monitor-manager: Find the supported scales from one passed

When passing scales over D-Bus, we'll loose some precision. To set the
correct scale, use the configured scale and look up the one actually
supported by the monitor mode, and use that. To match the supported one
with the configured one, the difference must be within rounding error
range.

https://bugzilla.gnome.org/show_bug.cgi?id=765011

src/backends/meta-monitor-manager.c

@@ -1884,26 +1884,76 @@ create_monitor_config_from_variant (MetaMonitorManager *manager,
 }
 
 static gboolean
-derive_logical_monitor_size (GList                       *monitor_configs,
+find_monitor_mode_scale (MetaMonitorManager          *manager,
+                         MetaLogicalMonitorLayoutMode layout_mode,
+                         MetaMonitorConfig           *monitor_config,
+                         float                        scale,
+                         float                       *out_scale,
+                         GError                     **error)
+{
+  MetaMonitorSpec *monitor_spec;
+  MetaMonitor *monitor;
+  MetaMonitorModeSpec *monitor_mode_spec;
+  MetaMonitorMode *monitor_mode;
+  g_autofree float *supported_scales = NULL;
+  int n_supported_scales;
+  int i;
+
+  monitor_spec = monitor_config->monitor_spec;
+  monitor = meta_monitor_manager_get_monitor_from_spec (manager,
+                                                        monitor_spec);
+  if (!monitor)
+    {
+      g_set_error (error, G_IO_ERROR, G_IO_ERROR_FAILED,
+                   "Monitor not found");
+      return FALSE;
+    }
+
+  monitor_mode_spec = monitor_config->mode_spec;
+  monitor_mode = meta_monitor_get_mode_from_spec (monitor,
+                                                  monitor_mode_spec);
+  if (!monitor_mode)
+    {
+      g_set_error (error, G_IO_ERROR, G_IO_ERROR_FAILED,
+                   "Monitor mode not found");
+      return FALSE;
+    }
+
+  supported_scales =
+    meta_monitor_manager_calculate_supported_scales (manager, layout_mode,
+                                                     monitor, monitor_mode,
+                                                     &n_supported_scales);
+
+  for (i = 0; i < n_supported_scales; i++)
+    {
+      float supported_scale = supported_scales[i];
+
+      if (fabsf (supported_scale - scale) < FLT_EPSILON)
+        {
+          *out_scale = supported_scale;
+          return TRUE;
+        }
+    }
+
+  g_set_error (error, G_IO_ERROR, G_IO_ERROR_FAILED,
+               "Scale %g not valid for resolution %dx%d",
+               scale,
+               monitor_mode_spec->width,
+               monitor_mode_spec->height);
+  return FALSE;
+}
+
+static gboolean
+derive_logical_monitor_size (MetaMonitorConfig           *monitor_config,
                              int                         *out_width,
                              int                         *out_height,
-                             double                       scale,
+                             float                        scale,
                              MetaMonitorTransform         transform,
                              MetaLogicalMonitorLayoutMode layout_mode,
                              GError                     **error)
 {
-  MetaMonitorConfig *monitor_config;
   int width, height;
 
-  if (!monitor_configs)
-    {
-      g_set_error (error, G_IO_ERROR, G_IO_ERROR_FAILED,
-                   "Empty logical monitor");
-      return FALSE;
-    }
-
-  monitor_config = monitor_configs->data;
-
   if (meta_monitor_transform_is_rotated (transform))
     {
       width = monitor_config->mode_spec->height;
@@ -1918,14 +1968,6 @@ derive_logical_monitor_size (GList                       *monitor_configs,
   switch (layout_mode)
     {
     case META_LOGICAL_MONITOR_LAYOUT_MODE_LOGICAL:
-      if (fmodf (width, scale) != 0.0 || fmodf (height, scale) != 0.0)
-        {
-          g_set_error (error, G_IO_ERROR, G_IO_ERROR_FAILED,
-                       "Scale %g not valid for resolution %dx%d",
-                       scale, width, height);
-          return FALSE;
-        }
-
       width /= scale;
       height /= scale;
       break;
@@ -1947,19 +1989,22 @@ create_logical_monitor_config_from_variant (MetaMonitorManager          *manager
 {
   MetaLogicalMonitorConfig *logical_monitor_config;
   int x, y, width, height;
-  double scale;
+  double scale_d;
+  float scale;
   MetaMonitorTransform transform;
   gboolean is_primary;
   GVariantIter *monitor_configs_iter;
   GList *monitor_configs = NULL;
+  MetaMonitorConfig *monitor_config;
 
   g_variant_get (logical_monitor_config_variant, LOGICAL_MONITOR_CONFIG_FORMAT,
                  &x,
                  &y,
-                 &scale,
+                 &scale_d,
                  &transform,
                  &is_primary,
                  &monitor_configs_iter);
+  scale = (float) scale_d;
 
   while (TRUE)
     {
@@ -1986,7 +2031,23 @@ create_logical_monitor_config_from_variant (MetaMonitorManager          *manager
     }
   g_variant_iter_free (monitor_configs_iter);
 
-  if (!derive_logical_monitor_size (monitor_configs, &width, &height,
+  if (!monitor_configs)
+    {
+      g_set_error (error, G_IO_ERROR, G_IO_ERROR_FAILED,
+                   "Empty logical monitor");
+      goto err;
+    }
+
+  monitor_config = monitor_configs->data;
+  if (!find_monitor_mode_scale (manager,
+                                layout_mode,
+                                monitor_config,
+                                scale,
+                                &scale,
+                                error))
+    goto err;
+
+  if (!derive_logical_monitor_size (monitor_config, &width, &height,
                                     scale, transform, layout_mode, error))
     goto err;