net_transport.go
func (n *NetworkTransport) returnConn(conn *netConn) {
	n.connPoolLock.Lock()
	defer n.connPoolLock.Unlock()

	key := conn.target
	conns, _ := n.connPool[key]

	if !n.IsShutdown() && len(conns) < n.maxPool {
		n.connPool[key] = append(conns, conn)
	} else {
		conn.Release()
	}
}
func (n *NetworkTransport) returnConn(conn *netConn) {
	n.connPoolLock.Lock()
	defer n.connPoolLock.Unlock()

	key := conn.target
	conns, _ := n.connPool[key]

	if !n.IsShutdown() && len(conns) < n.maxPool {
		n.connPool[key] = append(conns, conn)
	} else {
		conn.Release()
	}
}
// returnConn returns a connection back to the pool.