config: Fix the memory and other management for MetaMonitorConfig

The code in MetaMonitorConfig was really complex and was trying to do
way too much, using multiple different variables to determine where
things were stored, and trying to do fancy tricks to transfer
ownership.

Add a refcounting system to help simplify this, and clean up the logic.
Simply along the way, this fixes multiple bugs in the monitor config
logic, most notably bug #734889, which was my original goal to fix.

src/backends/meta-monitor-config.c

@@ -67,6 +67,7 @@ typedef struct {
 } MetaOutputConfig;
 
 typedef struct {
+  guint refcount;
   MetaOutputKey *keys;
   MetaOutputConfig *outputs;
   unsigned int n_outputs;
@@ -77,7 +78,6 @@ struct _MetaMonitorConfig {
 
   GHashTable *configs;
   MetaConfiguration *current;
-  gboolean current_is_stored;
   gboolean current_is_for_laptop_lid;
   MetaConfiguration *previous;
 
@@ -124,12 +124,30 @@ config_clear (MetaConfiguration *config)
   g_free (config->outputs);
 }
 
+static MetaConfiguration *
+config_ref (MetaConfiguration *config)
+{
+  config->refcount++;
+  return config;
+}
+
 static void
-config_free (gpointer config)
+config_unref (MetaConfiguration *config)
+{
+  if (--config->refcount == 0)
     {
       config_clear (config);
       g_slice_free (MetaConfiguration, config);
     }
+}
+
+static MetaConfiguration *
+config_new (void)
+{
+  MetaConfiguration *config = g_slice_new0 (MetaConfiguration);
+  config->refcount = 1;
+  return config;
+}
 
 static unsigned long
 output_key_hash (const MetaOutputKey *key)
@@ -220,7 +238,7 @@ meta_monitor_config_init (MetaMonitorConfig *self)
   const char *filename;
   char *path;
 
-  self->configs = g_hash_table_new_full (config_hash, config_equal, NULL, config_free);
+  self->configs = g_hash_table_new_full (config_hash, config_equal, NULL, (GDestroyNotify) config_unref);
 
   filename = g_getenv ("MUTTER_MONITOR_FILENAME");
   if (filename == NULL)
@@ -830,11 +848,19 @@ meta_monitor_config_get_stored (MetaMonitorConfig *self,
   return stored;
 }
 
+static void
+set_current (MetaMonitorConfig *self,
+             MetaConfiguration *config)
+{
+  g_clear_pointer (&self->previous, (GDestroyNotify) config_unref);
+  self->previous = self->current;
+  self->current = config_ref (config);
+}
+
 static gboolean
 apply_configuration (MetaMonitorConfig  *self,
                      MetaConfiguration  *config,
-		     MetaMonitorManager *manager,
+		     MetaMonitorManager *manager)
-                     gboolean            stored)
 {
   GPtrArray *crtcs, *outputs;
 
@@ -845,9 +871,7 @@ apply_configuration (MetaMonitorConfig  *self,
     {
       g_ptr_array_unref (crtcs);
       g_ptr_array_unref (outputs);
-      if (!stored)
+      config_unref (config);
-        config_free (config);
-
       return FALSE;
     }
 
@@ -855,44 +879,12 @@ apply_configuration (MetaMonitorConfig  *self,
                                             (MetaCRTCInfo**)crtcs->pdata, crtcs->len,
                                             (MetaOutputInfo**)outputs->pdata, outputs->len);
 
-  /* Stored (persistent) configurations override the previous one always.
+  set_current (self, config);
-     Also, we clear the previous configuration if the current one (which is
-     about to become previous) is stored, or if the current one has
-     different outputs.
-  */
-  if (stored ||
-      (self->current && self->current_is_stored))
-    {
-      if (self->previous)
-        config_free (self->previous);
-      self->previous = NULL;
-    }
-  else
-    {
-      /* Despite the name, config_equal() only checks the set of outputs,
-         not their modes
-      */
-      if (self->current && config_equal (self->current, config))
-        {
-          self->previous = self->current;
-        }
-      else
-        {
-          if (self->current)
-            config_free (self->current);
-          self->previous = NULL;
-        }
-    }
 
-  self->current = config;
-  self->current_is_stored = stored;
   /* If true, we'll be overridden at the end of this call
    * inside turn_off_laptop_display / apply_configuration_with_lid */
   self->current_is_for_laptop_lid = FALSE;
 
-  if (self->current == self->previous)
-    self->previous = NULL;
-
   g_ptr_array_unref (crtcs);
   g_ptr_array_unref (outputs);
   return TRUE;
@@ -934,7 +926,7 @@ make_laptop_lid_config (MetaConfiguration  *reference)
 
   g_assert (reference->n_outputs > 1);
 
-  new = g_slice_new0 (MetaConfiguration);
+  new = config_new ();
   new->n_outputs = reference->n_outputs;
   new->keys = g_new0 (MetaOutputKey, reference->n_outputs);
   new->outputs = g_new0 (MetaOutputConfig, reference->n_outputs);
@@ -998,16 +990,21 @@ apply_configuration_with_lid (MetaMonitorConfig  *self,
       config->n_outputs > 1 &&
       laptop_display_is_on (config))
     {
-      if (apply_configuration (self, make_laptop_lid_config (config), manager, FALSE))
+      MetaConfiguration *laptop_lid_config = make_laptop_lid_config (config);
+      if (apply_configuration (self, laptop_lid_config, manager))
         {
           self->current_is_for_laptop_lid = TRUE;
+          config_unref (laptop_lid_config);
           return TRUE;
         }
       else
+        {
+          config_unref (laptop_lid_config);
           return FALSE;
         }
+    }
   else
-    return apply_configuration (self, config, manager, TRUE);
+    return apply_configuration (self, config, manager);
 }
 
 gboolean
@@ -1084,7 +1081,7 @@ make_default_config (MetaMonitorConfig *self,
   MetaConfiguration *ret;
   MetaOutput *primary;
 
-  ret = g_slice_new (MetaConfiguration);
+  ret = config_new ();
   make_config_key (ret, outputs, n_outputs, -1);
   ret->outputs = g_new0 (MetaOutputConfig, n_outputs);
 
@@ -1215,7 +1212,7 @@ ensure_at_least_one_output (MetaMonitorConfig  *self,
 
   /* Oh no, we don't! Activate the primary one and disable everything else */
 
-  ret = g_slice_new (MetaConfiguration);
+  ret = config_new ();
   make_config_key (ret, outputs, n_outputs, -1);
   ret->outputs = g_new0 (MetaOutputConfig, n_outputs);
 
@@ -1242,7 +1239,8 @@ ensure_at_least_one_output (MetaMonitorConfig  *self,
         }
     }
 
-  apply_configuration (self, ret, manager, FALSE);
+  apply_configuration (self, ret, manager);
+  config_unref (ret);
   return FALSE;
 }
 
@@ -1307,7 +1305,7 @@ meta_monitor_config_update_current (MetaMonitorConfig  *self,
 
   outputs = meta_monitor_manager_get_outputs (manager, &n_outputs);
 
-  current = g_slice_new (MetaConfiguration);
+  current = config_new ();
   current->n_outputs = n_outputs;
   current->outputs = g_new0 (MetaOutputConfig, n_outputs);
   current->keys = g_new0 (MetaOutputKey, n_outputs);
@@ -1320,15 +1318,11 @@ meta_monitor_config_update_current (MetaMonitorConfig  *self,
 
   if (self->current && config_equal_full (current, self->current))
     {
-      config_free (current);
+      config_unref (current);
       return;
     }
 
-  if (self->current && !self->current_is_stored)
+  set_current (self, current);
-    config_free (self->current);
-
-  self->current = current;
-  self->current_is_stored = FALSE;
 }
 
 void
@@ -1336,7 +1330,17 @@ meta_monitor_config_restore_previous (MetaMonitorConfig  *self,
                                       MetaMonitorManager *manager)
 {
   if (self->previous)
-    apply_configuration (self, self->previous, manager, FALSE);
+    {
+      /* The user chose to restore the previous configuration. In this
+       * case, restore the previous configuration. */
+      MetaConfiguration *prev_config = config_ref (self->previous);
+      apply_configuration (self, prev_config, manager);
+      config_unref (prev_config);
+
+      /* After this, self->previous contains the rejected configuration.
+       * Since it was rejected, nuke it. */
+      g_clear_pointer (&self->previous, (GDestroyNotify) config_unref);
+    }
   else
     {
       if (!meta_monitor_config_apply_stored (self, manager))
@@ -1354,7 +1358,7 @@ turn_off_laptop_display (MetaMonitorConfig  *self,
     return;
 
   new = make_laptop_lid_config (self->current);
-  apply_configuration (self, new, manager, FALSE);
+  apply_configuration (self, new, manager);
   self->current_is_for_laptop_lid = TRUE;
 }
 
@@ -1513,16 +1517,7 @@ meta_monitor_config_save (MetaMonitorConfig *self)
 void
 meta_monitor_config_make_persistent (MetaMonitorConfig *self)
 {
-  if (self->current_is_stored)
+  g_hash_table_replace (self->configs, self->current, config_ref (self->current));
-    return;
-
-  self->current_is_stored = TRUE;
-  g_hash_table_replace (self->configs, self->current, self->current);
-
-  if (self->previous)
-    config_free (self->previous);
-  self->previous = NULL;
-
   meta_monitor_config_save (self);
 }