// has not yet completed. See [Conn.SetDeadline], [Conn.SetReadDeadline], and
// [Conn.SetWriteDeadline].
func (c *Conn) Write(b []byte) (int, error) {
	// interlock with Close below
	for {
		x := c.activeCall.Load()
		if x&1 != 0 {
			return 0, net.ErrClosed
		}
		if c.activeCall.CompareAndSwap(x, x+2) {
			break
		}
	}
	defer c.activeCall.Add(-2)