Use sync.Once instead of bool flag

grace.go

@@ -59,15 +59,12 @@ type deadliner interface {
 // Allows for us to notice when the connection is closed.
 type conn struct {
 	net.Conn
-	wg     *sync.WaitGroup
-	closed bool
+	wg   *sync.WaitGroup
+	once sync.Once
 }
 
-func (c *conn) Close() error {
-	if !c.closed {
-		c.closed = true
-		defer c.wg.Done()
-	}
+func (c conn) Close() error {
+	c.once.Do(func() { c.wg.Done() })
 	return c.Conn.Close()
 }
 
@@ -137,7 +134,7 @@ func (l *listener) Accept() (net.Conn, error) {
 		}
 		return nil, err
 	}
-	return &conn{Conn: c, wg: &l.wg}, nil
+	return conn{Conn: c, wg: &l.wg}, nil
 }
 
 type Process struct {