GDesktopAppInfo: avoid inotify on missing dirs

Some desktop file directories, like /usr/local/share/applications may be
missing on some systems.

When we try to inotify on these directories, this will result in a
every-4-seconds poll being setup which is quite bad.

This is an issue that should be fixed in inotify itself but the problem
is much larger there.  For now, we can work around it in GDesktopAppInfo
by refusing to monitor missing directories.

We may get some spurious notifications of changes in the case that
/usr/local/share or /usr/local/share/applications is created without
actually adding desktop files, but spurious changes can already be
reported in other cases, so that's OK.  We won't get (user-visible)
notification for a simple case of a completely unrelated file being
created (however we cannot avoid the wakeup in this case due to how
inotify works).  That's probably pretty theoretical, though, since files
in /usr don't change much and for the home directory we're likely to
have at least ~/.config and ~/.local existing.

https://bugzilla.gnome.org/show_bug.cgi?id=736350

gio/gdesktopappinfo.c

@@ -137,6 +137,7 @@ G_DEFINE_TYPE_WITH_CODE (GDesktopAppInfo, g_desktop_app_info, G_TYPE_OBJECT,
 typedef struct
 {
   gchar                      *path;
+  gchar                      *alternatively_watching;
   gboolean                    is_config;
   gboolean                    is_setup;
   GLocalDirectoryMonitor     *monitor;
@@ -155,6 +156,52 @@ static GMutex          desktop_file_dir_lock;
 /* Monitor 'changed' signal handler {{{2 */
 static void desktop_file_dir_reset (DesktopFileDir *dir);
 
+/*< internal >
+ * desktop_file_dir_get_alternative_dir:
+ * @dir: a #DesktopFileDir
+ *
+ * Gets the "alternative" directory to monitor in case the path
+ * doesn't exist.
+ *
+ * If the path exists this will return NULL, otherwise it will return a
+ * parent directory of the path.
+ *
+ * This is used to avoid inotify on a non-existent directory (which
+ * results in polling).
+ *
+ * See https://bugzilla.gnome.org/show_bug.cgi?id=522314 for more info.
+ */
+static gchar *
+desktop_file_dir_get_alternative_dir (DesktopFileDir *dir)
+{
+  gchar *parent;
+
+  /* If the directory itself exists then we need no alternative. */
+  if (g_access (dir->path, R_OK | X_OK) == 0)
+    return NULL;
+
+  /* Otherwise, try the parent directories until we find one. */
+  parent = g_path_get_dirname (dir->path);
+
+  while (g_access (parent, R_OK | X_OK) != 0)
+    {
+      gchar *tmp = parent;
+
+      parent = g_path_get_dirname (tmp);
+
+      /* If somehow we get to '/' or '.' then just stop... */
+      if (g_str_equal (parent, tmp))
+        {
+          g_free (tmp);
+          break;
+        }
+
+      g_free (tmp);
+    }
+
+  return parent;
+}
+
 static void
 desktop_file_dir_changed (GFileMonitor      *monitor,
                           GFile             *file,
@@ -163,6 +210,7 @@ desktop_file_dir_changed (GFileMonitor      *monitor,
                           gpointer           user_data)
 {
   DesktopFileDir *dir = user_data;
+  gboolean do_nothing = FALSE;
 
   /* We are not interested in receiving notifications forever just
    * because someone asked about one desktop file once.
@@ -170,14 +218,29 @@ desktop_file_dir_changed (GFileMonitor      *monitor,
    * After we receive the first notification, reset the dir, destroying
    * the monitor.  We will take this as a hint, next time that we are
    * asked, that we need to check if everything is up to date.
+   *
+   * If this is a notification for a parent directory (because the
+   * desktop directory didn't exist) then we shouldn't fire the signal
+   * unless something actually changed.
    */
   g_mutex_lock (&desktop_file_dir_lock);
 
+  if (dir->alternatively_watching)
+    {
+      gchar *alternative_dir;
+
+      alternative_dir = desktop_file_dir_get_alternative_dir (dir);
+      do_nothing = alternative_dir && g_str_equal (dir->alternatively_watching, alternative_dir);
+      g_free (alternative_dir);
+    }
+
+  if (!do_nothing)
     desktop_file_dir_reset (dir);
 
   g_mutex_unlock (&desktop_file_dir_lock);
 
   /* Notify anyone else who may be interested */
+  if (!do_nothing)
     g_app_info_monitor_fire ();
 }
 
@@ -1207,6 +1270,12 @@ desktop_file_dir_create_for_config (GArray      *array,
 static void
 desktop_file_dir_reset (DesktopFileDir *dir)
 {
+  if (dir->alternatively_watching)
+    {
+      g_free (dir->alternatively_watching);
+      dir->alternatively_watching = NULL;
+    }
+
   if (dir->monitor)
     {
       g_signal_handlers_disconnect_by_func (dir->monitor, desktop_file_dir_changed, dir);
@@ -1252,10 +1321,23 @@ desktop_file_dir_reset (DesktopFileDir *dir)
 static void
 desktop_file_dir_init (DesktopFileDir *dir)
 {
+  const gchar *watch_dir;
+
   g_assert (!dir->is_setup);
 
+  g_assert (!dir->alternatively_watching);
   g_assert (!dir->monitor);
-  dir->monitor = g_local_directory_monitor_new_in_worker (dir->path, G_FILE_MONITOR_NONE, NULL);
+
+  dir->alternatively_watching = desktop_file_dir_get_alternative_dir (dir);
+  watch_dir = dir->alternatively_watching ? dir->alternatively_watching : dir->path;
+
+  /* There is a very thin race here if the watch_dir has been _removed_
+   * between when we checked for it and when we establish the watch.
+   * Removes probably don't happen in usual operation, and even if it
+   * does (and we catch the unlikely race), the only degradation is that
+   * we will fall back to polling.
+   */
+  dir->monitor = g_local_directory_monitor_new_in_worker (watch_dir, G_FILE_MONITOR_NONE, NULL);
 
   if (dir->monitor)
     {