backend: Add 'experimental-features' gsetting

This gsetting will allow the adding of keywords to a array, where each
keyword may enable an experimental feauter, if the given mutter version
supports that particular experimental feature. Emphasis is put on the
lack of guarantee that any such keyword has any effect. Currently no
keywords are defined.

https://bugzilla.gnome.org/show_bug.cgi?id=777732

data/org.gnome.mutter.gschema.xml.in

@@ -102,6 +102,20 @@
       </description>
     </key>
 
+    <key name="experimental-features" type="as">
+      <default>[]</default>
+      <summary>Enable experimental features</summary>
+      <description>
+        To enable experimental features, add the feature keyword to the list.
+        Whether the feature requires restarting the compositor depends on the
+        given feature. Any experimental feature is not required to still be
+        available, or configurable. Don't expect adding anything in this
+        setting to be future proof.
+
+        Currently possible keywords: (none)
+      </description>
+    </key>
+
     <child name="keybindings" schema="org.gnome.mutter.keybindings"/>
 
   </schema>

src/backends/meta-backend-private.h

@@ -97,6 +97,11 @@ struct _MetaBackendClass
 
 };
 
+typedef enum _MetaExperimentalFeature
+{
+  META_EXPERIMENTAL_FEATURE_NONE = 0,
+} MetaExperimentalFeature;
+
 void meta_init_backend (GType backend_gtype);
 
 ClutterBackend * meta_backend_get_clutter_backend (MetaBackend *backend);
@@ -146,6 +151,14 @@ ClutterBackend * meta_backend_get_clutter_backend (MetaBackend *backend);
 
 void meta_backend_monitors_changed (MetaBackend *backend);
 
+gboolean meta_backend_is_experimental_feature_enabled (MetaBackend            *backend,
+                                                       MetaExperimentalFeature feature);
+
+void meta_backend_override_experimental_features (MetaBackend *backend);
+
+void meta_backend_enable_experimental_feature (MetaBackend            *backend,
+                                               MetaExperimentalFeature feature);
+
 gboolean meta_is_stage_views_enabled (void);
 
 MetaInputSettings *meta_backend_get_input_settings (MetaBackend *backend);

src/backends/meta-backend.c

@@ -68,6 +68,10 @@ struct _MetaBackendPrivate
   MetaRenderer *renderer;
   MetaEgl *egl;
 
+  GSettings *mutter_settings;
+  MetaExperimentalFeature experimental_features;
+  gboolean experimental_features_overridden;
+
   ClutterBackend *clutter_backend;
   ClutterActor *stage;
 
@@ -403,6 +407,96 @@ meta_backend_real_get_relative_motion_deltas (MetaBackend *backend,
   return FALSE;
 }
 
+static gboolean
+experimental_features_handler (GVariant *features_variant,
+                               gpointer *result,
+                               gpointer  data)
+{
+  MetaBackend *backend = data;
+  MetaBackendPrivate *priv = meta_backend_get_instance_private (backend);
+  GVariantIter features_iter;
+  char *feature;
+  MetaExperimentalFeature features = META_EXPERIMENTAL_FEATURE_NONE;
+
+  if (priv->experimental_features_overridden)
+    {
+      *result = GINT_TO_POINTER (FALSE);
+      return TRUE;
+    }
+
+  g_variant_iter_init (&features_iter, features_variant);
+  while (g_variant_iter_loop (&features_iter, "s", &feature))
+    {
+      /* So far no experimental features defined. */
+      g_info ("Unknown experimental feature '%s'\n", feature);
+    }
+
+  if (features != priv->experimental_features)
+    {
+      priv->experimental_features = features;
+      *result = GINT_TO_POINTER (TRUE);
+    }
+  else
+    {
+      *result = GINT_TO_POINTER (FALSE);
+    }
+
+  return TRUE;
+}
+
+static gboolean
+update_experimental_features (MetaBackend *backend)
+{
+  MetaBackendPrivate *priv = meta_backend_get_instance_private (backend);
+
+  return GPOINTER_TO_INT (g_settings_get_mapped (priv->mutter_settings,
+                                                 "experimental-features",
+                                                 experimental_features_handler,
+                                                 backend));
+}
+
+static void
+mutter_settings_changed (GSettings   *settings,
+                         gchar       *key,
+                         MetaBackend *backend)
+{
+  gboolean changed;
+
+  if (!g_str_equal (key, "experimental-features"))
+    return;
+
+  changed = update_experimental_features (backend);
+  if (changed)
+    g_signal_emit_by_name (backend, "experimental-features-changed");
+}
+
+gboolean
+meta_backend_is_experimental_feature_enabled (MetaBackend            *backend,
+                                              MetaExperimentalFeature feature)
+{
+  MetaBackendPrivate *priv = meta_backend_get_instance_private (backend);
+
+  return !!(priv->experimental_features & feature);
+}
+
+void
+meta_backend_override_experimental_features (MetaBackend *backend)
+{
+  MetaBackendPrivate *priv = meta_backend_get_instance_private (backend);
+
+  priv->experimental_features = META_EXPERIMENTAL_FEATURE_NONE;
+  priv->experimental_features_overridden = TRUE;
+}
+
+void
+meta_backend_enable_experimental_feature (MetaBackend            *backend,
+                                          MetaExperimentalFeature feature)
+{
+  MetaBackendPrivate *priv = meta_backend_get_instance_private (backend);
+
+  priv->experimental_features |= feature;
+}
+
 static void
 meta_backend_class_init (MetaBackendClass *klass)
 {
@@ -435,6 +529,12 @@ meta_backend_class_init (MetaBackendClass *klass)
                 0,
                 NULL, NULL, NULL,
                 G_TYPE_NONE, 1, G_TYPE_INT);
+  g_signal_new ("experimental-features-changed",
+                G_TYPE_FROM_CLASS (object_class),
+                G_SIGNAL_RUN_LAST,
+                0,
+                NULL, NULL, NULL,
+                G_TYPE_NONE, 0);
 }
 
 static gboolean
@@ -445,6 +545,12 @@ meta_backend_initable_init (GInitable     *initable,
   MetaBackend *backend = META_BACKEND (initable);
   MetaBackendPrivate *priv = meta_backend_get_instance_private (backend);
 
+  priv->mutter_settings = g_settings_new ("org.gnome.mutter");
+  g_signal_connect (priv->mutter_settings, "changed",
+                    G_CALLBACK (mutter_settings_changed),
+                    backend);
+  update_experimental_features (backend);
+
   priv->egl = g_object_new (META_TYPE_EGL, NULL);
 
   priv->renderer = META_BACKEND_GET_CLASS (backend)->create_renderer (backend);