Add TTL server option

This deprecates the Server.TTL method which has a data race
(detected with Go's race detector).

Fixes libp2p#18

server.go

@@ -22,9 +22,36 @@ const (
 
 var defaultTTL uint32 = 3200
 
+type serverOpts struct {
+	ttl uint32
+}
+
+func applyServerOpts(options ...ServerOption) serverOpts {
+	// Apply default configuration and load supplied options.
+	var conf = serverOpts{
+		ttl: defaultTTL,
+	}
+	for _, o := range options {
+		if o != nil {
+			o(&conf)
+		}
+	}
+	return conf
+}
+
+// ServerOption fills the option struct.
+type ServerOption func(*serverOpts)
+
+// TTL sets the TTL for DNS replies.
+func TTL(ttl uint32) ServerOption {
+	return func(o *serverOpts) {
+		o.ttl = ttl
+	}
+}
+
 // Register a service by given arguments. This call will take the system's hostname
 // and lookup IP by that hostname.
-func Register(instance, service, domain string, port int, text []string, ifaces []net.Interface) (*Server, error) {
+func Register(instance, service, domain string, port int, text []string, ifaces []net.Interface, opts ...ServerOption) (*Server, error) {
 	entry := newServiceEntry(instance, service, domain)
 	entry.Port = port
 	entry.Text = text
@@ -68,7 +95,7 @@ func Register(instance, service, domain string, port int, text []string, ifaces
 		return nil, fmt.Errorf("could not determine host IP addresses")
 	}
 
-	s, err := newServer(ifaces)
+	s, err := newServer(ifaces, applyServerOpts(opts...))
 	if err != nil {
 		return nil, err
 	}
@@ -81,7 +108,7 @@ func Register(instance, service, domain string, port int, text []string, ifaces
 
 // RegisterProxy registers a service proxy. This call will skip the hostname/IP lookup and
 // will use the provided values.
-func RegisterProxy(instance, service, domain string, port int, host string, ips []string, text []string, ifaces []net.Interface) (*Server, error) {
+func RegisterProxy(instance, service, domain string, port int, host string, ips []string, text []string, ifaces []net.Interface, opts ...ServerOption) (*Server, error) {
 	entry := newServiceEntry(instance, service, domain)
 	entry.Port = port
 	entry.Text = text
@@ -124,7 +151,7 @@ func RegisterProxy(instance, service, domain string, port int, host string, ips
 		ifaces = listMulticastInterfaces()
 	}
 
-	s, err := newServer(ifaces)
+	s, err := newServer(ifaces, applyServerOpts(opts...))
 	if err != nil {
 		return nil, err
 	}
@@ -154,7 +181,7 @@ type Server struct {
 }
 
 // Constructs server structure
-func newServer(ifaces []net.Interface) (*Server, error) {
+func newServer(ifaces []net.Interface, opts serverOpts) (*Server, error) {
 	ipv4conn, err4 := joinUdp4Multicast(ifaces)
 	if err4 != nil {
 		log.Printf("[zeroconf] no suitable IPv4 interface: %s", err4.Error())
@@ -172,7 +199,7 @@ func newServer(ifaces []net.Interface) (*Server, error) {
 		ipv4conn:       ipv4conn,
 		ipv6conn:       ipv6conn,
 		ifaces:         ifaces,
-		ttl:            defaultTTL,
+		ttl:            opts.ttl,
 		shouldShutdown: make(chan struct{}),
 	}
 
@@ -199,6 +226,8 @@ func (s *Server) SetText(text []string) {
 }
 
 // TTL sets the TTL for DNS replies
+//
+// Deprecated: This method is racy. Use the TTL server option instead.
 func (s *Server) TTL(ttl uint32) {
 	s.ttl = ttl
 }