have to wait this long on every call.
-	mDNSTimeout = 250 * time.Millisecond
+	mDNSTimeout = 500 * time.Millisecond
 	MaxDuration = time.Duration(math.MaxInt64)
-	ChannelSize = 16
+	MailboxSize = 16
 )
 
 var (
@@ -88,7 +88,7 @@ func (c *MDNSClient) Start(ifi *net.Interface) error {
 	c.server = &dns.Server{Listener: nil, PacketConn: multicast, Handler: dns.HandlerFunc(handleMDNS)}
 	go c.server.ActivateAndServe()
 
-	queryChan := make(chan *MDNSInteraction, ChannelSize)
+	queryChan := make(chan *MDNSInteraction, MailboxSize)
 	c.queryChan = queryChan
 	go c.queryLoop(queryChan)
 
@@ -216,13 +216,13 @@ func (c *MDNSClient) handleSendQuery(q mDNSQueryInfo) {
 			name: q.name,
 			id:   m.Id,
 		}
-		c.inflight[q.name] = query
 		_, err = c.conn.WriteTo(buf, c.addr)
 		if err != nil {
 			q.responseCh <- &ResponseA{Err: err}
 			close(q.responseCh)
 			return
 		}
+		c.inflight[q.name] = query
 	}
 	info := &responseInfo{
 		ch:      q.responseCh,
@@ -237,10 +237,13 @@ func (c *MDNSClient) handleResponse(r *dns.Msg) {
 	for _, answer := range r.Answer {
 		switch rr := answer.(type) {
 		case *dns.A:
-			if query, found := c.inflight[rr.Hdr.Name]; found {
+			name := rr.Hdr.Name
+			if query, found := c.inflight[name]; found {
 				for _, resp := range query.responseInfos {
 					resp.ch <- &ResponseA{Name: rr.Hdr.Name, Addr: rr.A}
+					close(resp.ch)
 				}
+				delete(c.inflight, name)
 			} else {
 				// We've received a response that didn't match a query
 				// Do we want to cache it?

server/server.go

@@ -41,10 +41,9 @@ func queryHandler(zone Zone, mdnsClient *MDNSClient) dns.HandlerFunc {
 		} else {
 			Debug.Printf("Failed lookup for %s; sending mDNS query", q.Name)
 			// We don't know the answer; see if someone else does
-			channel := make(chan *ResponseA, ChannelSize)
+			channel := make(chan *ResponseA)
 			replies := make([]net.IP, 0)
 			go func() {
-				// Loop terminates when channel is closed by MDNSClient on timeout
 				for resp := range channel {
 					Debug.Printf("Got address response %s to query %s addr %s", resp.Name, q.Name, resp.Addr)
 					replies = append(replies, resp.Addr)