Ensure that the WaitGroup isn't marked done twice when edgecase errors occurr

grace.go

@@ -60,10 +60,14 @@ type deadliner interface {
 type conn struct {
 	net.Conn
 	wg     *sync.WaitGroup
+	closed bool
 }
 
-func (c conn) Close() error {
+func (c *conn) Close() error {
+	if !c.closed {
+		c.closed = true
 		defer c.wg.Done()
+	}
 	return c.Conn.Close()
 }
 
@@ -133,7 +137,7 @@ 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 {