defer aa.Close()
	bb := NewFile(uintptr(bfd), b.Name())
	defer bb.Close()

	// This would fail on Linux in case the copy_file_range syscall was used because it doesn't
	// support destination files opened with O_APPEND, see
	// https://man7.org/linux/man-pages/man2/copy_file_range.2.html#ERRORS
	_, err = io.Copy(aa, bb)
	if err != nil {
		t.Fatal(err)
	}

	buf, err := ReadFile(aa.Name())
	if err != nil {

	// Now call ReadFrom (through io.Copy), which will hopefully call
	// poll.CopyFileRange.
	n, err := io.Copy(dst, realsrc)
	if err != nil {
		t.Fatal(err)
	}

	// If we didn't have a limit, we should have called poll.CopyFileRange
	// with the right file descriptor arguments.
	if limit > 0 && !hook.called {
		t.Fatal("never called poll.CopyFileRange")
	}
	if hook.called && hook.dstfd != int(dst.Fd()) {