switch {
	case errors.Is(err, syscall.EPROTONOSUPPORT): // Not supported: >= v5.6
	case errors.Is(err, syscall.EINVAL): // Not supported: < v5.6
	case err == nil: // Supported and no error
		poll.CloseFunc(s)
		fallthrough
	default:
		// another error: MPTCP was not available but it might be later
		mptcpAvailable = true
	}

	major, minor := unix.KernelVersion()
	// SOL_MPTCP only supported from kernel 5.16

	if err != nil {
		if errcall != "" {
			err = wrapSyscallError(errcall, err)
		}
		return nil, err
	}

	if netfd, err = newFD(d, fd.family, fd.sotype, fd.net); err != nil {
		poll.CloseFunc(d)
		return nil, err
	}
	if err = netfd.init(); err != nil {
		netfd.Close()
		return nil, err
	}
	lsa, _ := syscall.Getsockname(netfd.pfd.Sysfd)

		return syscall.EINVAL
	}
	// Poller may want to unregister fd in readiness notification mechanism,
	// so this must be executed before fd.CloseFunc.
	fd.pd.close()
	var err error
	switch fd.kind {
	case kindNet:
		// The net package uses the CloseFunc variable for testing.
		err = CloseFunc(fd.Sysfd)
	default:
		err = syscall.CloseHandle(fd.Sysfd)
	}
	fd.Sysfd = syscall.InvalidHandle
	runtime_Semrelease(&fd.csema)

			err = wrapSyscallError(errcall, err)
		}
		return nil, err
	}

	// Associate our new socket with IOCP.
	netfd, err := newFD(s, fd.family, fd.sotype, fd.net)
	if err != nil {
		poll.CloseFunc(s)
		return nil, err
	}
	if err := netfd.init(); err != nil {
		fd.Close()
		return nil, err
	}

	// Get local and peer addr out of AcceptEx buffer.
	var lrsa, rrsa *syscall.RawSockaddrAny

	// If the descriptor is indeed closed, using a loop would race
	// with some other goroutine opening a new descriptor.
	// (The Linux kernel guarantees that it is closed on an EINTR error.)
	return CloseFunc(fd)
}

// Copy creates a copy of the FD.
//
// The FD instance points to the same underlying file descriptor. The file
// descriptor isn't closed until all FD instances that refer to it have been
// closed/destroyed.

type changeImplConn struct {
	net.Conn
	writeFunc func([]byte) (int, error)
	closeFunc func() error
}

func (w *changeImplConn) Write(p []byte) (n int, err error) {
	if w.writeFunc != nil {
		return w.writeFunc(p)
	}
	return w.Conn.Write(p)
}

func (w *changeImplConn) Close() error {
	if w.closeFunc != nil {
		return w.closeFunc()
	}
	return w.Conn.Close()
}

// no remaining references.
func (fd *FD) destroy() error {
	// Poller may want to unregister fd in readiness notification mechanism,
	// so this must be executed before CloseFunc.
	fd.pd.close()

	err := fd.SysFile.destroy(fd.Sysfd)

	fd.Sysfd = -1
	runtime_Semrelease(&fd.csema)
	return err
}

// Close closes the FD. The underlying file descriptor is closed by the

	cst.tr.CloseIdleConnections()
	a2 := get()
	if a1 == a2 {
		t.Errorf("didn't close connection")
	}
}

type noteCloseConn struct {
	net.Conn
	closeFunc func()
}

func (x noteCloseConn) Close() error {
	x.closeFunc()
	return x.Conn.Close()
}

type testErrorReader struct{ t *testing.T }

func (r testErrorReader) Read(p []byte) (n int, err error) {

type rwTestConn struct {
	io.Reader
	io.Writer
	noopConn

	closeFunc func() error // called if non-nil
	closec    chan bool    // else, if non-nil, send value to it on close
}

func (c *rwTestConn) Close() error {
	if c.closeFunc != nil {
		return c.closeFunc()
	}
	select {
	case c.closec <- true:
	default:
	}
	return nil
}

type testConn struct {