/*

 * Copyright © 2009, 2010 Codethink Limited

 * Copyright © 2010 Red Hat, Inc.

 *

 * SPDX-License-Identifier: LGPL-2.1-or-later

 *

 * 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 2.1 of the License, 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, see <http://www.gnu.org/licenses/>.

 *

 * Authors: Ryan Lortie <desrt@desrt.ca>

 *          Matthias Clasen <mclasen@redhat.com>

 */

#include "config.h"

#include "gsettingsbackendinternal.h"

#include "gsimplepermission.h"

#include "giomodule-priv.h"

#include <string.h>

#include <stdlib.h>

#include <glib.h>

#include <glibintl.h>

typedef struct _GSettingsBackendClosure GSettingsBackendClosure;

typedef struct _GSettingsBackendWatch   GSettingsBackendWatch;

struct _GSettingsBackendPrivate

{

  GSettingsBackendWatch *watches;

  GMutex lock;

};

G_DEFINE_ABSTRACT_TYPE_WITH_PRIVATE (GSettingsBackend, g_settings_backend, G_TYPE_OBJECT)

/* For g_settings_backend_sync_default(), we only want to actually do

 * the sync if the backend already exists.  This avoids us creating an

 * entire GSettingsBackend in order to call a do-nothing sync()

 * operation on it.  This variable lets us avoid that.

 */

static gboolean g_settings_has_backend;

/**

 * GSettingsBackend:

 *

 * 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 [class@Gio.Settings], 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

 * [struct@GLib.Tree]. These trees always have strings as keys and

 * [struct@GLib.Variant] as values.

 *

 * 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

 * `gio/gsettingsbackend.h`.

 **/

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;

}

struct _GSettingsBackendWatch

{

  /* Always access the target via the weak reference */

  GWeakRef                       target;

  /* The pointer is only for comparison from the weak notify,

   * at which point the target might already be close to

   * destroyed. It's not safe to use it for anything anymore

   * at that point */

  GObject                       *target_ptr;

  const GSettingsListenerVTable *vtable;

  GMainContext                  *context;

  GSettingsBackendWatch         *next;

};

struct _GSettingsBackendClosure

{

  void (*function) (GObject           *target,

                    GSettingsBackend  *backend,

                    const gchar       *name,

                    gpointer           origin_tag,

                    gchar            **names);

  GMainContext      *context;

  GObject           *target;

  GSettingsBackend  *backend;

  gchar             *name;

  gpointer           origin_tag;

  gchar            **names;

};

static void

g_settings_backend_watch_weak_notify (gpointer  data,

                                      GObject  *where_the_object_was)

{

  GSettingsBackend *backend = data;

  GSettingsBackendWatch **ptr;

  /* search and remove */

  g_mutex_lock (&backend->priv->lock);

  for (ptr = &backend->priv->watches; *ptr; ptr = &(*ptr)->next)

    if ((*ptr)->target_ptr == where_the_object_was)

      {

        GSettingsBackendWatch *tmp = *ptr;

        *ptr = tmp->next;

        g_weak_ref_clear (&tmp->target);

        g_slice_free (GSettingsBackendWatch, tmp);

        g_mutex_unlock (&backend->priv->lock);

        return;

      }

  /* we didn't find it.  that shouldn't happen. */

  g_assert_not_reached ();

}

/*< private >

 * g_settings_backend_watch:

 * @backend: a #GSettingsBackend

 * @target: the GObject (typically GSettings instance) to call back to

 * @context: (nullable): a #GMainContext, or %NULL

 * ...: callbacks...

 *

 * Registers a new watch on a #GSettingsBackend.

 *

 * note: %NULL @context does not mean "default main context" but rather,

 * "it is okay to dispatch in any context".  If the default main context

 * is specifically desired then it must be given.

 *

 * note also: if you want to get meaningful values for the @origin_tag

 * that appears as an argument to some of the callbacks, you *must* have

 * @context as %NULL.  Otherwise, you are subject to cross-thread

 * dispatching and whatever owned @origin_tag at the time that the event

 * occurred may no longer own it.  This is a problem if you consider that

 * you may now be the new owner of that address and mistakenly think

 * that the event in question originated from yourself.

 *

 * tl;dr: If you give a non-%NULL @context then you must ignore the

 * value of @origin_tag given to any callbacks.

 **/

void

g_settings_backend_watch (GSettingsBackend              *backend,

                          const GSettingsListenerVTable *vtable,

                          GObject                       *target,

                          GMainContext                  *context)

{

  GSettingsBackendWatch *watch;

  /* For purposes of discussion, we assume that our target is a

   * GSettings instance.

   *

   * Our strategy to defend against the final reference dropping on the

   * GSettings object in a thread other than the one that is doing the

   * dispatching is as follows:

   *

   *  1) hold a strong reference on the GSettings during an outstanding

   *     dispatch.  This ensures that the delivery is always possible while

   *     the GSettings object is alive, and if this was the last reference

   *     then it will be dropped from the dispatch thread.

   *

   *  2) hold a weak reference on the GSettings at other times.  This

   *     allows us to receive early notification of pending destruction

   *     of the object.  At this point, it is still safe to obtain a

   *     reference on the GObject to keep it alive, so #1 will work up

   *     to that point.  After that point, we'll have been able to drop

   *     the watch from the list.

   *

   * Note, in particular, that it's not possible to simply have an

   * "unwatch" function that gets called from the finalize function of

   * the GSettings instance because, by that point it is no longer

   * possible to keep the object alive using g_object_ref() and we would

   * have no way of knowing this.

   *

   * Note also that we need to hold a reference on the main context here

   * since the GSettings instance may be finalized before the closure runs.

   *

   * All access to the list holds a mutex.  We have some strategies to

   * avoid some of the pain that would be associated with that.

   */

  watch = g_slice_new (GSettingsBackendWatch);

  watch->context = context;

  watch->vtable = vtable;

  g_weak_ref_init (&watch->target, target);

  watch->target_ptr = target;

  g_object_weak_ref (target, g_settings_backend_watch_weak_notify, backend);

  /* linked list prepend */

  g_mutex_lock (&backend->priv->lock);

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

  backend->priv->watches = watch;

  g_mutex_unlock (&backend->priv->lock);

}

void

g_settings_backend_unwatch (GSettingsBackend *backend,

                            GObject          *target)

{

  /* Our caller surely owns a reference on 'target', so the order of

   * these two calls is unimportant.

   */

  g_object_weak_unref (target, g_settings_backend_watch_weak_notify, backend);

  g_settings_backend_watch_weak_notify (backend, target);

}

static gboolean

g_settings_backend_invoke_closure (gpointer user_data)

{

  GSettingsBackendClosure *closure = user_data;

  closure->function (closure->target, closure->backend, closure->name,

                     closure->origin_tag, closure->names);

  if (closure->context)

    g_main_context_unref (closure->context);

  g_object_unref (closure->backend);

  g_object_unref (closure->target);

  g_strfreev (closure->names);

  g_free (closure->name);

  g_slice_free (GSettingsBackendClosure, closure);

  return FALSE;

}

static void

g_settings_backend_dispatch_signal (GSettingsBackend    *backend,

                                    gsize                function_offset,

                                    const gchar         *name,

                                    gpointer             origin_tag,

                                    const gchar * const *names)

{

  GSettingsBackendWatch *watch;

  GSList *closures = NULL;

  /* We're in a little bit of a tricky situation here.  We need to hold

   * a lock while traversing the list, but we don't want to hold the

   * lock while calling back into user code.

   *

   * We work around this by creating a bunch of GSettingsBackendClosure

   * objects while holding the lock and dispatching them after.  We

   * never touch the list without holding the lock.

   */

  g_mutex_lock (&backend->priv->lock);

  for (watch = backend->priv->watches; watch; watch = watch->next)

    {

      GSettingsBackendClosure *closure;

      GObject *target = g_weak_ref_get (&watch->target);

      /* If the target was destroyed in the meantime, just skip it here */

      if (!target)

        continue;

      closure = g_slice_new (GSettingsBackendClosure);

      closure->context = watch->context;

      if (closure->context)

        g_main_context_ref (closure->context);

      closure->backend = g_object_ref (backend);

      closure->target = g_steal_pointer (&target);

      closure->function = G_STRUCT_MEMBER (void *, watch->vtable,

                                           function_offset);

      closure->name = g_strdup (name);

      closure->origin_tag = origin_tag;

      closure->names = g_strdupv ((gchar **) names);

      closures = g_slist_prepend (closures, closure);

    }

  g_mutex_unlock (&backend->priv->lock);

  while (closures)

    {

      GSettingsBackendClosure *closure = closures->data;

      if (closure->context)

        g_main_context_invoke (closure->context,

                               g_settings_backend_invoke_closure,

                               closure);

      else

        g_settings_backend_invoke_closure (closure);

      closures = g_slist_delete_link (closures, closures);

    }

}

/**

 * 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 occurring 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)

{

  g_return_if_fail (G_IS_SETTINGS_BACKEND (backend));

  g_return_if_fail (is_key (key));

  g_settings_backend_dispatch_signal (backend,

                                      G_STRUCT_OFFSET (GSettingsListenerVTable,

                                                       changed),

                                      key, origin_tag, NULL);

}

/**

 * g_settings_backend_keys_changed:

 * @backend: a #GSettingsBackend implementation

 * @path: the path containing the changes

 * @items: (array zero-terminated=1): 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)

{

  g_return_if_fail (G_IS_SETTINGS_BACKEND (backend));

  g_return_if_fail (is_path (path));

  /* XXX: should do stricter checking (ie: inspect each item) */

  g_return_if_fail (items != NULL);

  g_settings_backend_dispatch_signal (backend,

                                      G_STRUCT_OFFSET (GSettingsListenerVTable,

                                                       keys_changed),

                                      path, origin_tag, items);

}

/**

 * 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)

{

  g_return_if_fail (G_IS_SETTINGS_BACKEND (backend));

  g_return_if_fail (is_path (path));

  g_settings_backend_dispatch_signal (backend,

                                      G_STRUCT_OFFSET (GSettingsListenerVTable,

                                                       path_changed),

                                      path, origin_tag, NULL);

}

/**

 * 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)

{

  g_return_if_fail (G_IS_SETTINGS_BACKEND (backend));

  g_return_if_fail (is_key (key));

  g_settings_backend_dispatch_signal (backend,

                                      G_STRUCT_OFFSET (GSettingsListenerVTable,

                                                       writable_changed),

                                      key, NULL, NULL);

}

/**

 * 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)

{

  g_return_if_fail (G_IS_SETTINGS_BACKEND (backend));

  g_return_if_fail (is_path (path));

  g_settings_backend_dispatch_signal (backend,

                                      G_STRUCT_OFFSET (GSettingsListenerVTable,

                                                       path_writable_changed),

                                      path, NULL, NULL);

}

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: (out): the location to save the path

 * @keys: (out) (transfer container) (array zero-terminated=1): the

 *        location to save the relative keys

 * @values: (out) (optional) (transfer container) (array zero-terminated=1):

 *          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)

{

  const gchar **keys;

  gchar *path;

  g_return_if_fail (G_IS_SETTINGS_BACKEND (backend));

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

#ifdef DEBUG_CHANGES

  {

    gint i;

    g_print ("----\n");

    g_print ("changed_tree(): prefix %s\n", path);

    for (i = 0; keys[i]; i++)

      g_print ("  %s\n", keys[i]);

    g_print ("----\n");

  }

#endif

  g_settings_backend_keys_changed (backend, path, keys, origin_tag);

  g_free (path);

  g_free (keys);

}

/*< private >

 * g_settings_backend_read:

 * @backend: a #GSettingsBackend implementation

 * @key: the key to read

 * @expected_type: a #GVariantType

 * @default_value: if the default value should be returned

 *

 * 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.

 *

 * The returned value will be of the type given in @expected_type.  If

 * the backend stored a value of a different type then %NULL will be

 * returned.

 *

 * If @default_value is %TRUE then this gets the default value from the

 * backend (ie: the one that the backend would contain if

 * g_settings_reset() were called).

 *

 * Returns: (nullable) (transfer full): the value that was read, or %NULL

 */

GVariant *

g_settings_backend_read (GSettingsBackend   *backend,

                         const gchar        *key,

                         const GVariantType *expected_type,

                         gboolean            default_value)

{

  GVariant *value;

  value = G_SETTINGS_BACKEND_GET_CLASS (backend)

    ->read (backend, key, expected_type, default_value);

  if (value != NULL)

    value = g_variant_take_ref (value);

  if G_UNLIKELY (value && !g_variant_is_of_type (value, expected_type))

    {

      g_variant_unref (value);

      value = NULL;

    }

  return value;

}

/*< private >

 * g_settings_backend_read_user_value:

 * @backend: a #GSettingsBackend implementation

 * @key: the key to read

 * @expected_type: a #GVariantType

 *

 * Reads the 'user value' of a key.

 *

 * This is the value of the key that the user has control over and has

 * set for themselves.  Put another way: if the user did not set the

 * value for themselves, then this will return %NULL (even if the

 * sysadmin has provided a default value).

 *

 * Returns: (nullable) (transfer full): the value that was read, or %NULL

 */

GVariant *

g_settings_backend_read_user_value (GSettingsBackend   *backend,

                                    const gchar        *key,

                                    const GVariantType *expected_type)

{

  GVariant *value;

  value = G_SETTINGS_BACKEND_GET_CLASS (backend)

    ->read_user_value (backend, key, expected_type);

  if (value != NULL)

    value = g_variant_take_ref (value);

  if G_UNLIKELY (value && !g_variant_is_of_type (value, expected_type))

    {

      g_variant_unref (value);

      value = NULL;

    }

  return 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

 *

 * 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.

 *

 * If @value has a floating reference, it will be sunk.

 *

 * Returns: %TRUE if the write succeeded, %FALSE if the key was not writable

 */

gboolean

g_settings_backend_write (GSettingsBackend *backend,

                          const gchar      *key,

                          GVariant         *value,

                          gpointer          origin_tag)

{

  gboolean success;

  g_variant_ref_sink (value);

  success = G_SETTINGS_BACKEND_GET_CLASS (backend)

    ->write (backend, key, value, origin_tag);

  g_variant_unref (value);

  return success;

}

/*< private >

 * g_settings_backend_write_tree:

 * @backend: a #GSettingsBackend implementation

 * @tree: 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_tree (GSettingsBackend *backend,

                               GTree            *tree,

                               gpointer          origin_tag)

{

  return G_SETTINGS_BACKEND_GET_CLASS (backend)

    ->write_tree (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_get_writable:

 * @backend: a #GSettingsBackend implementation

 * @key: the name of a key

 *

 * 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.

 *

 * Returns: %TRUE if the key is writable

 */

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_finalize (GObject *object)

{

  GSettingsBackend *backend = G_SETTINGS_BACKEND (object);

  g_mutex_clear (&backend->priv->lock);

  G_OBJECT_CLASS (g_settings_backend_parent_class)

    ->finalize (object);

}

static void

ignore_subscription (GSettingsBackend *backend,

                     const gchar      *key)

{

}

static GVariant *

g_settings_backend_real_read_user_value (GSettingsBackend   *backend,

                                         const gchar        *key,

                                         const GVariantType *expected_type)

{

  return g_settings_backend_read (backend, key, expected_type, FALSE);

}

static void

g_settings_backend_init (GSettingsBackend *backend)

{

  backend->priv = g_settings_backend_get_instance_private (backend);

  g_mutex_init (&backend->priv->lock);

}

static void

g_settings_backend_class_init (GSettingsBackendClass *class)

{

  GObjectClass *gobject_class = G_OBJECT_CLASS (class);

  class->subscribe = ignore_subscription;

  class->unsubscribe = ignore_subscription;

  class->read_user_value = g_settings_backend_real_read_user_value;

  gobject_class->finalize = g_settings_backend_finalize;

}

static void

g_settings_backend_variant_unref0 (gpointer data)

{

  if (data != NULL)

    g_variant_unref (data);

}

/*< private >

 * g_settings_backend_create_tree:

 *

 * 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().

 *

 * Returns: a new #GTree

 */

GTree *

g_settings_backend_create_tree (void)

{

  return g_tree_new_full ((GCompareDataFunc) strcmp, NULL,

                          g_free, g_settings_backend_variant_unref0);

}

static gboolean

g_settings_backend_verify (gpointer impl)

{

  GSettingsBackend *backend = impl;

  if (strcmp (G_OBJECT_TYPE_NAME (backend), "GMemorySettingsBackend") == 0 &&

      g_strcmp0 (g_getenv ("GSETTINGS_BACKEND"), "memory") != 0)

    {

      g_message ("Using the 'memory' GSettings backend.  Your settings "

     "will not be saved or shared with other applications.");

    }

  g_settings_has_backend = TRUE;

  return TRUE;

}

/* We need to cache the default #GSettingsBackend for the entire process

 * lifetime, especially if the backend is #GMemorySettingsBackend: it needs to

 * keep the in-memory settings around even while there are no #GSettings

 * instances alive. */

static GSettingsBackend *settings_backend_default_singleton = NULL;  /* (owned) (atomic) */

/**

 * g_settings_backend_get_default:

 *

 * Returns the default #GSettingsBackend. It is possible to override

 * the default by setting the `GSETTINGS_BACKEND` environment variable

 * to the name of a settings backend.

 *

 * The user gets a reference to the backend.

 *

 * Returns: (not nullable) (transfer full): the default #GSettingsBackend,

 *     which will be a dummy (memory) settings backend if no other settings

 *     backend is available.

 *

 * Since: 2.28

 */

GSettingsBackend *

g_settings_backend_get_default (void)

{

  if (g_once_init_enter_pointer (&settings_backend_default_singleton))

    {

      GSettingsBackend *singleton;