shell-global: Make saving of persistent state asynchronous

This is an expensive operation that is best avoided in the main loop. Given
the call doesn't care much about returning error or status, it can just
be made async within.

Every operation on a given file will be destructive wrt previous
operations on the same file, so we just cancel any pending operation on
it before batching the current one.

Closes: https://gitlab.gnome.org/GNOME/gnome-shell/issues/815

src/shell-global.c

@@ -85,6 +85,8 @@ struct _ShellGlobal {
   GSList *leisure_closures;
   guint leisure_function_id;
 
+  GHashTable *save_ops;
+
   gboolean has_modal;
   gboolean frame_timestamps;
   gboolean frame_finish_timestamp;
@@ -311,6 +313,10 @@ shell_global_init (ShellGlobal *global)
                                      NULL);
 
   g_strfreev (search_path);
+
+  global->save_ops = g_hash_table_new_full (g_file_hash,
+                                            (GEqualFunc) g_file_equal,
+                                            g_object_unref, g_object_unref);
 }
 
 static void
@@ -330,6 +336,8 @@ shell_global_finalize (GObject *object)
   g_free (global->imagedir);
   g_free (global->userdatadir);
 
+  g_hash_table_unref (global->save_ops);
+
   G_OBJECT_CLASS(shell_global_parent_class)->finalize (object);
 }
 
@@ -1541,20 +1549,77 @@ shell_global_get_session_mode (ShellGlobal *global)
 }
 
 static void
-save_variant (GFile      *dir,
+delete_variant_cb (GObject      *object,
-              const char *property_name,
+                   GAsyncResult *result,
-              GVariant   *variant)
+                   gpointer      user_data)
+{
+  ShellGlobal *global = user_data;
+  GError *error = NULL;
+
+  if (!g_file_delete_finish (G_FILE (object), result, &error))
+    {
+      if (!g_error_matches (error, G_IO_ERROR, G_IO_ERROR_CANCELLED))
+        {
+          g_warning ("Could not delete runtime/persistent state file: %s\n",
+                     error->message);
+        }
+
+      g_error_free (error);
+    }
+
+  g_hash_table_remove (global->save_ops, object);
+}
+
+static void
+replace_variant_cb (GObject      *object,
+                    GAsyncResult *result,
+                    gpointer      user_data)
+{
+  ShellGlobal *global = user_data;
+  GError *error = NULL;
+
+  if (!g_file_replace_contents_finish (G_FILE (object), result, NULL, &error))
+    {
+      if (!g_error_matches (error, G_IO_ERROR, G_IO_ERROR_CANCELLED))
+        {
+          g_warning ("Could not replace runtime/persistent state file: %s\n",
+                     error->message);
+        }
+
+      g_error_free (error);
+    }
+
+  g_hash_table_remove (global->save_ops, object);
+}
+
+static void
+save_variant (ShellGlobal *global,
+              GFile       *dir,
+              const char  *property_name,
+              GVariant    *variant)
 {
   GFile *path = g_file_get_child (dir, property_name);
+  GCancellable *cancellable;
+
+  cancellable = g_hash_table_lookup (global->save_ops, path);
+  g_cancellable_cancel (cancellable);
+
+  cancellable = g_cancellable_new ();
+  g_hash_table_insert (global->save_ops, g_object_ref (path), cancellable);
 
   if (variant == NULL || g_variant_get_data (variant) == NULL)
-    (void) g_file_delete (path, NULL, NULL);
+    {
+      g_file_delete_async (path, G_PRIORITY_DEFAULT, cancellable,
+                           delete_variant_cb, global);
+    }
   else
     {
-      gsize size = g_variant_get_size (variant);
+      g_file_replace_contents_async (path,
-      g_file_replace_contents (path, g_variant_get_data (variant), size,
+                                     g_variant_get_data (variant),
-                               NULL, FALSE, G_FILE_CREATE_REPLACE_DESTINATION,
+                                     g_variant_get_size (variant),
-                               NULL, NULL, NULL);
+                                     NULL, FALSE,
+                                     G_FILE_CREATE_REPLACE_DESTINATION,
+                                     cancellable, replace_variant_cb, global);
     }
 
   g_object_unref (path);
@@ -1608,7 +1673,7 @@ shell_global_set_runtime_state (ShellGlobal  *global,
                                 const char   *property_name,
                                 GVariant     *variant)
 {
-  save_variant (global->runtime_state_path, property_name, variant);
+  save_variant (global, global->runtime_state_path, property_name, variant);
 }
 
 /**
@@ -1643,7 +1708,7 @@ shell_global_set_persistent_state (ShellGlobal *global,
                                    const char  *property_name,
                                    GVariant    *variant)
 {
-  save_variant (global->userdatadir_path, property_name, variant);
+  save_variant (global, global->userdatadir_path, property_name, variant);
 }
 
 /**