vnc: deal with surface NULL pointers

Secondary displays in multihead setups are allowed to have a NULL
DisplaySurface.  Typically user interfaces handle this by hiding the
window which shows the display in question.

This isn't an option for vnc though because it simply hasn't a concept
of windows or outputs.  So handle the situation by showing a placeholder
DisplaySurface instead.  Also check in console_select whenever a surface
is preset in the first place before requesting an update.

This fixes a segfault which can be triggered by switching to an unused
display (via vtrl-alt-<nr>) in a multihead setup, for example using
-device virtio-vga,max_outputs=2.

Cc: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Tested-by: Christian Borntraeger <borntraeger@de.ibm.com>
Message-id: 20180308161803.6152-1-kraxel@redhat.com

1 files changed, 2 insertions, 0 deletions

diff --git a/include/ui/console.h b/include/ui/console.h
index aae9e44cb3..5fca9afcbc 100644
--- a/include/ui/console.h
+++ b/include/ui/console.h
@@ -260,6 +260,8 @@ DisplaySurface *qemu_create_displaysurface_guestmem(int width, int height,
                                                     pixman_format_code_t format,
                                                     int linesize,
                                                     uint64_t addr);
+DisplaySurface *qemu_create_message_surface(int w, int h,
+                                            const char *msg);
 PixelFormat qemu_default_pixelformat(int bpp);
 
 DisplaySurface *qemu_create_displaysurface(int width, int height);