add once for done

README.md

@@ -15,6 +15,7 @@
 
 - 2016/08/24
 	> delete session.go:Session:wg(atomic.WaitGroup). Add session.go:Session:grNum instead to prevent from (Session)Close() block on session.go:Session:wg.Wait()
+	> add once for session.go:Session:done(chan struct{})
 	> version: 0.2.05
 
 - 2016/08/23

session.go

@@ -84,6 +84,7 @@ type Session struct {
 	*gettyConn
 	pkgHandler ReadWriter
 	listener   EventListener
+	once       sync.Once
 	done       chan struct{}
 	readerDone chan struct{} // end reader
 
@@ -239,7 +240,8 @@ func (this *Session) stop() {
 	case <-this.done:
 		return
 	default:
-		close(this.done)
+		// defeat "panic: close of closed channel"
+		this.once.Do(func() { close(this.done) })
 	}
 }