derp: don't start sending keepalives to client until after serverInfo sent

Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

derp/derp_server.go

@@ -122,16 +122,11 @@ func (s *Server) accept(nc net.Conn, brw *bufio.ReadWriter) error {
 	if clientInfo != nil {
 		c.info = *clientInfo
 	}
-	go func() {
-		if err := c.keepAlive(ctx); err != nil {
-			s.logf("derp: %s: client %x: keep alive failed: %v", nc.RemoteAddr(), c.key, err)
-		}
-	}()
 
 	defer func() {
 		s.mu.Lock()
 		curClient := s.clients[c.key]
-		if curClient != nil && curClient.nc == nc {
+		if curClient == c {
 			s.logf("derp: %s: client %x: removing connection", nc.RemoteAddr(), c.key)
 			delete(s.clients, c.key)
 		}
@@ -159,6 +154,12 @@ func (s *Server) accept(nc net.Conn, brw *bufio.ReadWriter) error {
 		s.logf("derp: %s: client %x: adding connection, replacing %s", nc.RemoteAddr(), c.key, oldClient.nc.RemoteAddr())
 	}
 
+	go func() {
+		if err := c.keepAlive(ctx); err != nil {
+			s.logf("derp: %s: client %x: keep alive failed: %v", nc.RemoteAddr(), c.key, err)
+		}
+	}()
+
 	for {
 		dstKey, contents, err := s.recvPacket(c.br)
 		if err != nil {