Shift top-left display to origin

When using a custom fullscreen display configuration, it's possible to
specify that e.g. a single screen should be fullscreen on client
monitor #4. Since we send down absolute positions and disable alignment
when all windows are in fullscreen, we can send configurations with a
very large offset to the top-left corner. This could result in the guest
trying to create a screen that was much larger than necessary. For
example when sending a configuration of 1280x1024+4240+0, the guest
would need to allocate a screen of size 5520x1024, which might fail if
video memory was too low. To avoid this issue, we shift all displays
so that the minimum X coordinate for all screens is at x=0, and the
minimum y coordinate is at y=0.

src/virt-viewer-session-spice.c

@@ -804,7 +804,7 @@ virt_viewer_session_spice_fullscreen_auto_conf(VirtViewerSessionSpice *self)
     GdkScreen *screen = gdk_screen_get_default();
     SpiceMainChannel* cmain = virt_viewer_session_spice_get_main_channel(self);
     VirtViewerApp *app = NULL;
-    GdkRectangle dest;
+    GdkRectangle *displays;
     gboolean agent_connected;
     gint i;
     gsize ndisplays = 0;
@@ -838,19 +838,29 @@ virt_viewer_session_spice_fullscreen_auto_conf(VirtViewerSessionSpice *self)
 
     ndisplays = virt_viewer_app_get_n_initial_displays(app);
     g_debug("Performing full screen auto-conf, %" G_GSIZE_FORMAT " host monitors", ndisplays);
+    displays = g_new0(GdkRectangle, ndisplays);
 
     for (i = 0; i < ndisplays; i++) {
+        GdkRectangle* rect = &displays[i];
         gint j = virt_viewer_app_get_initial_monitor_for_display(app, i);
         if (j == -1)
             continue;
 
-        gdk_screen_get_monitor_geometry(screen, j, &dest);
-        g_debug("Set SPICE display %d to (%d,%d)-(%dx%d)",
-                  i, dest.x, dest.y, dest.width, dest.height);
-        spice_main_set_display(cmain, i, dest.x, dest.y, dest.width, dest.height);
-        spice_main_set_display_enabled(cmain, i, TRUE);
+        gdk_screen_get_monitor_geometry(screen, j, rect);
     }
 
+    virt_viewer_shift_monitors_to_origin(displays, ndisplays);
+
+    for (i = 0; i < ndisplays; i++) {
+        GdkRectangle *rect = &displays[i];
+
+        spice_main_set_display(cmain, i, rect->x, rect->y, rect->width, rect->height);
+        spice_main_set_display_enabled(cmain, i, TRUE);
+        g_debug("Set SPICE display %d to (%d,%d)-(%dx%d)",
+                  i, rect->x, rect->y, rect->width, rect->height);
+    }
+    g_free(displays);
+
     spice_main_send_monitor_config(cmain);
     self->priv->did_auto_conf = TRUE;
     return TRUE;

src/virt-viewer-session.c

@@ -381,6 +381,8 @@ virt_viewer_session_on_monitor_geometry_changed(VirtViewerSession* self,
     if (!all_fullscreen)
         virt_viewer_align_monitors_linear(monitors, nmonitors);
 
+    virt_viewer_shift_monitors_to_origin(monitors, nmonitors);
+
     klass->apply_monitor_geometry(self, monitors, nmonitors);
     g_free(monitors);
 }

src/virt-viewer-util.c

@@ -538,6 +538,48 @@ virt_viewer_align_monitors_linear(GdkRectangle *displays, guint ndisplays)
     g_free(sorted_displays);
 }
 
+/* Shift all displays so that the monitor origin is at (0,0). This reduces the
+ * size of the screen that will be required on the guest when all client
+ * monitors are fullscreen but do not begin at the origin. For example, instead
+ * of sending down the following configuration:
+ *   1280x1024+4240+0
+ * (which implies that the guest screen must be at least 5520x1024), we'd send
+ *   1280x1024+0+0
+ * (which implies the guest screen only needs to be 1280x1024). The first
+ * version might fail if the guest video memory is not large enough to handle a
+ * screen of that size.
+ */
+void
+virt_viewer_shift_monitors_to_origin(GdkRectangle *displays, guint ndisplays)
+{
+    gint xmin = G_MAXINT;
+    gint ymin = G_MAXINT;
+    gint i;
+
+    g_return_if_fail(ndisplays > 0);
+
+    for (i = 0; i < ndisplays; i++) {
+        GdkRectangle *display = &displays[i];
+        if (display->width > 0 && display->height > 0) {
+            xmin = MIN(xmin, display->x);
+            ymin = MIN(ymin, display->y);
+        }
+    }
+    g_return_if_fail(xmin < G_MAXINT && ymin < G_MAXINT);
+
+    if (xmin > 0 || ymin > 0) {
+        g_debug("%s: Shifting all monitors by (%i, %i)", G_STRFUNC, xmin, ymin);
+        for (i = 0; i < ndisplays; i++) {
+            GdkRectangle *display = &displays[i];
+            if (display->width > 0 && display->height > 0) {
+                display->x -= xmin;
+                display->y -= ymin;
+            }
+        }
+    }
+}
+
+
 /*
  * Local variables:
  *  c-indent-level: 4

src/virt-viewer-util.h

@@ -58,6 +58,7 @@ gint virt_viewer_compare_version(const gchar *s1, const gchar *s2);
 
 /* monitor alignment */
 void virt_viewer_align_monitors_linear(GdkRectangle *displays, guint ndisplays);
+void virt_viewer_shift_monitors_to_origin(GdkRectangle *displays, guint ndisplays);
 
 #endif