GSettingsBackend: add path checking helpers

Add helpers for checking if a string is a valid GSettingsBackend
[path|key|dir].

These helpers match equivalent functions found in dconf and will be used
for assertion checking when adding GSettingsBackendChangeset.

docs/reference/gio/gio-sections.txt

@@ -2497,6 +2497,9 @@ G_TYPE_FILE_DESCRIPTOR_BASED
 GSettingsBackend
 GSettingsBackendClass
 G_SETTINGS_BACKEND_EXTENSION_POINT_NAME
+g_settings_backend_is_path
+g_settings_backend_is_key
+g_settings_backend_is_dir
 g_settings_backend_get_default
 g_settings_backend_changed
 g_settings_backend_path_changed

gio/gsettingsbackend.c

@@ -1044,3 +1044,90 @@ g_settings_backend_sync_default (void)
         class->sync (backend);
     }
 }
+
+/**
+ * g_settings_backend_is_path:
+ * @string: a string
+ *
+ * Checks if @string is a valid #GSettingsBackend path.  Paths must
+ * start with '/' and not contain '//'.
+ *
+ * A path may be either a key or a dir.  See g_settings_backend_is_key()
+ * and g_settings_backend_is_dir() for examples of each.
+ *
+ * Returns: %TRUE if @string is a path
+ **/
+gboolean
+g_settings_backend_is_path (const gchar *string)
+{
+  if (string[0] != '/')
+    return FALSE;
+
+  if (strstr (string, "//"))
+    return FALSE;
+
+  return TRUE;
+}
+
+/**
+ * g_settings_backend_is_key:
+ * @string: a string
+ *
+ * Checks if @string is a valid #GSettingsBackend key.  Keys must
+ * start with '/', not contain '//' and not end with '/'.
+ *
+ * A key is the potential location of a single value within a
+ * #GSettingsBackend.
+ *
+ * "/a", "/a/b" and "/a/b/c" are examples of keys.  "", "/", "a", "a/b",
+ * "//a/b", "/a//b", and "/a/" are examples of strings that are not
+ * keys.
+ *
+ * Returns: %TRUE if @string is a key
+ **/
+gboolean
+g_settings_backend_is_key (const gchar *string)
+{
+  if (string[0] != '/')
+    return FALSE;
+
+  if (strstr (string, "//"))
+    return FALSE;
+
+  if (g_str_has_suffix (string, "/"))
+    return FALSE;
+
+  return TRUE;
+}
+
+/**
+ * g_settings_backend_is_dir:
+ * @string: a string
+ *
+ * Checks if @string is a valid #GSettingsBackend dir.  dirs must start
+ * and end with '/' and not contain '//'.
+ *
+ * A dir refers to a subtree of the database that can contain other dirs
+ * or keys.  If @string is a dir, then it will be a prefix of any key or
+ * dir contained within it.
+ *
+ * "/", "/a/" and "/a/b/" are examples of dirs.  "", "a/", "a/b/",
+ * "//a/b/", "/a//b/" and "/a" are examples of strings that are not
+ * dirs.
+ *
+ * Returns: %TRUE if @string is a dir
+ **/
+gboolean
+g_settings_backend_is_dir (const gchar *string)
+{
+  if (string[0] != '/')
+    return FALSE;
+
+  if (strstr (string, "//"))
+    return FALSE;
+
+  if (!g_str_has_suffix (string, "/"))
+    return FALSE;
+
+  return TRUE;
+}

gio/gsettingsbackend.h

@@ -169,6 +169,13 @@ GSettingsBackend *      g_null_settings_backend_new                     (void);
 GLIB_AVAILABLE_IN_ALL
 GSettingsBackend *      g_memory_settings_backend_new                   (void);
 
+GLIB_AVAILABLE_IN_2_48
+gboolean                g_settings_backend_is_path                      (const gchar         *string);
+GLIB_AVAILABLE_IN_2_48
+gboolean                g_settings_backend_is_key                       (const gchar         *string);
+GLIB_AVAILABLE_IN_2_48
+gboolean                g_settings_backend_is_dir                       (const gchar         *string);
+
 G_DEFINE_AUTOPTR_CLEANUP_FUNC(GSettingsBackend, g_object_unref)
 G_END_DECLS