gio/gsettingsbackend.c

/*
 * Copyright © 2009, 2010 Codethink Limited
 * Copyright © 2010 Red Hat, Inc.
 *
 * This library is free software; you can redistribute it and/or
 * modify it under the terms of the GNU Lesser General Public
 * License as published by the Free Software Foundation; either
 * version 2 of the licence, or (at your option) any later version.
 *
 * This library is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
 * Lesser General Public License for more details.
 *
 * You should have received a copy of the GNU Lesser General Public
 * License along with this library; if not, write to the
 * Free Software Foundation, Inc., 59 Temple Place - Suite 330,
 * Boston, MA 02111-1307, USA.
 *
 * Authors: Ryan Lortie <desrt@desrt.ca>
 *          Matthias Clasen <mclasen@redhat.com>
 */

#include "config.h"

#include "gsettingsbackendinternal.h"
#include "gnullsettingsbackend.h"
#include "giomodule-priv.h"
#include "gio-marshal.h"

#include <string.h>
#include <stdlib.h>
#include <glib.h>
#include <glibintl.h>

#include "gioalias.h"

G_DEFINE_ABSTRACT_TYPE (GSettingsBackend, g_settings_backend, G_TYPE_OBJECT)

typedef struct _GSettingsBackendWatch GSettingsBackendWatch;

struct _GSettingsBackendPrivate
{
  GSettingsBackendWatch *watches;
  gchar *context;
};

enum
{
  PROP_0,
  PROP_CONTEXT
};

/**
 * SECTION:gsettingsbackend
 * @title: GSettingsBackend
 * @short_description: an interface for settings backend implementations
 * @include: gio/gsettingsbackend.h
 * @see_also: #GSettings, #GIOExtensionPoint
 *
 * The #GSettingsBackend interface defines a generic interface for
 * non-strictly-typed data that is stored in a hierarchy. To implement
 * an alternative storage backend for #GSettings, you need to implement
 * the #GSettingsBackend interface and then make it implement the
 * extension point #G_SETTINGS_BACKEND_EXTENSION_POINT_NAME.
 *
 * The interface defines methods for reading and writing values, a
 * method for determining if writing of certain values will fail
 * (lockdown) and a change notification mechanism.
 *
 * The semantics of the interface are very precisely defined and
 * implementations must carefully adhere to the expectations of
 * callers that are documented on each of the interface methods.
 *
 * Some of the GSettingsBackend functions accept or return a #GTree.
 * These trees always have strings as keys and #GVariant as values.
 * g_settings_backend_create_tree() is a convenience function to create
 * suitable trees.
 *
 * <note><para>
 * The #GSettingsBackend API is exported to allow third-party
 * implementations, but does not carry the same stability guarantees
 * as the public GIO API. For this reason, you have to define the
 * C preprocessor symbol #G_SETTINGS_ENABLE_BACKEND before including
 * <filename>gio/gsettingsbackend.h</filename>
 * </para></note>
 **/

struct _GSettingsBackendWatch
{
  GSettingsBackendChangedFunc              changed;
  GSettingsBackendPathChangedFunc          path_changed;
  GSettingsBackendKeysChangedFunc          keys_changed;
  GSettingsBackendWritableChangedFunc      writable_changed;
  GSettingsBackendPathWritableChangedFunc  path_writable_changed;
  gpointer                                 user_data;

  GSettingsBackendWatch                   *next;
};

void
g_settings_backend_watch (GSettingsBackend                        *backend,
                          GSettingsBackendChangedFunc              changed,
                          GSettingsBackendPathChangedFunc          path_changed,
                          GSettingsBackendKeysChangedFunc          keys_changed,
                          GSettingsBackendWritableChangedFunc      writable_changed,
                          GSettingsBackendPathWritableChangedFunc  path_writable_changed,
                          gpointer                                 user_data)
{
  GSettingsBackendWatch *watch;

  watch = g_slice_new (GSettingsBackendWatch);
  watch->changed = changed;
  watch->path_changed = path_changed;
  watch->keys_changed = keys_changed;
  watch->writable_changed = writable_changed;
  watch->path_writable_changed = path_writable_changed;
  watch->user_data = user_data;

  watch->next = backend->priv->watches;
  backend->priv->watches = watch;
}

void
g_settings_backend_unwatch (GSettingsBackend *backend,
                            gpointer          user_data)
{
  GSettingsBackendWatch **ptr;

  for (ptr = &backend->priv->watches; *ptr; ptr = &(*ptr)->next)
    if ((*ptr)->user_data == user_data)
      {
        GSettingsBackendWatch *tmp = *ptr;

        *ptr = tmp->next;
        g_slice_free (GSettingsBackendWatch, tmp);

        return;
      }

  g_assert_not_reached ();
}

static gboolean
is_key (const gchar *key)
{
  gint length;
  gint i;

  g_return_val_if_fail (key != NULL, FALSE);
  g_return_val_if_fail (key[0] == '/', FALSE);

  for (i = 1; key[i]; i++)
    g_return_val_if_fail (key[i] != '/' || key[i + 1] != '/', FALSE);

  length = i;

  g_return_val_if_fail (key[length - 1] != '/', FALSE);

  return TRUE;
}

static gboolean
is_path (const gchar *path)
{
  gint length;
  gint i;

  g_return_val_if_fail (path != NULL, FALSE);
  g_return_val_if_fail (path[0] == '/', FALSE);

  for (i = 1; path[i]; i++)
    g_return_val_if_fail (path[i] != '/' || path[i + 1] != '/', FALSE);

  length = i;

  g_return_val_if_fail (path[length - 1] == '/', FALSE);

  return TRUE;
}

/**
 * g_settings_backend_changed:
 * @backend: a #GSettingsBackend implementation
 * @key: the name of the key
 * @origin_tag: the origin tag
 *
 * Signals that a single key has possibly changed.  Backend
 * implementations should call this if a key has possibly changed its
 * value.
 *
 * @key must be a valid key (ie: starting with a slash, not containing
 * '//', and not ending with a slash).
 *
 * The implementation must call this function during any call to
 * g_settings_backend_write(), before the call returns (except in the
 * case that no keys are actually changed and it cares to detect this
 * fact).  It may not rely on the existence of a mainloop for
 * dispatching the signal later.
 *
 * The implementation may call this function at any other time it likes
 * in response to other events (such as changes occuring outside of the
 * program).  These calls may originate from a mainloop or may originate
 * in response to any other action (including from calls to
 * g_settings_backend_write()).
 *
 * In the case that this call is in response to a call to
 * g_settings_backend_write() then @origin_tag must be set to the same
 * value that was passed to that call.
 *
 * Since: 2.26
 **/
void
g_settings_backend_changed (GSettingsBackend *backend,
                            const gchar      *key,
                            gpointer          origin_tag)
{
  GSettingsBackendWatch *watch;

  g_return_if_fail (G_IS_SETTINGS_BACKEND (backend));
  g_return_if_fail (is_key (key));

  for (watch = backend->priv->watches; watch; watch = watch->next)
    watch->changed (backend, key, origin_tag, watch->user_data);
}

/**
 * g_settings_backend_keys_changed:
 * @backend: a #GSettingsBackend implementation
 * @path: the path containing the changes
 * @items: the %NULL-terminated list of changed keys
 * @origin_tag: the origin tag
 *
 * Signals that a list of keys have possibly changed.  Backend
 * implementations should call this if keys have possibly changed their
 * values.
 *
 * @path must be a valid path (ie: starting and ending with a slash and
 * not containing '//').  Each string in @items must form a valid key
 * name when @path is prefixed to it (ie: each item must not start or
 * end with '/' and must not contain '//').
 *
 * The meaning of this signal is that any of the key names resulting
 * from the contatenation of @path with each item in @items may have
 * changed.
 *
 * The same rules for when notifications must occur apply as per
 * g_settings_backend_changed().  These two calls can be used
 * interchangeably if exactly one item has changed (although in that
 * case g_settings_backend_changed() is definitely preferred).
 *
 * For efficiency reasons, the implementation should strive for @path to
 * be as long as possible (ie: the longest common prefix of all of the
 * keys that were changed) but this is not strictly required.
 *
 * Since: 2.26
 */
void
g_settings_backend_keys_changed (GSettingsBackend    *backend,
                                 const gchar         *path,
                                 gchar const * const *items,
                                 gpointer             origin_tag)
{
  GSettingsBackendWatch *watch;

  g_return_if_fail (G_IS_SETTINGS_BACKEND (backend));
  g_return_if_fail (path[0] == '\0' || is_path (path));
  g_return_if_fail (items != NULL);

  for (watch = backend->priv->watches; watch; watch = watch->next)
    watch->keys_changed (backend, path, items, origin_tag, watch->user_data);
}

/**
 * g_settings_backend_path_changed:
 * @backend: a #GSettingsBackend implementation
 * @path: the path containing the changes
 * @origin_tag: the origin tag
 *
 * Signals that all keys below a given path may have possibly changed.
 * Backend implementations should call this if an entire path of keys
 * have possibly changed their values.
 *
 * @path must be a valid path (ie: starting and ending with a slash and
 * not containing '//').
 *
 * The meaning of this signal is that any of the key which has a name
 * starting with @path may have changed.
 *
 * The same rules for when notifications must occur apply as per
 * g_settings_backend_changed().  This call might be an appropriate
 * reasponse to a 'reset' call but implementations are also free to
 * explicitly list the keys that were affected by that call if they can
 * easily do so.
 *
 * For efficiency reasons, the implementation should strive for @path to
 * be as long as possible (ie: the longest common prefix of all of the
 * keys that were changed) but this is not strictly required.  As an
 * example, if this function is called with the path of "/" then every
 * single key in the application will be notified of a possible change.
 *
 * Since: 2.26
 */
void
g_settings_backend_path_changed (GSettingsBackend *backend,
                                 const gchar      *path,
                                 gpointer          origin_tag)
{
  GSettingsBackendWatch *watch;

  g_return_if_fail (G_IS_SETTINGS_BACKEND (backend));
  g_return_if_fail (is_path (path));

  for (watch = backend->priv->watches; watch; watch = watch->next)
    watch->path_changed (backend, path, origin_tag, watch->user_data);
}

/**
 * g_settings_backend_writable_changed:
 * @backend: a #GSettingsBackend implementation
 * @key: the name of the key
 *
 * Signals that the writability of a single key has possibly changed.
 *
 * Since GSettings performs no locking operations for itself, this call
 * will always be made in response to external events.
 *
 * Since: 2.26
 **/
void
g_settings_backend_writable_changed (GSettingsBackend *backend,
                                     const gchar      *key)
{
  GSettingsBackendWatch *watch;

  g_return_if_fail (G_IS_SETTINGS_BACKEND (backend));
  g_return_if_fail (is_key (key));

  for (watch = backend->priv->watches; watch; watch = watch->next)
    watch->writable_changed (backend, key, watch->user_data);
}

/**
 * g_settings_backend_path_writable_changed:
 * @backend: a #GSettingsBackend implementation
 * @path: the name of the path
 *
 * Signals that the writability of all keys below a given path may have
 * changed.
 *
 * Since GSettings performs no locking operations for itself, this call
 * will always be made in response to external events.
 *
 * Since: 2.26
 **/
void
g_settings_backend_path_writable_changed (GSettingsBackend *backend,
                                          const gchar      *path)
{
  GSettingsBackendWatch *watch;

  g_return_if_fail (G_IS_SETTINGS_BACKEND (backend));
  g_return_if_fail (is_path (path));

  for (watch = backend->priv->watches; watch; watch = watch->next)
    watch->path_writable_changed (backend, path, watch->user_data);
}

typedef struct
{
  const gchar **keys;
  GVariant **values;
  gint prefix_len;
  gchar *prefix;
} FlattenState;

static gboolean
g_settings_backend_flatten_one (gpointer key,
                                gpointer value,
                                gpointer user_data)
{
  FlattenState *state = user_data;
  const gchar *skey = key;
  gint i;

  g_return_val_if_fail (is_key (key), TRUE);

  /* calculate longest common prefix */
  if (state->prefix == NULL)
    {
      gchar *last_byte;

      /* first key?  just take the prefix up to the last '/' */
      state->prefix = g_strdup (skey);
      last_byte = strrchr (state->prefix, '/') + 1;
      state->prefix_len = last_byte - state->prefix;
      *last_byte = '\0';
    }
  else
    {
      /* find the first character that does not match.  we will
       * definitely find one because the prefix ends in '/' and the key
       * does not.  also: no two keys in the tree are the same.
       */
      for (i = 0; state->prefix[i] == skey[i]; i++);

      /* check if we need to shorten the prefix */
      if (state->prefix[i] != '\0')
        {
          /* find the nearest '/', terminate after it */
          while (state->prefix[i - 1] != '/')
            i--;

          state->prefix[i] = '\0';
          state->prefix_len = i;
        }
    }


  /* save the entire item into the array.
   * the prefixes will be removed later.
   */
  *state->keys++ = key;

  if (state->values)
    *state->values++ = value;

  return FALSE;
}

/**
 * g_settings_backend_flatten_tree:
 * @tree: a #GTree containing the changes
 * @path: the location to save the path
 * @keys: the location to save the relative keys
 * @values: the location to save the values, or %NULL
 *
 * Calculate the longest common prefix of all keys in a tree and write
 * out an array of the key names relative to that prefix and,
 * optionally, the value to store at each of those keys.
 *
 * You must free the value returned in @path, @keys and @values using
 * g_free().  You should not attempt to free or unref the contents of
 * @keys or @values.
 *
 * Since: 2.26
 **/
void
g_settings_backend_flatten_tree (GTree         *tree,
                                 gchar        **path,
                                 const gchar ***keys,
                                 GVariant    ***values)
{
  FlattenState state = { 0, };
  gsize nnodes;

  nnodes = g_tree_nnodes (tree);

  *keys = state.keys = g_new (const gchar *, nnodes + 1);
  state.keys[nnodes] = NULL;

  if (values != NULL)
    {
      *values = state.values = g_new (GVariant *, nnodes + 1);
      state.values[nnodes] = NULL;
    }

  g_tree_foreach (tree, g_settings_backend_flatten_one, &state);
  g_return_if_fail (*keys + nnodes == state.keys);

  *path = state.prefix;
  while (nnodes--)
    *--state.keys += state.prefix_len;
}

/**
 * g_settings_backend_changed_tree:
 * @backend: a #GSettingsBackend implementation
 * @tree: a #GTree containing the changes
 * @origin_tag: the origin tag
 *
 * This call is a convenience wrapper.  It gets the list of changes from
 * @tree, computes the longest common prefix and calls
 * g_settings_backend_changed().
 *
 * Since: 2.26
 **/
void
g_settings_backend_changed_tree (GSettingsBackend *backend,
                                 GTree            *tree,
                                 gpointer          origin_tag)
{
  GSettingsBackendWatch *watch;
  const gchar **keys;
  gchar *path;

  g_return_if_fail (G_IS_SETTINGS_BACKEND (backend));

  g_settings_backend_flatten_tree (tree, &path, &keys, NULL);

  for (watch = backend->priv->watches; watch; watch = watch->next)
    watch->keys_changed (backend, path, keys, origin_tag, watch->user_data);

  g_free (path);
  g_free (keys);
}

/*< private >
 * g_settings_backend_read:
 * @backend: a #GSettingsBackend implementation
 * @key: the key to read
 * @expected_type: a #GVariantType hint
 * @returns: the value that was read, or %NULL
 *
 * Reads a key. This call will never block.
 *
 * If the key exists, the value associated with it will be returned.
 * If the key does not exist, %NULL will be returned.
 *
 * If @expected_type is given, it serves as a type hint to the backend.
 * If you expect a key of a certain type then you should give
 * @expected_type to increase your chances of getting it.  Some backends
 * may ignore this argument and return values of a different type; it is
 * mostly used by backends that don't store strong type information.
 */
GVariant *
g_settings_backend_read (GSettingsBackend   *backend,
                         const gchar        *key,
                         const GVariantType *expected_type,
                         gboolean            default_value)
{
  return G_SETTINGS_BACKEND_GET_CLASS (backend)
    ->read (backend, key, expected_type, default_value);
}

/*< private >
 * g_settings_backend_write:
 * @backend: a #GSettingsBackend implementation
 * @key: the name of the key
 * @value: a #GVariant value to write to this key
 * @origin_tag: the origin tag
 * @returns: %TRUE if the write succeeded, %FALSE if the key was not writable
 *
 * Writes exactly one key.
 *
 * This call does not fail.  During this call a
 * #GSettingsBackend::changed signal will be emitted if the value of the
 * key has changed.  The updated key value will be visible to any signal
 * callbacks.
 *
 * One possible method that an implementation might deal with failures is
 * to emit a second "changed" signal (either during this call, or later)
 * to indicate that the affected keys have suddenly "changed back" to their
 * old values.
 */
gboolean
g_settings_backend_write (GSettingsBackend *backend,
                          const gchar      *key,
                          GVariant         *value,
                          gpointer          origin_tag)
{
  return G_SETTINGS_BACKEND_GET_CLASS (backend)
    ->write (backend, key, value, origin_tag);
}

/*< private >
 * g_settings_backend_write_keys:
 * @backend: a #GSettingsBackend implementation
 * @values: a #GTree containing key-value pairs to write
 * @origin_tag: the origin tag
 *
 * Writes one or more keys.  This call will never block.
 *
 * The key of each item in the tree is the key name to write to and the
 * value is a #GVariant to write.  The proper type of #GTree for this
 * call can be created with g_settings_backend_create_tree().  This call
 * might take a reference to the tree; you must not modified the #GTree
 * after passing it to this call.
 *
 * This call does not fail.  During this call a #GSettingsBackend::changed
 * signal will be emitted if any keys have been changed.  The new values of
 * all updated keys will be visible to any signal callbacks.
 *
 * One possible method that an implementation might deal with failures is
 * to emit a second "changed" signal (either during this call, or later)
 * to indicate that the affected keys have suddenly "changed back" to their
 * old values.
 */
gboolean
g_settings_backend_write_keys (GSettingsBackend *backend,
                               GTree            *tree,
                               gpointer          origin_tag)
{
  return G_SETTINGS_BACKEND_GET_CLASS (backend)
    ->write_keys (backend, tree, origin_tag);
}

/*< private >
 * g_settings_backend_reset:
 * @backend: a #GSettingsBackend implementation
 * @key: the name of a key
 * @origin_tag: the origin tag
 *
 * "Resets" the named key to its "default" value (ie: after system-wide
 * defaults, mandatory keys, etc. have been taken into account) or possibly
 * unsets it.
 */
void
g_settings_backend_reset (GSettingsBackend *backend,
                          const gchar      *key,
                          gpointer          origin_tag)
{
  G_SETTINGS_BACKEND_GET_CLASS (backend)
    ->reset (backend, key, origin_tag);
}

/*< private >
 * g_settings_backend_reset_path:
 * @backend: a #GSettingsBackend implementation
 * @name: the name of a key or path
 * @origin_tag: the origin tag
 *
 * "Resets" the named path.  This means that every key under the path is
 * reset.
 */
void
g_settings_backend_reset_path (GSettingsBackend *backend,
                               const gchar      *path,
                               gpointer          origin_tag)
{
  G_SETTINGS_BACKEND_GET_CLASS (backend)
    ->reset_path (backend, path, origin_tag);
}

/*< private >
 * g_settings_backend_get_writable:
 * @backend: a #GSettingsBackend implementation
 * @key: the name of a key
 * @returns: %TRUE if the key is writable
 *
 * Finds out if a key is available for writing to.  This is the
 * interface through which 'lockdown' is implemented.  Locked down
 * keys will have %FALSE returned by this call.
 *
 * You should not write to locked-down keys, but if you do, the
 * implementation will deal with it.
 */
gboolean
g_settings_backend_get_writable (GSettingsBackend *backend,
                                 const gchar      *key)
{
  return G_SETTINGS_BACKEND_GET_CLASS (backend)
    ->get_writable (backend, key);
}

/*< private >
 * g_settings_backend_unsubscribe:
 * @backend: a #GSettingsBackend
 * @name: a key or path to subscribe to
 *
 * Reverses the effect of a previous call to
 * g_settings_backend_subscribe().
 */
void
g_settings_backend_unsubscribe (GSettingsBackend *backend,
                                const char       *name)
{
  G_SETTINGS_BACKEND_GET_CLASS (backend)
    ->unsubscribe (backend, name);
}

/*< private >
 * g_settings_backend_subscribe:
 * @backend: a #GSettingsBackend
 * @name: a key or path to subscribe to
 *
 * Requests that change signals be emitted for events on @name.
 */
void
g_settings_backend_subscribe (GSettingsBackend *backend,
                              const gchar      *name)
{
  G_SETTINGS_BACKEND_GET_CLASS (backend)
    ->subscribe (backend, name);
}

static void
g_settings_backend_set_property (GObject         *object,
                                 guint            prop_id,
                                 const GValue    *value,
                                 GParamSpec      *pspec)
{
  GSettingsBackend *backend = G_SETTINGS_BACKEND (object);

  switch (prop_id)
    {
    case PROP_CONTEXT:
      backend->priv->context = g_value_dup_string (value);
      break;

    default:
      G_OBJECT_WARN_INVALID_PROPERTY_ID (object, prop_id, pspec);
      break;
    }
}

static void
g_settings_backend_get_property (GObject    *object,
                                 guint       prop_id,
                                 GValue     *value,
                                 GParamSpec *pspec)
{
  GSettingsBackend *backend = G_SETTINGS_BACKEND (object);

  switch (prop_id)
    {
    case PROP_CONTEXT:
      g_value_set_string (value, backend->priv->context);
      break;

    default:
      G_OBJECT_WARN_INVALID_PROPERTY_ID (object, prop_id, pspec);
      break;
    }
}

static void
g_settings_backend_finalize (GObject *object)
{
  GSettingsBackend *backend = G_SETTINGS_BACKEND (object);

  g_free (backend->priv->context);

  G_OBJECT_CLASS (g_settings_backend_parent_class)->finalize (object);
}

static void
ignore_subscription (GSettingsBackend *backend,
                     const gchar      *key)
{
}

static void
g_settings_backend_init (GSettingsBackend *backend)
{
  backend->priv = G_TYPE_INSTANCE_GET_PRIVATE (backend,
                                               G_TYPE_SETTINGS_BACKEND,
                                               GSettingsBackendPrivate);
}

static void
g_settings_backend_class_init (GSettingsBackendClass *class)
{
  GObjectClass *gobject_class = G_OBJECT_CLASS (class);

  class->subscribe = ignore_subscription;
  class->unsubscribe = ignore_subscription;

  gobject_class->get_property = g_settings_backend_get_property;
  gobject_class->set_property = g_settings_backend_set_property;
  gobject_class->finalize = g_settings_backend_finalize;

  g_type_class_add_private (class, sizeof (GSettingsBackendPrivate));

  /**
   * GSettingsBackend:context:
   *
   * The "context" property gives a hint to the backend as to
   * what storage to use. It is up to the implementation to make
   * use of this information.
   *
   * E.g. DConf supports "user", "system", "defaults" and "login"
   * contexts.
   *
   * If your backend supports different contexts, you should also
   * provide an implementation of the supports_context() class
   * function in #GSettingsBackendClass.
   */
  g_object_class_install_property (gobject_class, PROP_CONTEXT,
    g_param_spec_string ("context", P_("Context"),
                         P_("An identifier to decide which storage to use"),
                         "", G_PARAM_READWRITE |
                         G_PARAM_CONSTRUCT_ONLY | G_PARAM_STATIC_STRINGS));

}

/*< private >
 * g_settings_backend_create_tree:
 * @returns: a new #GTree
 *
 * This is a convenience function for creating a tree that is compatible
 * with g_settings_backend_write().  It merely calls g_tree_new_full()
 * with strcmp(), g_free() and g_variant_unref().
 */
GTree *
g_settings_backend_create_tree (void)
{
  return g_tree_new_full ((GCompareDataFunc) strcmp, NULL,
                          g_free, (GDestroyNotify) g_variant_unref);
}


static gpointer
get_default_backend (const gchar *context)
{
  GIOExtension *extension = NULL;
  GIOExtensionPoint *point;
  GList *extensions;
  const gchar *env;
  GType type;

  _g_io_modules_ensure_loaded ();

  point = g_io_extension_point_lookup (G_SETTINGS_BACKEND_EXTENSION_POINT_NAME);

  if ((env = getenv ("GSETTINGS_BACKEND")))
    {
      extension = g_io_extension_point_get_extension_by_name (point, env);

      if (extension == NULL)
        g_warning ("Can't find GSettings backend '%s' given in "
                   "GSETTINGS_BACKEND environment variable", env);
    }

  if (extension == NULL)
    {
      extensions = g_io_extension_point_get_extensions (point);

      if (extensions == NULL)
        g_error ("No GSettingsBackend implementations exist.");

      extension = extensions->data;
    }

  if (context[0] != '\0') /* (context != "") */
    {
      GSettingsBackendClass *backend_class;
      GTypeClass *class;

      class = g_io_extension_ref_class (extension);
      backend_class = G_SETTINGS_BACKEND_CLASS (class);

      if (backend_class->supports_context == NULL ||
          !backend_class->supports_context (context))
        {
          g_type_class_unref (class);
          return NULL;
        }

      g_type_class_unref (class);
    }

  type = g_io_extension_get_type (extension);

  return g_object_new (type, "context", context, NULL);
}

static GHashTable *g_settings_backends;

/*< private >
 * g_settings_backend_get_with_context:
 * @context: a context that might be used by the backend to determine
 *     which storage to use, or %NULL to use the default storage
 * @returns: the default #GSettingsBackend
 *
 * Returns the default #GSettingsBackend. It is possible to override
 * the default by setting the <envar>GSETTINGS_BACKEND</envar>
 * environment variable to the name of a settings backend.
 *
 * The @context parameter can be used to indicate that a different
 * than the default storage is desired. E.g. the DConf backend lets
 * you use "user", "system", "defaults" and "login" as contexts.
 *
 * If @context is not supported by the implementation, this function
 * returns an instance of the #GSettingsMemoryBackend.
 * See g_settings_backend_supports_context(),
 *
 * The user does not own the return value and it must not be freed.
 */
GSettingsBackend *
g_settings_backend_get_with_context (const gchar *context)
{
  GSettingsBackend *backend;

  g_return_val_if_fail (context != NULL, NULL);

  _g_io_modules_ensure_extension_points_registered ();

  if (g_settings_backends == NULL)
    g_settings_backends = g_hash_table_new (g_str_hash, g_str_equal);

  backend = g_hash_table_lookup (g_settings_backends, context);

  if (!backend)
    {
      backend = get_default_backend (context);

      if (!backend)
        backend = g_null_settings_backend_new ();

      g_hash_table_insert (g_settings_backends, g_strdup (context), backend);
    }

  return g_object_ref (backend);
}

/*< private >
 * g_settings_backend_supports_context:
 * @context: a context string that might be passed to
 *     g_settings_backend_new_with_context()
 * @returns: #TRUE if @context is supported
 *
 * Determines if the given context is supported by the default
 * GSettingsBackend implementation.
 */
gboolean
g_settings_backend_supports_context (const gchar *context)
{
  GSettingsBackend *backend;

  g_return_val_if_fail (context != NULL, FALSE);

  backend = get_default_backend (context);

  if (backend)
    {
      g_object_unref (backend);
      return TRUE;
    }

  return FALSE;
}

/**
 * g_settings_backend_setup:
 * @context: a context string (not %NULL or "")
 * @backend: a #GSettingsBackend
 *
 * Sets up @backend for use with #GSettings.
 *
 * If you create a #GSettings with its context property set to @context
 * then it will use the backend given to this function.  See
 * g_settings_new_with_context().
 *
 * The backend must be set up before any settings objects are created
 * for the named context.
 *
 * It is not possible to specify a backend for the default context.
 *
 * This function takes a reference on @backend and never releases it.
 *
 * Since: 2.26
 **/
void
g_settings_backend_setup (const gchar      *context,
                          GSettingsBackend *backend)
{
  g_return_if_fail (context[0] != '\0');
  g_return_if_fail (G_IS_SETTINGS_BACKEND (backend));

  if (g_settings_backends == NULL)
    g_settings_backends = g_hash_table_new (g_str_hash, g_str_equal);

  if (g_hash_table_lookup (g_settings_backends, context))
    g_error ("A GSettingsBackend already exists for context '%s'", context);

  g_hash_table_insert (g_settings_backends,
                       g_strdup (context),
                       g_object_ref (backend));
}

#define __G_SETTINGS_BACKEND_C__
#include "gioaliasdef.c"