monitor-manager: Consider external layout before default linear config

In case of no existing configuration, we use a default layout of
aligning attached displays horizontally. This sidesteps any layout
configuration that is done externally, for instance via xorg.conf,
which is not desirable. Instead, base the initial configuration on
the existing layout if it passes some sanity checks before falling
back to the default linear config.

src/backends/meta-monitor-config-manager.c

@@ -559,6 +559,83 @@ create_preferred_logical_monitor_config (MetaMonitorManager          *monitor_ma
   return logical_monitor_config;
 }
 
+static MetaLogicalMonitorConfig *
+create_logical_monitor_config_from_output (MetaMonitorManager           *monitor_manager,
+                                           MetaMonitor                  *monitor,
+                                           MetaLogicalMonitorConfig     *primary_logical_monitor_config,
+                                           MetaLogicalMonitorLayoutMode  layout_mode)
+{
+    MetaOutput *output;
+    MetaCrtc *crtc;
+
+    output = meta_monitor_get_main_output (monitor);
+    crtc = meta_output_get_assigned_crtc (output);
+    return create_preferred_logical_monitor_config (monitor_manager,
+                                                    monitor,
+                                                    crtc->rect.x,
+                                                    crtc->rect.y,
+                                                    primary_logical_monitor_config,
+                                                    layout_mode);
+}
+
+MetaMonitorsConfig *
+meta_monitor_config_manager_create_current (MetaMonitorConfigManager *config_manager)
+{
+  MetaMonitorManager *monitor_manager = config_manager->monitor_manager;
+  GList *logical_monitor_configs;
+  MetaMonitor *primary_monitor;
+  MetaLogicalMonitorLayoutMode layout_mode;
+  MetaLogicalMonitorConfig *primary_logical_monitor_config;
+  GList *monitors;
+  GList *l;
+
+  if (meta_monitor_config_store_get_config_count (config_manager->config_store) > 0)
+    return NULL;
+
+  primary_monitor = find_primary_monitor (monitor_manager);
+  if (!primary_monitor || !meta_monitor_is_active (primary_monitor))
+    return NULL;
+
+  layout_mode = meta_monitor_manager_get_default_layout_mode (monitor_manager);
+
+  primary_logical_monitor_config =
+    create_logical_monitor_config_from_output (monitor_manager,
+                                               primary_monitor,
+                                               NULL,
+                                               layout_mode);
+
+  primary_logical_monitor_config->is_primary = TRUE;
+  logical_monitor_configs = g_list_append (NULL,
+                                           primary_logical_monitor_config);
+
+  monitors = meta_monitor_manager_get_monitors (monitor_manager);
+  for (l = monitors; l; l = l->next)
+    {
+      MetaMonitor *monitor = l->data;
+      MetaLogicalMonitorConfig *logical_monitor_config;
+
+      if (monitor == primary_monitor)
+        continue;
+
+      if (!meta_monitor_is_active (monitor))
+        continue;
+
+      logical_monitor_config =
+        create_logical_monitor_config_from_output (monitor_manager,
+                                                   monitor,
+                                                   primary_logical_monitor_config,
+                                                   layout_mode);
+
+      logical_monitor_configs = g_list_append (logical_monitor_configs,
+                                               logical_monitor_config);
+    }
+
+  return meta_monitors_config_new (monitor_manager,
+                                   logical_monitor_configs,
+                                   layout_mode,
+                                   META_MONITORS_CONFIG_FLAG_NONE);
+}
+
 MetaMonitorsConfig *
 meta_monitor_config_manager_create_linear (MetaMonitorConfigManager *config_manager)
 {

src/backends/meta-monitor-config-manager.h

@@ -87,6 +87,7 @@ gboolean meta_monitor_config_manager_assign (MetaMonitorManager *manager,
 
 MetaMonitorsConfig * meta_monitor_config_manager_get_stored (MetaMonitorConfigManager *config_manager);
 
+MetaMonitorsConfig * meta_monitor_config_manager_create_current (MetaMonitorConfigManager *config_manager);
 MetaMonitorsConfig * meta_monitor_config_manager_create_linear (MetaMonitorConfigManager *config_manager);
 
 MetaMonitorsConfig * meta_monitor_config_manager_create_fallback (MetaMonitorConfigManager *config_manager);

src/backends/meta-monitor-manager.c

@@ -576,6 +576,25 @@ meta_monitor_manager_ensure_configured (MetaMonitorManager *manager)
       g_clear_object (&config);
     }
 
+  config = meta_monitor_config_manager_create_current (manager->config_manager);
+  if (config)
+    {
+      if (!meta_monitor_manager_apply_monitors_config (manager,
+                                                       config,
+                                                       method,
+                                                       &error))
+        {
+          g_clear_object (&config);
+          g_warning ("Failed to use current monitor configuration: %s",
+                     error->message);
+          g_clear_error (&error);
+        }
+      else
+        {
+          goto done;
+        }
+    }
+
   config = meta_monitor_config_manager_create_linear (manager->config_manager);
   if (config)
     {