dstoff, err := dst.Seek(0, io.SeekCurrent)
	if err != nil {
		t.Fatal(err)
	}
	srcoff, err := src.Seek(0, io.SeekCurrent)
	if err != nil {
		t.Fatal(err)
	}
	if dstoff != srcoff {
		t.Errorf("offsets differ: dstoff = %d, srcoff = %d", dstoff, srcoff)
	}
	if dstoff != int64(len(data)) {
		t.Errorf("dstoff = %d, want %d", dstoff, len(data))
	}
	if n != int64(len(data)) {

	var (
		srcPix        []uint8
		dstPix        []uint8
		stride        int
		rect          image.Rectangle
		bytesPerPixel int
	)
	switch target := dst.(type) {
	case *image.Alpha:
		srcPix = src.(*image.Alpha).Pix
		dstPix, stride, rect = target.Pix, target.Stride, target.Rect
		bytesPerPixel = 1
	case *image.Alpha16:
		srcPix = src.(*image.Alpha16).Pix

	var hookFd int
	switch fdType {
	case "src":
		hookFd = hook.srcfd
	case "dst":
		hookFd = hook.dstfd
	default:
		t.Fatalf("unknown fdType %q", fdType)
	}
	if err := rc.Control(func(fd uintptr) {
		if hook.called && hookFd != int(fd) {
			t.Fatalf("wrong %s file descriptor: got %d, want %d", fdType, hook.dstfd, int(fd))
		}
	}); err != nil {
		t.Fatalf("syscall.RawConn.Control error: %v", err)
	}
}

	}()
	var (
		called     bool
		gotHandled bool
		gotFD      *poll.FD
	)
	poll.TestHookDidSendFile = func(dstFD *poll.FD, src int, written int64, err error, handled bool) {
		if called {
			t.Error("internal/poll.SendFile called multiple times, want one call")
		}
		called = true
		gotHandled = handled
		gotFD = dstFD
	}
	f()
	if !called {
		t.Error("internal/poll.SendFile was not called, want it to be")
		return
	}

// dst block.
func scale(dst *block, src *[4]block) {
	for i := 0; i < 4; i++ {
		dstOff := (i&2)<<4 | (i&1)<<2
		for y := 0; y < 4; y++ {
			for x := 0; x < 4; x++ {
				j := 16*y + 2*x
				sum := src[i][j] + src[i][j+1] + src[i][j+8] + src[i][j+9]
				dst[8*y+x+dstOff] = (sum + 2) >> 2
			}
		}
	}
}

// sosHeaderY is the SOS marker "\xff\xda" followed by 8 bytes:

		}
		d0 += ddelta
		s0 += sdelta
	}
}

// drawCopySrc copies bytes to dstPix from srcPix. These arguments roughly
// correspond to the Pix fields of the image package's concrete image.Image
// implementations, but are offset (dstPix is dst.Pix[dpOffset:] not dst.Pix).
func drawCopySrc(
	dstPix []byte, dstStride int, r image.Rectangle,
	srcPix []byte, srcStride int, sp image.Point,
	bytesPerRow int) {

	var err error

	// get an object of type used by this message
	dstX, err := dst.UnmarshalNew()
	if err != nil {
		return nil, err
	}

	// get an object of type used by this message
	srcX, err := src.UnmarshalNew()
	if err != nil {
		return nil, err
	}

	// Merge the two typed protos
	merge.Merge(dstX, srcX)

	// Convert the merged proto back to dst